Properties of Matter: Opposites Attract

3 Properties of Matter: Opposites Attract Learning Objectives As you work through this chapter you will learn how to: < classify materials as pure substances or mixtures. < distinguish between chemical and physical changes. < describe the subatomic composition of atoms and ions. < identify fundamental properties of the chemical elements. < distinguish between ionic and molecular compounds. < write correct chemical formulas and names for compounds. 3.1 Classification of Matter Two of the principal goals of chemistry are to identify the composition of matter, any substance that has mass and occupies space, and to provide an explanation for the chemical and physical properties of various substances. Chemical properties are those that involve a substance undergoing a change into a new substance. In contrast, physical properties are inherent characteristics of a substance and do not involve a change in composition. For example, two of the chemical properties of aluminum are its tendency to react with acids to produce hydrogen gas and its tendency to react with oxygen with the release of a large amount of energy. Aluminum’s physical properties include its silver-gray metallic appearance, a density of 2.7 g/cm3 and a melting point of 660 EC. Chemists have developed a useful classification system that allows us to anticipate the composition and properties of a wide range of common materials. The first stage of this classification system is shown in Figure 3.1. 61 62 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT matter pure substances mixtures Figure 3.1 Basic classification of matter Pure substances have a fixed composition and, because of this, have very specific properties like our aluminum example. Since the composition is always the same, a unique chemical formula can be written for a pure substance. There are two categories of pure substances, elements and compounds. Chemical elements are the basic building blocks of matter. An atom is the smallest unit of an element that can exist. The atoms of each element are unique in their composition and properties, and, according to atomic theory, all atoms of a given element have the same chemical properties. Presently there are 118 known elements.1 Each element has a unique name and chemical symbol. The names and symbols for a number of common elements are shown in Table 3.1. It is important that you learn the names and symbols for these elements. You will be using them throughout the course. pictures of common elements 1 There are 94 naturally-occurring elements and 24 elements that have been made by nuclear reactions. More information can be found at http://www.webelements.com/. CHAPTER 3 63 PROPERTIES OF MATTER: OPPOSITES ATTRACT Table 3.1 Names and Chemical Symbols of Common Elements Names and Symbols of Common Elements‡ Element Symbol Element Symbol Element Symbol Aluminum Al Gold Au Phosphorus P Argon Ar Helium He Platinum Pt Arsenic As Hydrogen H Potassium K Barium Ba Iodine I Rubidium Rb Boron B Iron Fe Silicon Si Bromine Br Lead Pb Silver Ag Cadmium Cd Lithium Li Sodium Na Calcium Ca Magnesium Mg Strontium Sr Carbon C Manganese Mn Sulfur S Chlorine Cl Mercury Hg Tin Sn Chromium Cr Neon Ne Titanium Ti Cobalt Co Nickel Ni Uranium U Copper Cu Nitrogen N Zinc Zn Fluorine F Oxygen O ‡ The chemical symbols for the elements are either one or two letters. The single letter symbol or the first letter of a two-letter symbol is always capitalized. Some symbols are derived from earlier names (usually in Latin) for the element; for example, Na for sodium comes from the Latin natrium. Chemical compounds are pure substances containing two or more chemically combined elements. Hydrogen peroxide and sodium chloride are examples of chemical compounds. Compounds are formed by chemical reactions between elements and/or other compounds. 3.2 How Matter Changes A chemical reaction, or chemical change, is a process that produces one or more new substances by rearrangement of the atoms present. Usually the new substances 64 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT produced in chemical reactions have properties very different than the original material so you can readily identify that a chemical change has occurred. However, in some instances the formation of a new substance may be difficult to detect and a chemical analysis is needed to confirm this. When an iron nail rusts it reacts with oxygen in the air to form a reddish-brown material that flakes off the surface of the nail. This rust has a very different appearance and properties compared to the nail so you expect that a new substance was formed by a chemical change. In a similar way, the formation of carbon dioxide and water in the combustion of gasoline suggests that a chemical reaction has occurred. On the other hand, a physical change is a process that does not produce a new substance. The melting of ice is a physical change because the ice and the liquid that forms are both water even though its appearance has changed. When sugar is dissolved in water the composition of both the sugar and the water do not change and no new substance is formed. If you allow the water to evaporate from a glass of sugar water, the solid that remains is sugar. If you gently heat the sugar water and allow the gas that comes off to condense in a separate container you can recover the water. You can distinguish between chemical and physical changes by remembering: Chemical changes produce new substances, physical changes do not. Check for Understanding 3.1 1. Solutions Indicate whether each of the following is primarily a chemical change or a physical change. a) painting a wall d) grating cheese b) boiling water e) burning a match c) frying an egg 2. Indicate one chemical property and one physical property of the element hydrogen. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 3.3 Mixtures In contrast to pure substances, mixtures have a variable composition so their properties are not as well defined and they do not have a specific chemical formula. They are created by mechanically blending together different substances. For example, you can create a potting soil mixture by combining various amounts of sand and peat moss depending upon the needs of your plants. As you change the proportions of sand and peat moss, properties like drainage change. This variation in properties with a variation in composition is typical of all mixtures. Mixtures are further classified as homogeneous or heterogeneous. A homogeneous mixture, also known as a solution, has a uniform composition throughout. It, like pure substances, consists of a single phase.2 If a teaspoon of salt is completely dissolved in water and thoroughly mixed, it forms a homogeneous mixture. A sample from any part of this mixture will have the same composition. However, if you put an excessive amount of salt in a glass of water, not all of the salt will dissolve no matter how much mixing you do. You have created a heterogeneous mixture. A heterogeneous mixture does not have a uniform composition throughout. In this case, there is a liquid phase with the salt dissolved in the water that has a very different composition than the undissolved salt portion. Heterogeneous mixtures have two or more distinct phases. Figure 3.2 shows an expanded classification scheme for matter. Metal alloys like steel are solid homogeneous mixtures of elements. Steel is primarily iron mixed with a small amount of carbon. White gold is an alloy of gold, nickel and palladium. In both instances the composition of the mixture is uniform throughout and therefore only a single phase is present. pictures of homogeneous and heterogeneous mixtures 2 A phase refers to a portion of a substance with a specific composition and hence appearance. It does not have to be a different physical state (solid, liquid or gas) than another phase. 65 66 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT matter pure substances elements compounds mixtures homogeneous heterogeneous Figure 3.2 Expanded classification scheme for matter Sometimes it is difficult to identify distinct phases when the phases occur on the microscopic level. For example, how would you classify blood? First, you expect blood to be a mixture because there are many different blood cells and substances in blood, and the composition of blood can vary for a given individual from hour to hour (for example, your blood sugar level is not constant throughout the day). Classifying blood as homogeneous or heterogeneous is not as obvious. It looks rather uniform, however, under a microscope a drop of blood clearly displays different phases; there is a liquid phase that contains solids (cells and cell fragments) of different composition. Consequently, blood is classified as a heterogeneous mixture. For our discussions we will consider a mixture as heterogeneous if it displays more than one phase upon visual inspection with the naked eye. When chemists perform experiments they try to control as many variables as possible. One important control is having information about all of the substances involved in an experiment. Thus, chemists use pure substances in their experiments as much as possible. Since the composition of a pure substance is known, the chemist has more information about what is involved in the experiment than if a mixture is used. A CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT chemist may also choose to use a homogeneous mixture (a solution) if it is carefully prepared with a known composition. Most materials that you encounter every day are not pure substances but are mixtures. Can you think of any common items around the home that are pure substances? Check for Understanding 3.2 1. 2. Solutions Indicate whether each of the following is a pure substance or a mixture. a) gold c) rubbing alcohol e) carbon monoxide b) flour d) stainless steel f) lead Indicate whether each of the following is a homogeneous or heterogeneous mixture. a) wood b) Jell-O c) vodka d) silver-plated jewelry 3.4 Atomic Structure Have you ever walked across a carpet on a dry day and gotten a shock when you touched someone or something? The shock you felt was a result of static electricity. Static electricity is the accumulation of electric charge on an object. The shock occurred as the accumulated charge was released. This is the same phenomenon that occurs with lightning discharges. One might wonder where this charge comes from. 67 68 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT There are two forms of electric charge in nature, positive and negative. You may know that opposite charges (positive and negative) attract and that like charges repel each other. The electrostatic force (F) of attraction or repulsion between two charges is described by Coulomb’s law, which can be represented mathematically by F  k q1 q2 r2 (3.1) where k is a numerical constant; q1 and q2 are the magnitudes (sizes) of the electric charges; r is the distance between charge centers. This equation indicates that the force of attraction or repulsion increases as the size of the charges increases (bigger q1 and q2) and as the charges get closer (smaller r). This law, first published in 1783 by the French physicist Charles Augustin de Coulomb, is an especially important one in chemistry. Essentially all of the interactions that are of interest to chemists involve the attractions and repulsions of electric charges. As incredibly small as atoms are, they are composed of even smaller units called subatomic particles. The electron is the lightest, stable subatomic particle that is known. It carries an electric charge that is believed to be the basic negative charge in nature. All atoms contain electrons, however, since atoms are electrically neutral, there must also be a source of positive electric charge present. A proton is the subatomic particle with a positive electric charge, equal in magnitude to the charge of an electron. The number of electrons in an atom always equals the number of protons present. There is another subatomic particle found in atoms, the neutron. A neutron has no net electric charge. These subatomic particles differ in a number ways other than their electric charge. The proton and neutron are very similar in mass (the neutron mass is just slightly larger) and they are almost 2000 times heavier than the electron. Protons and neutrons are found in the very dense center of the atom called the atomic nucleus while the electrons are in motion outside the nucleus. These properties are summarized in Table 3.2. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 69 Table 3.2 Electric charge, location and mass properties of the proton, neutron and electron Particle (symbol) Location in atom Relative electric charge‡ proton (p) nucleus +1 1.673 x 10-24 neutron (n) nucleus 0 1.675 x 10-24 Mass (g) electron (e-) outside the nucleus -1 9.109 x 10-28 ‡ The SI unit of electric charge is the coulomb (C). The magnitude of the electric charge carried by a proton or an electron equals 1.602 x 10-19 C. As was mentioned, an individual atom is incredibly small. In fact, you could fit about 5 million atoms across the period at the end of this sentence. Yet these incredibly small entities make up all matter. A useful physical model is to picture the electrons outside the nucleus forming a cloud of negative charge resembling a sphere (see Fig. 3.3). This cloud represents the size of the atom. The diameter of a typical atom is on the order of 10-10 m (100 pm). The nucleus, at the center of this charge cloud, is even smaller with a diameter on the order of 10-15 m. Figure 3.3 Representation of an atom (not to scale) In order for an electric charge to accumulate on an object, either positive charge (protons) or negative charge (electrons) must be gained or lost. Electrons are more readily removed or added to atoms because the primary force holding them in place is the electrostatic attraction of the electrons to the positive charge of the nucleus. Protons, on 70 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT the other hand, are bound together in the nucleus by the nuclear force. The nuclear force is an attractive force that acts between protons and neutrons. It is so strong that it prevents the positively-charged protons from repelling each other and leaving the nucleus. The number of protons and neutrons in an atom is not altered by chemical or physical changes, only by nuclear reactions. As you walk across the carpet on a dry day, your body picks up extra electrons that accumulate on your person until you touch some other object and the excess electrons are released when you get a shock. Although the atom’s nucleus is important, it is the number and arrangement of the electrons in an atom that determine the chemical properties of an element. You might wonder why the electrons are not eventually drawn to the opposite charge of the protons and end up in the nucleus. This question puzzled scientists for many years around the turn of the twentieth century. A simple but somewhat misleading explanation is that the motion of the electrons is what keeps them from being drawn into the nucleus. However, the factors responsible for atomic structure are much more complex than this. We will discuss some of this complexity in Chapter 7. 3.5 Chemical Elements and the Periodic Table The atoms of each chemical element are unique in terms of subatomic particle composition. The atomic number (Z) is defined as the number of protons that are present in an atom. This number is what defines an atom as being a specific element. Each element has a unique atomic number. For example, if an atom has 6 protons it must be an atom of carbon because the atomic number of carbon is 6. Since the number of electrons always equals the number of protons in an atom, knowing the atomic number also provides us with the electron count. Thus, you can associate a specific number of protons and electrons with each element. In order to fully characterize the subatomic particle composition of an atom the number of neutrons must also be known. This is available from the mass number (A) of an atom. The mass number is the total number of protons plus neutrons in an atom. The number of neutrons is obtained by subtracting the atomic number from the mass number (number of neutrons = A - Z). CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT A shorthand notation (see Fig. 3.4) is used to provide information about the composition of an atom. The notation consists of the symbol of the element with the mass number on the upper left and the atomic number on the lower left. Since the chemical symbol fixes the atomic number, the lower number is often omitted. Thus, 14C stands for an atom of carbon that has 6 protons, 6 electrons and 8 neutrons. Figure 3.4 Shorthand notation for designating specific atoms One of the most familiar images in chemistry textbooks, classrooms and laboratories is the periodic table of the elements. It is a visual representation of the chemical elements arranged into groups (vertical columns) and periods (horizontal rows). When the elements are arranged this way, it reflects regular trends in their properties. The Russian chemist Dmitri Mendeleev is credited with first proposing, in 1869, a periodic table consistent with the modern version. His arrangement of the known elements of the time allowed him to anticipate the discovery of as yet unknown elements. Figure 3.5 shows a simple version of the modern table. The elements are arranged according to increasing atomic number starting with hydrogen (Z=1) and moving from the left to right across the periods. Each element is assigned a location which is designated with the chemical symbol and atomic number of the element. Additional information about each element is displayed on more detailed periodic tables. 71 72 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT metals nonmetals metalloids 1A 8A 1 H 2A 26 atomic number (Z) Fe chemical symbol 3A 4A 5A 6A 7A 2 He 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne 11 Na 12 Mg 3B 4B 5B 6B 7B  8B 2B 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 55 Cs 56 Ba 57 La* 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 87 Fr 88 Ra 89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 Ac‡ Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo à 1B * 58 Ce 59 Pr 60 Nd 61 62 Pm Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er ‡ 90 Th 91 Pa 92 U 93 Np 95 96 Am Cm 97 Bk 98 Cf 99 Es 100 101 102 103 Fm Md No Lr 94 Pu Figure 3.5 Periodic Table of the elements 69 Tm 70 Yb 71 Lu CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Elements in the same group tend to have similar physical and chemical properties. Notice that in Figure 3.5 the groups (or families) are labeled with a number and a letter designation.3 The periods are numbered consecutively 1 through 7 (not shown) starting with the top row. The “A groups” are known as the main group elements and the “B groups” are the transition elements. Several of the A groups have common names. Group Common name 1A alkali metals (except hydrogen) 2A alkaline earth metals 7A halogens 8A noble gases The most important distinction to make among the chemical elements is that between the metals and nonmetals. Note that the elements to the left of the bold “stair step” line are the metallic elements. They have the physical and chemical properties that you expect for metals like copper and gold. They tend to be shiny, good conductors of electricity and heat, dense and malleable (able to be shaped). All of the metals are solids at room temperature except mercury which is a liquid. Chemically, metals tend to react with acids, oxygen and nonmetals. In contrast, the nonmetallic elements tend to be brittle nonconductors with low melting points and densities. Many nonmetals, like oxygen and nitrogen, are gases at room temperature. Chemically they tend to react with metals and with other nonmetals. There are a few elements, the metalloids, that exist in several physical forms. One form tends to have properties similar to those of metals while another form has properties characteristic of nonmetals. Many of the metalloids, for example, silicon (Si) and germanium (Ge), are semiconductor materials and are important in the technology industry. The chemistry of the metalloids will not be emphasized in this class. 3 On some periodic tables the groups are numbered 1 through 18, starting on the left. 73 74 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT For most elements the smallest stable unit is an individual atom of the element. For some nonmetals, though, the basic unit is not an atom but a two-atom unit called a diatomic molecule. A molecule is two or more atoms chemically bonded together. It is the smallest unit of a molecular compound, or of an element that does not consist of individual atoms. For example, a sample of oxygen gas does not consist of individual atoms of oxygen, but instead has diatomic molecules of oxygen with the chemical formula O2. The subscript (2) indicates the number of oxygen atoms in the molecule. The common diatomic elemental forms are H2, N2, O2, F2, Cl2, Br2 and I2. Unless you are specifically referring to a single atom of these elements, you should always use the chemical formula for the diatomic molecule to represent these elements. Check for Understanding 3.3 1. Indicate the chemical symbol for each of the following elements. a) silver 2. b) lead c) potassium d) silicon Indicate the name of each of the following elements. a) Br 3. Solutions b) Cu c) Au d) Ca Indicate name of the element and determine the number of protons, neutrons and electrons present for each of the following. You may consult the periodic table. a) 127 I b) 40 Ar c) 235 U CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 3.6 Gaining and Losing Electrons When an atom gains or loses electrons an imbalance of electric charge is created and a charged entity called an ion is formed. An ion is an atom or a group of atoms with a net electric charge. A cation is a positively-charged ion. An anion is a negativelycharged ion. In order to form a cation electrons must be removed from an atom. For example, a zinc atom normally has 30 protons, 30 electrons and some number of neutrons, and is electrically neutral. If two electrons are removed from a zinc atom there are now 30 positive charges in the nucleus and only 28 negative charges, so a cation with a +2 charge is formed. This zinc cation is represented like this: Zn2+. In a similar way, removing one electron from a potassium atom forms K+ (19 p, 18 e-), a potassium cation. Note that the number of neutrons present is not important as far as the ion charge is concerned so we can ignore them. In a similar way, electrons are added to an atom to form an anion. Adding one electron to a chlorine atom (17p, 17e-) forms the chloride ion Cl- (17p, 18e-). Likewise, adding two electrons to a sulfur atom (16p, 16e-) forms the sulfide ion S2- (16p, 18e-). All of the ions mentioned thus far are monatomic (single atom) ions. Notice that the anions are named differently than the cations. Monatomic cations are named using the full element name followed by the word “ion”. Zn2+ zinc ion K+ potassium ion Monatomic anions, however, are named using part of the element name with an -ide suffix and the word “ion”. Cl- chlorine + ide + ion ö chloride ion S2- sulfur + ide + ion ö sulfide ion 75 76 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Notice that in our examples so far, the cations were formed from atoms of metals and the anions were formed from atoms of nonmetals. This is an important trend for you to know. Metal atoms tend to lose electrons to form cations. Nonmetals tend to gain electrons to form anions. Since you are able to identify metallic and nonmetallic elements based on their location in the periodic table, you are now able to anticipate whether an atom is likely to form a cation or an anion. The major exception to these trends is hydrogen. Although it is classified as a nonmetal, it has a number of chemical properties similar to that of metals, including the tendency to lose its electron to form H+, a hydrogen ion. Now what about the ion charge? It would be very useful (and impressive!) if you could also anticipate the exact charge of a monatomic ion. Using the periodic table you can! The guidelines below are two other important trends for you to know. The “A” group metals tend to lose electrons to form cations with a charge equal to the group number. The nonmetals tend to gain electrons to form anions with a charge equal to the group number minus 8. These important trends are illustrated in Examples 3.1 and 3.2. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.1 Problem Write the correct chemical formula and the name for the likely monatomic ion formed from the element strontium. Solution Strontium (Sr) is a metal located in Group 2A. Therefore, it is expected to form a cation (since it is a metal) with a +2 charge (equal to the group number). The ion name is just the name of the element + ion. Sr2+ strontium ion Example 3.2 Problem Write the correct chemical formula and the name for the likely monatomic ion formed from the element nitrogen. Solution Nitrogen (N) is a nonmetal located in Group 5A. Therefore, it is expected to form an anion (since it is a nonmetal) with a -3 charge (equal to the group number - 8). The ion name is formed by adding -ide to part (nitrogen) of the element name. N3- nitride ion Unfortunately, this connection between the ion charge and group number does not work for the “B” group metals. In fact, many transition elements form more than one type of ion. For example, iron forms Fe2+ and Fe3+ ions. Along with different formulas, these two distinct ions of iron have different properties. In order to differentiate them, they are named with the element name following by a Roman numeral (in parentheses) 77 78 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT equal to the ion charge. Thus, Fe2+ is known as iron(II) ion (pronounced iron two ion) and Fe3+ is iron(III) ion. A number of common transition element ions will be identified in Section 3.8. Check for Understanding 3.4 Solutions 1. How does one form Fe3+ from an Fe atom? 2. Write the correct chemical formula and the name for the likely monatomic ion formed from each of the following elements. You may consult the periodic table. a) iodine b) barium c) lithium d) selenium 3.7 Chemical Compounds As noted before, chemical compounds are pure substances formed by chemical reactions between elements and/or other compounds. Since compounds are pure substances, they have a specific composition. Regardless of the source of a compound, it is always found to consist of the same elements in the same proportions. These observations are summarized by the law of constant composition. All samples of a specific compound contain the same proportions of their constituent elements. This means that a unique chemical formula can be written for each compound. You may be familiar with the formulas for chemical compounds like H2O (water) and NaCl (sodium chloride). The chemical symbols in the formula indicate what elements are present. The subscripts indicate the amounts of the constituent elements. For example, the formula for water indicates that it is composed of two elements, hydrogen and oxygen, and there are 2 hydrogen atoms for every oxygen atom (the subscript 1 is CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT assumed when no subscript is used). Depending on the type of compound, the chemical formula will also indicate additional information that we will consider shortly. When elements chemically combine to form compounds the properties of the compound are different than those of the individual elements. For example, liquid water results from the combination of two colorless gases, hydrogen and oxygen. Sodium chloride, a relatively stable and harmless solid, is formed from a very reactive metal (Na) and a poisonous gas (Cl2). There are two general categories of compounds, ionic and molecular. It is very important to be able to identify into which category a specific compound belongs because this will enable you to anticipate important physical and chemical properties of the compound. But how will you be able to distinguish between them? The characteristics of ionic and molecular compounds, including how to identify and name them, are discussed in the next two sections. compounds ionic molecular Figure 3.6 General categories of chemical compounds 3.8 Ionic Compounds An ionic compound (also known as a salt) is formed when a metal reacts with a nonmetal in a chemical reaction. Ionic compounds consist of a 3-dimensional arrangement of cations (formed from the metal) and anions (formed from the nonmetal) that are held together by strong electrostatic attractions between the oppositely charged ions. These attractions are referred to as ionic bonds. Figure 3.7 is a representation of this array of ions where the smaller (purple) spheres represent the cations and the larger 79 80 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT (green) spheres represent the anions. One of the most common ionic compounds is sodium chloride (NaCl), the major component in table salt. Like sodium chloride, ionic compounds are solids at room temperature, and, when they dissolve in water they form a mixture that can conduct electricity. Figure 3.7 Packing of ions in an ionic compound Since both cations and anions are needed for an ionic compound, you will generally find both a metal and nonmetal(s) present. Usually the cation will be an ion formed from a metallic element.4 In fact, the presence of a metal is the main feature that distinguishes the ionic and molecular compounds that we will consider. If a metal is present, the compound is ionic. In ionic compounds the number of positive charges is exactly balanced by the number of negative charges so the ionic compound is electrically neutral. The chemical formula of an ionic compound indicates the simplest ratio of the ions needed to achieve a 4 We will encounter only one situation where the cation is not formed from a metallic element. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT balance of charges. Since an ionic compound is an array of ions that might be very large or very small depending on the sample you have, there is no basic unit of an ionic compound like atoms are for elements. Instead, the formula unit is used. The formula unit for an ionic compound is the cation-anion grouping represented by the ionic formula. For example, NaCl is the formula unit for sodium chloride. Formulas for ionic compounds are written in the general form (cation)x(anion)y where x is the smallest whole number of cations and y is the smallest whole number of anions needed for charge balance. It is also important to note that the charges of the ions are never shown explicitly in the ionic formula. Ionic compounds are named using the cation name followed by the anion name. Example 3.3 illustrates the naming of ionic compounds and Example 3.4 shows how to determine the chemical formula and name for an ionic compound. 81 82 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.3 Problem Write the correct name for the following ionic compounds. A. BaF2 B. Fe2S3 Solution Notice that these ionic compounds involve a combination of a metal and nonmetal. The compound name is obtained by using the cation name followed by the anion name. The subscripts used for charge balance do not play a role in the naming of ionic compounds. A. BaF2 Barium is a metal located in Group 2A. Therefore, it is expected to form a cation with a +2 charge (Ba2+) and is named barium ion. Fluorine is a nonmetal located in Group 7A. Therefore, it is expected to form an anion with a -1 charge (F-) and is named fluoride ion. The compound name is obtained by using the cation name followed by the anion name. Therefore, the name of BaF2 is barium fluoride. B. Fe2S3 Sulfur is a nonmetal located in Group 6A. Therefore, it is expected to form an anion with a -2 charge (S2-) and is named sulfide ion. We have already learned that iron forms Fe2+ and Fe3+ ions. To determine which ion is involved, look at the charge balance that is indicated in the formula. The three -2 anion charges are balanced by two cations. Therefore, the charge on the iron ion must be +3 and Fe3+ is named iron(III) ion. The compound name is obtained by using the cation name followed by the anion name. Therefore, the name of Fe2S3 is iron(III) sulfide. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.4 Problem Write the correct chemical formula and name for the compound formed between magnesium and bromine. Solution Magnesium (Mg), a metal, is reacting with bromine (Br2), a nonmetal, so one should expect an ionic compound to form. The formula will take the general form MgxBry. In order to get the correct subscripts you need to note the charge of the cation and anion involved. Magnesium is a metal located in Group 2A. Therefore, it is expected to form a cation with a +2 charge. Mg2+ magnesium ion Bromine is a nonmetal located in Group 7A. Therefore, it is expected to form an bromide ion anion with a -1 charge. BrIn order to achieve charge balance you will need 2 bromide ions (Br-) for each magnesium ion (Mg2+). Thus, the formula becomes MgBr2. The name is obtained by using the cation name followed by the anion name. Therefore, the name of MgBr2 is magnesium bromide. Note: When the charges of the cation and anion are not the same, you can use a shortcut to indicate charge balance with the smallest set of subscripts. Use the value of the anion charge as the subscript for the cation and use the value of the cation charge as the subscript for the anion. Mg2+ and Br-1 ö Mg1Br2 ö MgBr2 When the charges of the cation and anion are the same, a 1:1 ratio of ions is needed and both the cation and anion will have a subscript of 1. 83 84 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Thus far, we have only considered ions formed from single atoms. However, there are many instances where a group of atoms that are chemically bonded together either gains or loses electrons to form a polyatomic (many atom) ion. Polyatomic ions will be found in many of the compounds you will encounter in this course. Ionic compounds containing polyatomic ions are also named using the name of the cation followed by the name of the anion. The important difference to consider with polyatomic ions is that whenever more than one is needed to balance charge, the formula for the polyatomic ion is enclosed in parentheses before attaching the subscript. This eliminates confusion with any subscripts associated with the polyatomic ion. For example, the formula for calcium chlorate is Ca(ClO3)2 and Fe3(PO4)2 is the formula for iron(II) phosphate. Table 3.3 contains a list of names and formulas for common cations. Table 3.4 contains a similar list for common anions. It is important that you learn the names and formulas for all of these ions. Example 3.5 illustrates how to write the chemical formula for a compound that contains a polyatomic ion. Example 3.5 Problem Write the correct chemical formula and the name for the compound formed between Cu2+ and nitrate ion. Solution From Table 3.4 on p. 86, we see that nitrate has the formula NO3-. Therefore, it takes two nitrates for each Cu2+ to balance the charges. Since the nitrate ion is a polyatomic ion, it is enclosed in parentheses with the subscript 2. The formula is Cu(NO3)2. Copper is a transition element with more than one common ionic charge. The possible charges are shown in Table 3.3) on p. 85. Therefore, the name for Cu2+ is copper(II) and the name of the compound is copper(II) nitrate. Cu(NO3)2 copper(II) nitrate CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Table 3.3 Names and Formulas of Common Cations CATIONS Plus One Charge Symbol Name Symbol Name Symbol Name H+ hydrogen Mg2+ magnesium Mn2+ manganese(II) Na+ sodium Ca2+ calcium Sn2+ tin(II) K+ potassium Sr2+ strontium Pb2+ lead(II) Ag+ silver Ba2+ barium NH4+ ammonium‡ Zn2+ zinc Cd2+ cadmium Ni2+ nickel Plus One or Two Charge ‡ Plus Two Charge Plus Two or Three Charge Plus Three Charge Symbol Name Symbol Name Symbol Name Cu+ copper(I) Fe2+ iron(II) Al3+ aluminum Cu2+ copper(II) Fe3+ iron(III) Hg22+ mercury(I) Co2+ cobalt(II) Hg2+ mercury(II) Co3+ cobalt(III) Cr2+ chromium(II) Cr3+ chromium(III) Ammonium ion is the only cation we will encounter that does not involve a metallic element. 85 86 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Table 3.4 Names and Formulas of Common Anions ANIONS Minus One Charge ‡ Minus Two Charge Minus Three Charge Symbol Name Symbol Name Symbol Name F- fluoride O2- oxide PO43- phosphate Cl- chloride O22- peroxide Br- bromide S2- sulfide I- iodide SO32- sulfite OH- hydroxide SO42- sulfate NO2- nitrite CO32- carbonate NO3- nitrate C2O42- oxalate C2H3O2- acetate SiO32- silicate CN- cyanide CrO42- chromate ClO- hypochlorite Cr2O72- dichromate ClO2- chlorite S2O32- thiosulfate ClO3- chlorate HPO42- hydrogen phosphate ClO4- perchlorate HCO3- hydrogen carbonate‡ HSO3- hydrogen sulfite‡ HSO4- hydrogen sulfate‡ MnO4- permanganate These ions are also known as bicarbonate (HCO3-), bisulfite (HSO3-) and bisulfate (HSO4-). CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Check for Understanding 3.5 Solutions It is important to be able to solve the following problems using only a periodic table. 1. Indicate the chemical formula and name for the common ion formed from each of the following elements. a) silver 2. c) lead d) aluminum Indicate whether each of the following compounds is ionic or molecular. a) CS2 3. b) oxygen b) HCl c) MnO2 d) Ba(OH)2 Indicate the name or formula, as appropriate, for each of the following compounds. a) Al2O3 b) iron(III) chloride c) potassium carbonate d) Zn(NO2)2 3.9 Molecular Compounds Molecular compounds consist of molecules composed of two or more different atoms bonded together by covalent bonds. A covalent bond forms when two atomic nuclei have strong electrostatic attractions for the same electron charge cloud. The two nuclei are said to be sharing a pair of electrons when they form a covalent bond. Molecular compounds are composed of nonmetal atoms (including hydrogen) and usually do not contain metals. The chemical formula for a molecular compound indicates the number of atoms of each type that are present in each molecule of the compound. For example, the formula for the fuel propane is C3H8. This means that each molecule of propane contains 3 atoms of carbon and 8 atoms of hydrogen. 87 88 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Just as spheres were used to represent the packing of cations and anions in an ionic compound, they are frequently used to indicate the bonding of atoms in a molecule. In Figures 3.8 and 3.9 molecules of water (H2O) and methane (CH4) are represented using two different models. A ball-and-stick molecular model (Fig. 3.8) is the traditional means of illustrating how the atoms in a molecule are bonded together. Atoms are represented as spheres connected by sticks that represent the covalent bonds between atoms. The model for water (Fig. 3.8a) shows that the oxygen atom is bonded to the two hydrogen atoms and that the three atoms form a bent structure. A space-filling molecular model (Fig. 3.9) also indicates the orientation of the bonded atoms but is more indicative of the relative size of the atoms in the molecule. (a) (b) (a) (b) Figure 3.8 Ball-and-stick molecular model for (a) water and (b) methane Figure 3.9 Space-filling molecular model for (a) water and (b) methane Images by Alan Shusterman, Reed College, 2011, reproduced with permission. Images by Alan Shusterman, Reed College, 2011, reproduced with permission. Molecules are not static entities as suggested by these models. At room temperature the atoms in a molecule are constantly vibrating and the entire molecule is rotating. However, such concrete models serve an important role for chemists as they test hypotheses about the detailed structure and properties of complex molecular systems. For example, James Watson and Francis Crick, the scientists who in 1953 proposed the double helix structure of DNA, used models to work out the details of their proposed structure (see Fig. 3.10). CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Figure 3.10 The original DNA model constructed by Watson and Crick. James Watson Collection, Cold Spring Harbor Laboratory Archives, reproduced with permission. The properties of molecular compounds are quite distinct from those of ionic compounds. A molecular compound may be a solid, liquid or gas at room temperature. Small molecules (fewer than 10 atoms) tend to be gases or liquids while larger molecules tend to be liquids or solids. Pure molecular substances are nonconductors of electricity and tend to be nonconductors even when dissolved in water. Important exceptions are the acids. An acid is a molecular substance that when dissolved in water produces H+ ions. Acids and water form a solution that conducts electricity. A comparison of the properties of ionic and molecular compounds is given in Table 3.5. 89 90 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Table 3.5 Comparison of the Properties of Ionic and Molecular Compounds Property Ionic Compounds Molecular Compounds composition chemical combination of metals and nonmetals chemical combination of different nonmetals structure 3-dimensional packing of cations and anions discrete molecules composed of a specific number and type of atoms indicates smallest ratio of ions needed for charge balance indicates exact composition of a single molecule solid small molecules: gas or liquid chemical formula usual physical state at room temperature electrical conductivity large molecules: liquid or solid pure substance: nonconducting pure substance: nonconducting dissolved in water: conducting dissolved in water: nonconducting, except for acids There are three important classes of molecular compounds that are distinguished by their composition and the rules used to assign them names. Binary molecular compounds Binary molecular compounds consist of molecules containing two nonmetals and no hydrogen. The general chemical formula is XaYb where X and Y are the chemical symbols of the nonmetals and a and b indicate the specific number of each atom present in each molecule. Carbon dioxide is a binary molecular compound with the formula CO2. Each molecule of carbon dioxide contains 1 atom of carbon bonded to 2 atoms of oxygen. The name for this binary molecular compound uses the entire element name for the first nonmetal atom in the formula (carbon) plus part of the element name (stem) with an -ide suffix for the second nonmetal atom in the formula (ox-ide). Binary molecular compound names also indicate the number of atoms of each type present in each molecule of the compound. This is done by attaching a Latin or Greek number prefix to CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT the element name to indicate how many atoms of the element are present. The common prefixes, along with their numerical values, are shown below. Adding prefixes results in the name monocarbon dioxide. However, when there is only one atom of the first nonmetal element no prefix is used, so finally we have carbon dioxide. Prefix Number mono 1 di 2 tri 3 tetra 4 penta 5 hexa 6 hepta 7 octa 8 nona 9 deca 10 Now we can see that the name for a binary molecular compound is constructed of 5 parts. number prefix for first nonmetal + first element name number prefix for second nonmetal + stem of second element name + -ide suffix Use of the naming structure for binary molecular compounds is illustrated in Examples 3.6 and 3.7. 91 92 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.6 Problem Given the formula PCl3, how is this binary molecular compound named? Solution Since there is only one atom of the first nonmetal (P) no prefix is used before the element name (phosphorus). There are 3 atoms of the second nonmetal (Cl) so the prefix tri- is used along with part of element name (chlorine) and the suffix -ide. Putting these pieces together yields number prefix for first nonmetal no prefix needed number prefix for second nonmetal + first element name + phosphorus PCl3 tri- + + stem of second element name + chlor- + -ide suffix -ide phosphorus trichloride Example 3.7 Problem Given the name dinitrogen monoxide, what is the chemical formula of this binary molecular compound? Solution The prefix di- indicates there are 2 atoms of the first nonmetal nitrogen (N). The prefix mono- indicates there is 1 atom of the second nonmetal oxygen (O). Thus the formula is N2O. dinitrogen monoxide N2O Note: This example illustrates the practice of dropping the ending vowel (o or a) from a prefix when it is used with the element oxygen. This avoids awkward names like monooxide or pentaoxide. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Binary acids Binary acids are molecular compounds with the general formula HnX(aq) where X is a nonmetal element bonded to n hydrogen atoms. The (aq) designation stands for aqueous solution and indicates that the compound is dissolved in water. The acid secreted by the stomach to help us digest our food is a binary acid, HCl(aq). The name of this acid is hydrochloric acid. The names of binary acids consist of the 4-part structure shown below. hydro- prefix + stem of nonmetal element name + -ic suffix acid Examples 3.8 and 3.9 illustrate use of this naming convention for binary acids. 93 94 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.8 Problem Given the formula H2S(aq), how is this binary acid named? Solution The name begins with the prefix hydro- followed by the stem of the nonmetal element (S) name followed by the suffix -ic. In this case, the full name sulfur is used to avoid a harsh sounding name (hydrosulfic). The word acid is added to complete the name. Putting these pieces together yields hydro- prefix hydro- H2S(aq) + stem of nonmetal element name + sulfur + -ic suffix + -ic acid acid hydrosulfuric acid It is important to note that the subscript 2 for hydrogen in the formula does not affect the naming. CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 95 Example 3.9 Problem Given the name hydrofluoric acid, what is the formula for this binary acid? Solution The hydro- prefix indicates that this is a binary acid and the stem fluor- tells you that the nonmetal fluorine is bonded to hydrogen. Thus, the binary acid formula will look like HnF(aq). In order to determine the value for n, remember that in water this molecule will break up to form H+ ions and fluoride anions. The charge on the cation must be balanced by the charge on the anion that is formed. Since fluorine is in Group 7A we expect it to form an anion with a minus one charge (F-). Thus, only one H+ is needed for each F- so n = 1. So the correct formula for hydrofluoric acid is HF(aq). Oxoacids The third category of molecular compounds we will consider is that of oxoacids. An oxoacid has the general formula HmXOn. That is, an oxoacid contains at least one oxygen atom and at least one hydrogen atom bonded to oxygen, plus one atom of another nonmetal element (X). Carbonic acid (H2CO3) is an oxoacid formed by dissolving carbon dioxide (CO2) in water. This is the acid present in all carbonated beverages. Although oxoacids are generally dissolved in water, the (aq) designation is not normally used after the chemical formula as was the case for binary acids. This difference is because the name for an oxoacid is the same whether it is dissolved in water or not. However, the name of a binary acid is different when it is not dissolved in water. For example, the gas HCl is named hydrogen chloride (similar to the naming pattern for binary molecular compounds) instead of hydrochloric acid used for the dissolved form HCl(aq)5. 5 The physical state of a substance is often designated by using the first letter of the state (solid, liquid or gas) in parentheses after the chemical formula. Thus, hydrogen chloride gas would be written as HCl(g) and sodium chloride solid would be NaCl(s). 96 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT The name for an oxoacid is derived from the name of the anion that forms when one or more hydrogen ions break away from the original oxoacid molecule. For example, if you remove two H+ ions from a molecule of carbonic acid, you are left with CO32-. The name of this anion is carbonate (recall Table 3.4). To get the oxoacid name you replace the -ate suffix with -ic and add the word acid. Thus carbonate becomes carbonic and we add the word acid to form carbonic acid. When the resulting anion has an -ite suffix, this is replaced by -ous to give the acid name. This naming pattern for oxoacids is illustrated in Examples 3.10 and 3.11. Example 3.10 Problem Given the formula HNO2, how is this oxoacid named? Solution Recall that the name of an oxoacid is linked to the name of the anion formed when the acid loses H+. Removing a H+ ion from this molecule leaves the nitrite anion NO2-. The -ite suffix is replaced by -ous and the word acid is added. This results in the name nitrous acid. HNO2 nitrous acid CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Example 3.11 Problem Given the name perchloric acid, what is the chemical formula for this oxoacid? Solution You can recognize this as an oxoacid because hydro- is not in the acid name. The -ic suffix indicates the anion name is perchlorate (ClO4-). Since this anion has a -1 charge only one H+ is needed to create the neutral oxoacid molecule. Thus, the formula is HClO4. perchloric acid HClO4 Check for Understanding 3.6 1. Indicate to which category of molecular compounds each of the following belongs and write the chemical name of the compound. a) HBr(aq) 2. Solutions b) HClO2 c) SF6 d) H2SO4 Write the chemical formula for each of the following molecular compounds. a) carbon disulfide b) nitric acid chemical nomenclature flashcards and review tests c) hydroiodic acid 97 98 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 3.10 Compound Naming Summary It is very important that you are able to distinguish between ionic and molecular compounds and properly write the name and chemical formula for a compound. Consider the following steps when given the name or formula of a chemical compound. 1. Identify the compound as ionic or molecular. Look for the presence of a metal, it indicates an ionic compound. 2. Ionic compound names consist of the cation name followed by the anion name. Ionic compound formulas take the basic form (cation)x(anion)y. The ion subscripts are the smallest whole numbers needed to achieve charge balance. 3. If the compound is molecular, identify which type of molecular compound it is (binary, binary acid, oxoacid), then apply the appropriate naming structure to writing the name or chemical formula. binary molecular compounds number prefix for first nonmetal + number prefix for second nonmetal first element name + stem of second element name binary acids hydro- prefix + stem of nonmetal element name + oxoacids -ate anions ö -ic acids -ite anions ö -ous acids -ic suffix acid + -ide suffix CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT Chapter 3 Keywords Glossary chemistry proton ion matter neutron cation chemical property atomic nucleus anion physical property nuclear force monatomic ion pure substance atomic number law of constant composition chemical element mass number chemical formula compound periodic table ionic compounds atom groups salt chemical change periods ionic bonds chemical reaction main group elements formula unit mixture transition elements polyatomic ion homogeneous mixture alkali metals covalent bond solution alkaline earth metals ball-and-stick model heterogeneous mixture halogens space-filling model phase noble gases acid physical state metals binary molecular compound static electricity nonmetals binary acid electrostatic force metalloids aqueous solution Coulomb’s law diatomic molecule oxoacid electron molecule subatomic particle diatomic elemental forms Supplementary Chapter 3 Check for Understanding questions Chapter 3 Exercises 1. Which of the following are pure substances? (a) (b) (c) (d) (e) vitamin tablet glass carbon monoxide wine helium Answers 99 100 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT 2. What is the fundamental difference between pure substances and mixtures? 3. Distinguish between homogeneous and heterogeneous mixtures. Give a specific example of each. Do not use examples given in the text. 4. Explain the difference between a phase and a physical state. 5. Which of the following are homogeneous mixtures? (a) (b) (c) (d) (e) 6. Which one of the following describes a chemical property of gold? A. B. C. D. E. 7. Gold is a yellow metal. Gold is a nonreactive metal. Gold is a soft metal. Gold is a very dense metal. Gold is a good conductor of heat and electricity. Which of the following are primarily chemical changes? (a) (b) (c) (d) (e) (f) 8. chocolate chip ice cream clean, dry air popcorn mercury steel grinding rocks into sand leaves turning red in autumn instant coffee dissolving in water evaporating alcohol explosion of a firecracker hard-boiling of an egg Distinguish between the electrostatic force and the nuclear force. CHAPTER 3 9. PROPERTIES OF MATTER: OPPOSITES ATTRACT Complete the following table with the appropriate data. Unless indicated otherwise, assume each line refers to a neutral atom. Element Symbol Atomic number Mass number 25 55 Number of Number of Number of protons neutrons electrons bromine 44 197 Au3+ 42 31 15 33 18 10. Where is most of the mass of an atom found? 11. Describe how cations and anions are formed. 12. What are the two primary types of chemical compounds? How do they differ from each other? 13. Determine the name or formula for each of the following. barium acetate _________________ CuSO4 ______________________ oxalic acid ____________________ CCl4 ________________________ iron(III) hydroxide ______________ K2S _________________________ ammonium carbonate ____________ H2SO3 _______________________ sodium phosphate _______________ NO2 ________________________ 101 102 CHAPTER 3 14. PROPERTIES OF MATTER: OPPOSITES ATTRACT Identify each of the following substances as ionic or molecular. HCN _________ NH3 _________ CS2 _________ 15. KClO3 _________ Write the formula of the compound that is likely to form between: silver and oxygen ____________ zinc and iodine ____________ 16. How many hydrogen atoms are there in each molecule of (C2H5)2NH? 17. What is the fundamental difference between a chemical and physical change? 18. A single compound that is a liquid at room temperature is most likely what type of compound (ionic or molecular)? Indicate two likely general properties of this compound. 19. In your own words state the law of constant composition. What does this suggest about the effectiveness of generic drugs? 20. What is meant by an aqueous solution? To what specific category of matter do aqueous solutions belong? 21. Prepare an expanded classification scheme of matter that includes all of the distinctions made in this chapter. 22. Indicate at least five regular chemistry patterns described in this chapter. CHAPTER 3 23. PROPERTIES OF MATTER: OPPOSITES ATTRACT What are the differences between the substances in each of the following pairs? a) CO and Co b) SO3 and SO32- 24. The correct formula for a compound made from copper and oxygen is: A. CuO 25. B. Cu2O C. Cu2O3 D. Cu3O2 E. both A and B One of the following formulas is incorrect. Write the correct formula for this compound and determine its correct name. A. Ba(OH)2 B. KMnO4 C. Na2O D. CaCl 103 104 CHAPTER 3 PROPERTIES OF MATTER: OPPOSITES ATTRACT A.18 APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS Chapter 3 Check for Understanding 3.1 1. Indicate whether each of the following is primarily a chemical change or a physical change. a) painting a wall d) grating cheese b) boiling water e) burning a match c) frying an egg Answers: a) physical change d) physical change b) physical change e) chemical change c) chemical change Solutions Recall that the fundamental difference between physical and chemical changes is that a new substance is formed in a chemical change but not in a physical change. a) Although some chemical changes may accompany the drying of paint, the process of applying paint to a wall does not produce a new substance. Thus it is a physical change. b) When water boils and forms a gas it is still water with the same composition as in the liquid state. Thus it is a physical change, as are all changes of physical state. c) Many new substances are formed as an egg cooks. You sense this because the appearance and properties of the egg change significantly. Thus it is a chemical change. d) The grated cheese is not a new substance, only a change in its physical form. Thus it is a physical change. e) Many new substances are formed when a match burns. You can detect some of these by their different properties (for example, odor, appearance). Thus it is a chemical change. APPENDIX B 2. SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS A.19 Indicate one chemical property and one physical property of the element hydrogen. Answers: physical properties - colorless odorless gas, density less than air chemical property - reacts with oxygen gas to form water Solutions A physical property of a substance does not involve a change in its composition whereas a chemical property does. There are many physical and chemical properties of hydrogen. The most familiar ones are listed. Check for Understanding 3.2 1. Indicate whether each of the following is a pure substance or a mixture. a) gold c) rubbing alcohol e) carbon monoxide b) flour d) stainless steel f) lead Answers: a) pure substance d) mixture b) mixture e) pure substance c) mixture f) pure substance Solutions A pure substance (an element or compound) has a fixed composition while a mixture does not. a) Gold is a chemical element so it is a pure substance. b) Flour is a fine powder prepared from grains such as wheat and barley plus other additives such as iron and vitamins. There are various grades of flour. Thus it is a mixture. c) Rubbing alcohol is a homogeneous mixture of two compounds, isopropyl alcohol and water. d) Stainless steel is an alloy of iron, carbon, and usually other metals such as nickel. Alloys are homogeneous mixtures. A.20 APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS e) Carbon monoxide is a compound containing carbon and oxygen. Thus it is a pure substance. f) Lead is a chemical element so it is a pure substance. 2. Indicate whether each of the following is a homogeneous or heterogeneous mixture. a) wood Answers: b) Jell-O c) vodka d) silver-plated jewelry a) heterogeneous mixture b) homogeneous mixture c) homogeneous mixture d) heterogeneous mixture Solutions A homogeneous mixture has a single phase with a uniform composition throughout whereas a heterogeneous mixture has two or more phases of varying composition. a) The grain and knots found in wood indicate phases of different composition so wood is a heterogeneous mixture. b) Jell-O is a homogeneous mixture because it consists of a single phase. c) Vodka is a homogeneous mixture because it consists of a single phase. d) The silver plating over a cheaper metal core represents two phases. Thus silverplated jewelry is a heterogeneous mixture. Check for Understanding 3.3 1. Indicate the chemical symbol for each of the following elements. a) silver Answers: b) lead c) potassium a) Ag c) K b) Pb d) Si d) silicon APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS Solutions You should memorize the names and symbols of the common elements shown in Table 3.1. 2. Indicate the name of each of the following elements. a) Br Answers: b) Cu c) Au d) Ca a) bromine c) gold b) copper d) calcium Solutions You should memorize the names and symbols of the common elements shown in Table 3.1. 3. Indicate name of the element and determine the number of protons, neutrons and electrons present for each of the following. You may consult the periodic table. a) Answers: 127 I b) 40 Ar a) iodine c) 235 b) argon 127 I 40 Ar U c) uranium 235 U number of protons 53 18 92 number of neutrons 74 22 143 number of electrons 53 18 92 Solutions The number of protons equals the number of electrons and both are equal to the atomic number (Z) of the element. The number of neutrons equals the mass number (shown as the superscript) minus the atomic number. a) The atomic number of iodine is 53, so the number of protons equals 53 and the number of electrons equals 53. The number of neutrons for 127I is determined by 127-53 = 74. A.21 A.22 APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS b) The atomic number of argon is 18, so the number of protons equals 18 and the number of electrons equals 18. The number of neutrons for 40Ar is determined by 40-18 = 22. c) The atomic number of uranium is 92, so the number of protons equals 92 and the number of electrons equals 92. The number of neutrons for 235U is determined by 235-92 = 143. Check for Understanding 3.4 1. How does one form Fe3+ from an Fe atom? Answer: Remove 3 electrons from an iron atom. Solution Cations are formed from atoms by the removal of electrons. In order to get an ion of +3 charge, three electrons must be removed from an iron atom. 2. Write the correct chemical formula and the name for the likely monatomic ion formed from each of the following elements. You may consult the periodic table. a) iodine Answers: b) barium c) lithium d) selenium a) I- iodide ion b) Ba2+ barium ion c) Li+ lithium ion d) Se2- selenide ion Solutions a) Iodine is a nonmetal and nonmetals tend to gain electrons to form anions with a charge equal to the group number minus 8. Since iodine is in Group 7A its ion charge should be 7-8 = -1. Monatomic anions are named using part of the element name with an -ide suffix and the word “ion”. This leads to I- (iodide ion). b) Barium is a metal and the “A” group metals tend to lose electrons to form cations with a charge equal to the group number. Since barium is in Group 2A its ion charge should be +2. Monatomic cations are named using the full element name followed by the word “ion”. This leads to Ba2+ (barium ion). APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS A.23 c) Lithium is a metal and the “A” group metals tend to lose electrons to form cations with a charge equal to the group number. Since lithium is in Group 1A its ion charge should be +1. Monatomic cations are named using the full element name followed by the word “ion”. This leads to Li+ (lithium ion). d) Selenium is a nonmetal and nonmetals tend to gain electrons to form anions with a charge equal to the group number minus 8. Since selenium is in Group 6A its ion charge should be 6-8 = -2. Monatomic anions are named using part of the element name with an -ide suffix and the word “ion”. This leads to Se2- (selenide ion). Check for Understanding 3.5 It is important to be able to solve the following problems using only a periodic table. 1. Indicate the chemical formula and name for the common ion formed from each of the following elements. a) silver Answers: b) oxygen c) lead d) aluminum a) Ag+ silver ion b) O2- oxide ion c) Pb2+ lead(II) ion d) Al3+ aluminum ion Solutions a) Silver is a transition element. These metals tend to form cations of various charge. You should memorize the formula of the common silver ion (Ag+) that is shown in Table 3.3. b) Oxygen is a nonmetal and nonmetals tend to gain electrons to form anions with a charge equal to the group number minus 8. Since oxygen is in Group 6A its ion charge should be 6-8 = -2. Monatomic anions are named using part of the element name with an -ide suffix and the word “ion”. This leads to O2- (oxide ion). A.24 APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS c) Lead is a transition element. These metals tend to form cations of various charge. You should memorize the formula of the common lead ion (Pb2+) that is shown in Table 3.3. d) Aluminum is an “A” group metal and the “A” group metals tend to lose electrons to form cations with a charge equal to the group number. Since aluminum is in Group 3A its ion charge should be +3. Monatomic cations are named using the full element name followed by the word “ion”. This leads to Al3+ (aluminum ion). 2. Indicate whether each of the following compounds is ionic or molecular. a) CS2 Answers: b) HCl c) MnO2 d) Ba(OH)2 a) molecular b) molecular c) ionic d) ionic Solutions a) This combination of nonmetals forms a molecular compound. b) This combination of nonmetals forms a molecular compound. c) This combination of a metal (Mn) and nonmetal (O) forms an ionic compound. d) This combination of a metal (Ba) and nonmetals (O and H) forms an ionic compound. APPENDIX B 3. SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS A.25 Indicate the name or formula, as appropriate, for each of the following compounds. a) Al2O3 b) iron(III) chloride c) potassium carbonate d) Zn(NO2)2 Answers: a) aluminum oxide b) Fe(Cl)3 c) K2CO3 d) zinc nitrite Solutions Note that since all of these compounds involve a combination of a metal and nonmetal(s) they are ionic. Thus the name is gotten from the cation name followed by the anion name. The formula is gotten by indicating the simplest ratio of cations and anions to produce charge balance. a) The metal cation is aluminum ion and the anion name is oxide ion. Thus the compound name is aluminum oxide. b) The Roman numeral indicates that the cation is Fe3+. The chloride ion formula is Cl-. Since it takes three chloride ions to balance the charge on the cation, the compound formula is FeCl3. c) The metal cation is K+. The polyatomic anion formula is CO32-. Since it takes two cations to balance the charge on the anion, the compound formula is K2CO3. d) The metal cation is zinc ion and the polyatomic anion is nitrite ion. Thus the compound name is zinc nitrite. Check for Understanding 3.6 1. Indicate to which category of molecular compounds each of the following belongs and write the chemical name of the compound. a) HBr(aq) b) HClO2 c) SF6 d) H2SO4 A.26 APPENDIX B Answers: SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS a) binary acid - hydrobromic acid b) oxoacid - chlorous acid c) binary molecular compound - sulfur hexafluoride d) oxoacid acid - sulfuric acid Solutions a) This is a binary acid because it follows the general formula HnX(aq). The name begins with the prefix hydro- followed by the stem of the nonmetal element name (brom-) followed by the suffix -ic. The word acid is added to complete the name. Putting these pieces together results in the name hydrobromic acid. b) This is an oxoacid because it has the general formula HmXOn. The name of an oxoacid is linked to the name of the anion formed when the acid loses H+. Removing a H+ ion from this molecule leaves the chlorite anion ClO2-. The -ite suffix is replaced by -ous and the word acid is added. This results in the name chlorous acid. c) This is a binary molecular compound because it has the general chemical formula is XaYb where X and Y are nonmetals. Since there is only one atom of the first nonmetal (S) no prefix is used before the element name (sulfur). There are 6 atoms of the second nonmetal (F) so the prefix hexa- is used along with part of element name (fluorine) and the suffix -ide. Putting these pieces together results in the name sulfur hexafluoride. d) This is an oxoacid because it has the general formula HmXOn. The name of an oxoacid is linked to the name of the anion formed when the acid loses H+. Removing two H+ ions from this molecule leaves the sulfate anion SO42-. The -ate suffix is dropped and sulf- is replaced by sulfur before adding the -ic suffix and the word acid. This results in the name sulfuric acid. 2. Write the chemical formula for each of the following molecular compounds. a) carbon disulfide Answers: a) CS2 b) HNO3 c) HI(aq) b) nitric acid c) hydroiodic acid APPENDIX B SOLUTIONS TO CHECK FOR UNDERSTANDING PROBLEMS A.27 Solutions a) Since the name does not contain the word acid, this must be a binary molecular compound. Each molecule contains atoms of carbon and sulfur and has the general formula CaSb. The absence of a prefix for the first nonmetal (C) indicates there is 1 atom of this element in each molecule. The prefix di- indicates there are 2 atoms of the second nonmetal (S). Thus the formula is CS2. b) This as an oxoacid because hydro- is not in the acid name. The -ic suffix indicates the anion name is nitrate (NO3-). Since this anion has a -1 charge only one H+ is needed to create the neutral oxoacid molecule. Thus, the formula is HNO3. c) The hydro- prefix indicates that this is a binary acid and the stem iod- tells you that the nonmetal iodine is bonded to hydrogen. Thus, the binary acid formula will look like HnI(aq). In order to determine the value for n, remember that in water this molecule will break up to form H+ ions and iodide anions. The charge on the cation must be balanced by the charge on the anion that is formed. Since iodine is in Group 7A we expect it to form an anion with a minus one charge (I-). Thus, only one H+ is needed for each I- so n = 1 and the formula is HI(aq). APPENDIX C ANSWERS TO SELECTED END-OF-CHAPTER EXERCISES A.65 Chapter 3 1. (c) and (e) 2. Pure substances have a definite composition and mixtures do not. 3. Homogeneous mixtures have a uniform composition (single phase) throughout. Heterogeneous mixtures contain more than one phase. Examples homogeneous mixtures: 14-karat gold, gasoline, container of oxygen gas and helium heterogeneous mixtures: granola bar, vinaigrette salad dressing, cloudy sky 4. A phase is a portion of a substance having a uniform composition throughout. A physical state characterizes a substance as a solid, liquid or gas. 5. (b) and (e) 6. B 7. (b), (e) and (f) 8. The electrostatic force is the force acting between objects having a net electric charge. It can be attractive or repulsive. The nuclear force is the short-range attraction acting between subatomic particles (protons and neutrons) in the nucleus of an atom. A.66 APPENDIX C ANSWERS TO SELECTED END-OF-CHAPTER EXERCISES 9. Element manganese bromine gold arsenic phosphorus Atomic number Mass number 25 55 25 30 25 Br 35 79 35 44 35 Au3+ 79 197 79 118 76 As 33 75 33 42 33 31 3- 15 31 15 16 18 Symbol 55 Mn 79 197 75 P Number of Number of Number of protons neutrons electrons 10. in the nucleus of the atom 11. Cations are formed by loss of electrons. Anions are formed by the gain of electrons. 12. see Table 3.5 13. barium acetate Ba(C2H3O2)2 CuSO4 oxalic acid H2C2O4 CCl4 carbon tetrachloride copper(II) sulfate iron(III) hydroxide Fe(OH)3 K2S potassium sulfide ammonium carbonate (NH4)2CO3 H2SO3 _ sulfurous acid sodium phosphate Na3PO4 NO2 nitrogen dioxide 14. HCN molecular 15. silver and oxygen zinc and iodine NH3 molecular Ag2O ZnI2 CS2 molecular KClO3 ionic ___ APPENDIX C ANSWERS TO SELECTED END-OF-CHAPTER EXERCISES 16. 11 17. A chemical change results in the formation of at least one new substance. No new substances are formed in physical changes. 18. molecular - a nonconductor that does not contain any metallic elements 20. An aqueous solution is a homogeneous mixture that has one or more substances dissolved in water. A.67 21. Metals Elements Metalloids Nonmetals Pure Substances Ionic Compounds Binary Molecular Binary acids Matter Oxoacids homogeneous Mixtures heterogeneous 23. a) CO is a molecular compound (carbon monoxide) composed of carbon and oxygen. Co is a chemical element (cobalt). b) SO3 is a molecular compound (sulfur trioxide) composed of sulfur and oxygen. SO32- is an anion (sulfite). A.68 APPENDIX C 24. E 25. D ANSWERS TO SELECTED END-OF-CHAPTER EXERCISES CaCl2 calcium chloride