Chemical Reactions Notes

CHEMICAL REACTIONS  Product : CONTENTS  CHEMICAL EQUATIONS A product is a new substance formed in a chemical reaction. For example, hydrogen and sodium hydroxide are the products of the reaction between sodium and water.  COVALENT BOND sodium  INTRODUCTION  BONDING IN METALS INTRODUCTION In the previous class, you have read about physical an chemical changes. Chemical changes result from chemical reaction taking placed between substances. In this chapter we shall deal with the chemical reactions and their representation in the form of chemical equations. The processes in which a substance or substances undergo change to produce new substances with new properties are known as chemical reactions. for example, when calcium carbonate is heated, calcium oxide (lime) and carbon dioxide are formed. The breaking up of calcium carbonate into calcium oxide and carbon dioxide is, thus, a chemical reaction because calcium carbonate changes into new substances, calcium oxide and carbon dioxide. Na  H 2O  water NaOH sodiumhy droxide The substance which takes part in a chemical reaction is called reactant. For example, in the breaking up of calcium carbonate into calcium oxide and carbon dioxide, calcium carbonate is the reactant. Similarly, sodium and water are the reactants when they react. H2 hydrogen Similarly, in the breaking up of calcium carbonate, calcium oxide and carbon dioxide are the products. CaCO 3 calcium carbonate  CaO calcium oxide  CO 2 carbon dioxide You know, atoms in a molecule are held together by a force of attraction called bond. The molecules do not participate directly in a chemical reaction. First they break down into atoms and these atoms then take part in the reaction. New bonds are formed between the atoms to form the products. That is, there take place rearrangement or regroupings of atoms in various ways to give products. For example, when ammonium cyanate is heated, different bonds in ammonium cyanate molecules are broken and new bonds are formed to produce urea. H 4 N — N  C  O  ammonium cyanate  Reactant :  O || H 2 N — C — NH 2 urea Here, we see that the molecular formulae of both ammonium cyanate and urea are the same, but their properties are quite different and they are two different compounds. Such compounds are known as isomers of each other and the reactions Eduull that produce such isomers are called isomerization reactions.  Valency : The number of electrons shared by an atom is called its valency. It is also called the combining capacity of an atom, e.g., Cl atom can share one valence electron, its valency is 1, Oxygen can share two valence electrons, its valency is 2. Nitrogen can share 3 valence electrons, its valency is 3, Carbon can share 4 valency electrons, therefore its valency is 4 and so on. It means if carbon combines with Chlorine, Carbon will share four valence electrons with four chlorine atoms, therefore the molecular formula of the covalent compound will be Element C Cl Valency 4 1 i.e., CCl4 is formula Some more examples are : Element H O H Valency 1 H2O Cl H S N H P 2 1 1 1 2 3 1 HCl H2 S NH3 Cl N 3 1 PCl3 5 2 N2 O5 Some Common Monoatomic Ions +1 Charge Name of ion Copper ion (Cuprous ion) Potassium ion Silver ion Formula Cu+ K+ Ag+ Sodium ion Lithium ion Na+ Li+ Aurous Au+ +2Charge Name of ion Barium ion Cobalt ion Strontium ion Iron (II) ion (Ferrous ion) *Copper (II) ion *Lead (II) ion Cadmium ion Magnesium ion Manganese (II) ion *Mercury (I) ion Zinc ion – 1 Charge Name of ion Bromide ion Chloride ion Fluoride ion Iodide ion Formula – Br Cl– F– I– – 2Charge Name of ion Oxide ion Sulphide ion Formula Ba2+ Co2+ Sr2+ Fe2+ Cu2+ Pb2+ Cd2+ Mg2+ Mn2+ +3 Charge Name of ion Aluminium ion Auric ion Chromium (III) ion Iron (III) ion (Ferric ion) Scandium ion Arsenic ion Bismuth ion Antimony ion Formula Al3+ Au3+ Cr3+ Fe3+ Sc3+ As3+ Bi3+ Sb3+ Hg 22 Zn2+ Formula 2– O S2– – 3 Charge Name of ion Nitride ion Phosphide ion Boride ion Formula N3– P3– B3–  These elements show more than one valency. So a Roman numeral shows their valency in a bracket. Some Common Polyatomic Ions – 1 Charge Name of ion Hydrogen carbonate or bicarbonate ion Hydrogen sulphate or (bisulphate ion) Formula HCO3 HSO 4 – 2Charge Name of ion Carbonate ion Manganate ion Formula CO32 MnO 24 Thiosulphate ion S2O32 Silicate ion SiO 32 – 3 Charge Name of ion Phosphate ion Arsenate ion Arsenite ion O Formula PO 34 AsO34 AsO33 Hydroxide ion OH– Sulphate ion SO 24 Nitrate ion NO3 Sulphite ion SO 32 Chlorate ion ClO3 Chromate ion Nitrite ion NO 2 Permanganate ion Acetate ion Cyanide ion Hypophosphite ion Meta aluminate ion Ammonium ion MnO4 CH3COO– Phosphite ion PO 33 CrO24 Borate ion BO33 Dichromate ion Cr2O72 Ferricyanide ion Hydrogen phosphate ion Oxalate ion HPO 24 [Fe(CN)6]3– C2O 24 CN– – 4 Charge H 2 PO 2 AlO2 Carbide ion +1 Charge Ferrocyanide ion C4– [Fe(CN)6]4– NH 4  Tests of Chemical Reaction : KNO3 potassium nitrate A chemical reaction must satisfy the following : (i) There must be either evolution or absorption of heat, i.e., a chemical reaction must be accompanied with change in temperature. (ii) The reaction must occur between fixed quantities of the reactants. (iii) There must not be either gain or loss of matter, i.e., a chemical reaction should follow the law of conservation of mass. (iv) The products obtained as a result of chemical reaction must have properties different from those of the reactants. CHEMICAL EQUATIONS  All chemical reactions are represented by chemical equations. A chemical equation is a shorthand representation of a chemical reaction using the symbols and formulae of substance involved in the chemical reaction. The symbols and formulae of the substances (elements or compounds) are arranged to show the reactants and products of a chemical reaction. Examples : 1. When potassium nitrate is heated, it gives potassium nitrite and oxygen. This reaction may be represented in the form of a chemical equation as follows.  KNO 2  O 2 potassium nitrite oxygen 2. Zinc and dilute sulphuric acid react to form zinc sulphate and hydrogen. This reaction is represented by a chemical equation as Zn + H2SO4  ZnSO4 + H2  Rules for writing chemical equation : Certain rules have to be followed while writing a chemical equation. 1. The reactants taking part in the reaction are written in terms of their symbols or molecular formulae on the left-hand side of the equation. 2. A plus (+) sign is added between the formulae of the reactants. 3. The products of reaction are written in terms of their symbols or molecular formulae on the right-hand side of the equation. 4. A plus (+) sign is added between the formulae of the products. 5. In between the reactants and the products an arrow sign () is inserted to show which way the reaction is occurring. A + B  C + D In this chemical equation, A and B are the reactants, and C and D are the products. The arrow indicates that the reaction proceeds towards the formation of C and D.  Balancing of Chemical Equation : Observe the following two chemical equations : on the products' side. Multiply Na by 2 on the reactants side so that they become equal on both side. The steps are as follows : Zn + H2SO4  ZnSO4 + H2 .....(i) Na + H2O  NaOH + H2 (i) Na + 2 H2O  NaOH + H2 .....(ii) (ii) Na + 2 H2O  2NaOH + H2 In equation (i), the number of atoms of Zn, H, S and O are equal on both sides, i.e., the equation is balanced  Balanced Equations : The equations in which atoms of various elements on the reactants' and the products side are equal. Equation (ii) is not balanced because the number of hydrogen atoms in not equal on both sides. It is called a skeleton chemical equation.  Reason of Balancing Equations : The number of atoms of elements on both sides of a chemical equation should be equal in accordance with the law of conservation of mass.  Balancing : The process of making atoms of various elements equal in an equation on either side is called balancing.  Steps in Balancing of Chemical Equations : A number of steps are involved in balancing a chemical equation, e.g., Na + H2O  NaOH + H2 Step-1 : Examine the number of atoms of different elements present in unbalanced equations. Number of Number of atoms in atoms in reactants products Na 1 1 H 2 3 O 1 1 Step-2 : Pick an element to balance the equation. In the above equation Na and O are balanced, Hydrogen is not. Step : To balance Hydrogen on both sides we need to multiply H2O by 2 which makes Hydrogen atoms equal to 4 on the reactants' side. To make Hydrogen 4 on the products' side, multiply NaOH by 2. Now oxygen has become 2 on both side. But Sodium atoms has become two (iii) 2 Na + 2 H2O  2NaOH + H2 The equation is now balanced. Example : Fe + H2O  Fe3O4 + H2 Step-1 : Fe Number of atoms in reactants 1 Number of atoms in products 3 H 2 2 O 1 4 Element Step-2 :Pick up the compound which has the maximum number of atoms whether a reactant or a product, and in that compound select the elements which has the highest number of atoms, e.g., we select Fe3O4 in the above equation : To balance oxygen atoms, In reactants Initial 1 (in H2O) To balance 1×4 In products 4 (in Fe3O4) 4×1 To equalise the number of atoms, we put the coefficient on the left side of the formula. A coefficient is a small whole number, like coefficients used in algebraic equations. You must keep in mind that we can put coefficients but we cannot change the subscripts in the formula, i.e., to balance Oxygen atoms, we can put the coefficient 4 as 4 H2O and not H2O4 or (H2O)4. Now the partly balanced equation becomes as follows : Fe(s) + 4 H2O(g)  Fe3O4(s) + H2(g) (Partly balanced) Step-3 : Pick up the second element to balance this partly balanced equation. Let us try to balance hydrogen atoms. In partly balanced equation. Atoms of Hydrogen. In reactants In products Edubull Initial 8 (in 8 H2O) 2 (in H4) Barium chloride BaCl2 Ba2+ and Cr To balance 8×1 2×4 Potassium nitrate KNO3 K+ and NO3 (NH4)2SO4 NH 4 and SO 24 Cupric sulphate CuSO4 Cu2+ and SO 24 Cupric chloride CuCl2 Cu2+ and Cl– To equalise the number of Hydrogen atoms, we use 4 as the coefficient of H2 in the products. Fe(s) + 4 H2O(g)  Fe3O4(s) + 4 H2 Step-4 : Pick up third element to be balanced. The element which is left to be balanced is Fe. In reactants In products Initial 1 (in Fe) 3 (in Fe3O4) To balance 1×3 3×1 To equalise, we use 3 as coefficient of Fe in reactants. 3Fe + 4H2O  Fe3O4 + 4H2 Atoms In reactants In products Fe 3 3 H 8 8 O 4 4 The equation is balanced because atoms of all the elements are equal on both sides. This method of balancing equation is known as hit and trial method.  Balancing of Ionic Equations : In these equations, charge balancing of atoms on both sides of the equation, e.g., Initial Cu2+(aq.) + H2S  CuS (s) + H+ (aq) Balanced Cu2+ (aq.) + H2S  CuS (s) + 2H+ (aq) We have balanced the charges. It was + 2 on LHS and we have made + 2 on RHS. Number of Hydrogen atoms, Cu and Sulphur atoms are also balanced on both sides. Compound Formula Ions involved Sodium chloride NaCl Na+ and Cl– Magnesium chloride MgCl2 Mg2+ and Cl– Magnesium oxide MgO Mg2+ and O2– Calcium chloride CaCl2 Ca2+ and Cl– Calcium oxide CaO Ca2+ and O2– NH4Cl NH 4 and Cl- Ammonium chloride Ammonium sulphate  Electrovalency : When an element forms electrovalent bond, its valency is known as electrovalency. The number of electrovalent or ionic bonds an atom can form is called its electrovalency. The electrovalency of an element is, therefore, equal to the number of electrons lost or gained by the atom to form an ion. Elements which lose electrons show positive electrovalency and those which gain electrons show negative electrovalency. For example, in the formation of sodium chloride (Na+Cl–), the electrovalency of sodium (Na) is +1, while that of chlorine (Cl) is – l. Elements which lose or gain one, two, three, ... , etc., electrons are said to be monovalent (orunivalent), divalent (or bivalent), trivalent, ... , etc., respectively. Monovalent elements : Na, CI, F Divalent elements : Mg, Ca, Ba, O Trivalent elements : Al, B  Characteristics compounds : of electrovalent or ionic 1. Electrovalent compounds are made up of positively and negatively charged ions. For example, sodium chloride (NaCl) is made up of Na+ and Cl– ions arranged in a definite order in three dimensions to form crystals. 2. Electrovalent compounds have high melting and boiling points. This is due to the presence of strong electrostatic forces of attraction between the positive and negative ions. A large amount of heat energy is required to break this force of attraction. Hence, the melting and boiling points of electrovalent compounds are high. 3. Electrovalent compounds are usually soluble in water but insoluble in organic solvents such as benzene, acetone, carbon disulphide and carbon tetrachloride. 4. Electrovalent compounds conduct electricity in molten state and in their aqueous solutions. In solid electrovalent compounds the ions are held together in fixed positions and cannot move. Hence, such compounds in the solid state do not conduct electricity. When an electrovalent compound is dissolved in water or is melted, the crystal structure breaks down. The ions now become free to move and can, therefore, conduct electricity. That the ionic compounds in molten state or in solution become conductors of electricity can be shown by the some specific activity. The shared electron pair always exists between the two atoms. The two dots between the two H atoms represent the pair of shared electrons. One pair of shared electrons gives a single bond. Such a bond is represented by a short line between the two atoms. Thus, a hydrogen molecule may be represented as in figure. H : H or H – H Once the bond is formed, the both atoms have a stable configuration of the noble gas helium. 2. Formation of a methane molecule (CH4) : A carbon atom has four electrons in its outermost shell (valence shell). It shares its valence electrons with those of four H atoms. Thus, an atom of carbon forms four single covalent bonds with four H atoms. The sharing of electrons between the two atoms takes place in such a way that both the atoms acquire the stable electronic configurations of their nearest noble gases. C + 4 H Pictorially, a methane molecule may be represented as in Figure. H There are three types of covalent bonds : H C H H 1. Single covalent bond 2. Double covalent bond 3. Triple covalent bond  Single covalent bond :  A single covalent bond is formed when one pair of electrons is shared between two atoms. H |  H C H or H—C—H | H H H THE COVALENT BOND The chemical bond farmed when two atoms share electrons between them is known as a covalent bond. H : H  Double covalent bond : A double covalent bond is formed when two pairs of electrons are shared between the two combining atoms. A sharing of two pairs of electrons is shown by marking two short lines between the symbols of the two atoms. EXAMPLES : EXAMPLES : 1. Formation of a hydrogen molecule (H2) : 1. Formation of an oxygen molecule (O2) : A molecule of hydrogen consists of two hydrogen atoms. Each hydrogen atom has one electron. When two atoms of hydrogen combine, one electron of each takes part in sharing. Thus, two electrons (one pair of electrons) are shared between the two atoms. H + H  H:H An atom of oxygen contains six electrons in its valence shell. It requires two more electrons to attain a stable eight-electron configuration (octet). This is achieved when each of the two oxygen atoms shares its two electrons with the other, resulting in the formation of a stable oxygen molecule. Edubul Pictorially, a nitrogen molecule can be represented as in figure. Double bond O + O  O O or O  O N N Pictorially, the oxygen molecule may be represented as in figure. O O 2. Formation of an ethylene molecule (C2H4) : In the formation of an ethylene molecule (C2H4), each of the two C atoms combines with two H atoms to form two single covalent bonds. The remaining two electrons of each C atom form a double bond between the two C atoms. H H 2 C + 4 H  C C or H H H H C  C H H (Ethylene molecule) Pictorially, a molecule of ethylene (C2H4) may be represented a in figure. H H C C H H   Triple covalent bond : A triple covalent bond is formed when three pairs of electrons (six electrons) are shared between the two combining atoms. A triple bond is shown by marking three short lines between the two symbols of the atoms. EXAMPLES : 1. Formation of a nitrogen molecule (N2) : An atom of nitrogen has five electrons in its valence shell. It requires three more electrons to attain the stable octet. This is achieved when two nitrogen atoms combine together by sharing three electrons each to form a nitrogen molecule. Triple bond N + N  N N or N  N 2. Formation of an acetylene molecule (C2H2) : In an acetylene molecule, two C atoms combine with two H atoms. Each C atom shares three of its valence electrons with the other C atom. One electron of each C atom is shared with one electron of a H atom. Thus, in a molecule of acetylene, there is a triple covalent bond between the two C atoms and each C atom is joined to one H atom by a single covalent bond. Pictorially, a molecule of acetylene may be represented as in figure. H C C H  Characteristics of covalent compounds : 1. Covalent compounds are made up of neutral molecules. Hence, the forces of attraction between the molecules are weaker than those found in ionic compounds. Therefore, covalent compounds are usually volatile liquids or gases. 2. The melting and the boiling points of covalent compounds are generally low. Since covalent compounds are made up of neutral molecules, the forces of attraction between the molecules are very weak. So, a comparatively small amount of heat energy is required to break these weak intermolecular forces of attraction. Hence, they have low melting and boiling points. 3. Covalent compounds are insoluble in water but soluble in organic solvents. 4. Covalent compounds do not conduct electricity. This is because they are made up of neutral molecules, not ions, and do not produce ions in the molten state or in aqueous solutions. BONDING IN METALS As you know, metals are hard solids and they are made up of atoms. It has been established that the atoms in a metal are very closely packed together. The force that holds the atoms closely together in a metal is known as the metallic bond. Metal atoms lose one, two or three electrons to form positively charged ions, called cations. Edbull The electrons thus lost move freely in the metal, i.e., these electrons become mobile, but the cations do not leave their positions. So in a metal lattice it is assumed that the metal ions are immersed in a sea of electrons. Due to the presence of mobile electrons, metals are good conductors of heat and electricity. POINT TO REMEMER The total number of electrons lost or gained by an atom to attain the stable configuration of the nearest noble gas is known as the valency of the atom. The number of electrovalent bonds by an atom can form is called its electrovalency. Covalent bonds are formed by the mutual sharing of electrons between the combining atoms. A chemical bond is the force of attraction which holds together the atoms in a molecule. Compounds containing covalent bonds are called covalent compounds. An electrovalent bond is formed as a result of complete transfer of electron(s) from one atom to another. The. total number of electrons shared by a given atom in the formation of covalent bonds to attain the stable configuration of its nearest noble gas is known as its covalency.  Atoms which can lose electrons and form positive ions are said to be electropositive. Atoms which can gain electrons and form negative ions are said to be electronegative. The force that holds the atoms closely together in a metal is known as the metallic bond.