Qualitative Analysis and Chemical Bonding Solids Lab Handout

Qualitative Analysis and Chemical Bonding Chemistry Big Idea 2, Investigation 6 An Advanced Inquiry Lab Introduction Looking for patterns in the properties of solids can help us understand how and why atoms join together to form compounds. What kinds of forces hold atoms together? How do these forces influence the properties of materials? Use your knowledge of the relationship between chemical bonding type and the properties of substances to determine the identity of mystery solids. Concepts • Chemical bonds • Ionic bonding Physical and chemical properties • Covalent bonding • Metallic bonding Electronegativity Background Groups of atoms are held together by attractive forces that we call chemical bonds. The origin of chemical bonds is reflected in the relationship between force and energy in the physical world. Think about the force of gravity-in order to overcome the force of attraction between an object and the Earth, we have to supply energy. Whether we climb a mountain or throw a ball high into the air, we have to supply energy. Similarly, in order to break a bond between two atoms, energy must be added to the system, usually in the form of heat, light or electdcity. The opposite is also tLue: whenever a bond is formed, energy is released. The term ionic bonding describes attractive forces between oppositely charged ions in an ionic compound. An ionic compound is formed when a metal reacts with a nonmetal to form positively charged cations and negatively charged anions, respectively. The oppositely charged ions are arranged in a tightly packed, extended three-dimensional structure called a crystal lattice (see Figure !). The net attractive forces between oppositely charged ions in the crystal structure are called ionic bonds. 8 Chloride anions 0 Sodium cations Figure 1. Crystal Structure of Sodium Chloride. Covalent bonding represents another type of attractive force between atoms. Covalent bonds are defined as the net attrac­ tive forces resulting from pairs of electrons that are shared between atoms (the shared electrons are attracted to the nuclei of both atoms in the bond). A group of atoms held together by covalent bonds is called a molecule. Atoms may share one, two or three pairs of electrons between them to form single, double and triple bonds, respectively. Substances held together by covalent bonds are usually divided into two groups based on whether individual (distinct) mol­ ecules exist or not. In a molecular solid, individual molecules in the solid state are attracted to each other by relatively weak inter­ molecular forces between the molecules. Covalent-network solids, on the other hand, consist of atoms fomring covalent bonds with each other in all directions. The result is an almost infinite network of strong covalent bonds-there are no individual molecules. LElectron Covalent bonds may be classified as polar �"cloud" or nonpolar. The element chlorine, for example, exists as a diatomic molecule, CL,. The two chlo­ Cl-Cl H-CI rine atoms are held together by a single covalent bond, with the two electrons in the bond shared Polar Bond between the two identical chlorine atoms. This Nonpolar Bond type of bond is called a ,u.mpolar covalent bond. Figure 2. Nonpolar versus Polar Covalent Bonds. The compound hydrogen chloride (HCl) consists of a hydrogen atom and a chlorine atom that a1so share a palf of electrons between them, Because the two atoms are different1 however, the electrons in the bond are not equally shared between the atoms. Chlorine has a greater electronegativity than hydro­ gen-it attracts the bonding electrons more strongly than hydrogen. The covalent bond between hydrogen and chlorine is an example of a polar bond. The distribution of bonding electrons in a nonpolar versus polar bond is shown in Figure 2. Notiee that the chlorine atom in HCJ has a partial negative charge (o-) while the hydrogen atom has a partial positive charge (I>'). The special properties of metals compared to nonmetals reflect their unique structure and bonding. Metals typically have a small number of valence electrons available for bonding. The valence electrons appear to be free to move among all of the metal atoms, some of which must exist or act as positively charged cations, ildetallic bonding describes the attractive forces that exist between closely packed metal cations and free.floating valence electrons in an extended three-dimensional structure. Experiment Overview The puqmse of this advanced inquiry lab is to identify twelve unknown solids based on systematic testing of their physi­ cal and chemical properties. The lab begins with an introductory activity to select measurable properties that will help identify the type of bonding in a solid. Given four solids representing the four types of chemical bonds-ionic, polar covalent, nonpolar covalent and metallic-students review the properties of each solid with a minimum of four tests. The results provide a basis for a guided-inquiry design of a flow chart procedure to distinguish and identify twelve unknown solids. Pre-Lab Questions I Melting Point, Solubility In Water, 25 °c Soluble Soluble Slightly soluble ' Insoluble i 1. Considering the data in the above table, explain the following observations based on the type of chemical bonding and intennolecular forces between atoms, molecu1es or ions in the solid state. Solid Sodium iodide, NaI Lactose, C1,H2:,0n Iodine, Tin, Sn 1z ' Bond Type Ionic Polar covalent Nonpolar covalent Metallic ; 661 203 114 232 0 c Color White White Dark gray Shiny gray a. Iodine has a much lower melting point than sodium iodide. b. Iodine is slightly soluble in water, wrule lactose is very soluble. 2. Predict the properties of the following solids. Solid ' Iodine chloride, ICI Cobalt nitrate, Co(NO3)2 Sulfur, S8 Lead, Pb Bond Type Melting Point High or Low Solubility In Water, 25 °C Materials Ethyl alcohol, CH3CH2OH, 20 mL Beaker, 100-mL Hexane, C6H l4' 20 mL Bunsen burner Hydrochloric acid solution, HCI, 0.1 M, 20 mL Conductivity meter or tester Sodium hydroxide solution, NaOH, 0.1 M, 20 mL Hot plate Water, distilled or deionized, 20 mL pH paper Known samples for the Introductory Activity Stirring rod Copper(II) sulfate, CuSO4, 2 g Test tube rack Dextrose monohydrate, C6H 12 O6H2O, 2 g Test tube holder Paraffin wax, C,H2, + 2 (n = 20-40), 2 g Test tubes, 6 Zinc, Zn, 2 g Thermometer Unlabeled samples, 6, 3 g each Tongs Aluminum dish Safety Precautions Hexane and ethyl alcohol are flammable organic solvents and dangerous fire risks. Keep away from flames, heat, and other sources of ignition. Cap the solvent bottles and work with hexane and ethyl alcohol in a fume hood or designated work area. Addition of a denaturant makes ethyl alcohol poisonous; it cannot be made nonpoisonous. Copper(II) sulfate is a skin and respira­ tory tract irritant and is toxic by ingestion. Graphite powder is a fire and inhalation risk. Dilute sodium hydroxide and acid solu­ tions are irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical­ resistant gloves, and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab. Please follow all laboratory safety guidelines. Introductory Activity Identifying Properties of Chemical Bonds 1. Four representative chemicals are provided for preliminary testing to identify physical and chemical properties that can be used for development of a qualitative analysis scheme. The chemicals and the type of bonding in each are: Copper(ll) sulfate-ionic bonding Paraffin wax-nonpolar covalent bonding Dextrose-polar covalent bonding Zinc-metallic bonding 2. Observe the color and appearance of each solid and perform the following qualitative tests on each: (a) solubility in water, solubility in hexane, and solubility in alcohol; (b) high or low melting point; (c) conductivity of solid and conductivity of aqueous solution; (d) pH of solution; (e) reaction with acid (0.1 M HCl) and reaction with base (0.1 M NaOH). 3. Consult the Materials list when developing small-scale or microscale procedures for these tests. Note that a total of 2 g of each solid and 20 rnL of each solvent are provided for testing. 4. To test the melting point of a substance, first place a small amount of each solid in separate locations in an aluminum evaporating dish. Hold the dish above the boiling water bath with tongs and observe if the solids melt at <100 °C. 5. If the substance did not melt at <100 °C, place a pea-size amount of solid in a borosilicate glass test tube. Heat the test tube in a burner flame for 1-2 minutes. Record observations. 6. Record the results of qualitative testing in a data table. -3-