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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Chapter Contents Chapter 7 Equilibrium Physical Equilibrium Equilibrium in Chemical Process Homogeneous Equilibria Heterogeneous Equilibria Applications of Equilibrium Constant Relationship between Equilibrium constant (K), Reaction Quotient (Q) and Gibbs Energy (G) Ionic Equilibria Ionization Constants of Weak Acids and Weak Bases Common ion effect in the ionisation of Acids and Bases Hydrolysis of Salts and the pH of their Solution Buffer Solution Solubility of Sparingly Soluble Salts Reversible processes : Reaction in which the products of the reaction can react with one another under same conditions of the reaction to give back the reactants e.g. formation of NH3, HI, SO3, etc. do not go to completion. Irreversible processes : Those in which the products formed do not react to give back the reactants e.g. 2KClO3  2KCl + 3O2. Such processes go to completion e.g. precipitation, the reactions involving evolution of gases, etc. Types of Equilibria (1) Physical equilibrium e.g., H2O(l) H2O(g) (2) Chemical equilibrium e.g., PCl5(g) PCl3(g) + Cl2(g) (3) Ionic equilibrium e.g., CH3COOH(aq.) CH3COO–(aq.) + H+(aq.) PHYSICAL EQUILIBRIUM Process Property becoming constant at equilibrium Solid Liquid Melting point at constant e.g., H2O(s) H2O(l) pressure Liquid Vapour Vapour pressure (e.g., pH2O) e.g., H2O() H2O(g) at constant temperature Solute (s) Solute (solution) Concentration of the solute e.g., Sugar(s) Sugar (solution) in the solution at the given temperature Gas(g) Gas(aq) Gas(aq) Gas(g)       at the given e.g., CO2(g) CO2(aq) temperature, e.g., 2 2 CO (aq) CO (g)       at the given temperature
2 Equilibrium NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 General Characteristics of Equilibria involving Physical processes (i) At equilibrium, some observable property of the system becomes constant. (ii) Equilibria involving gases can be attained only in closed vessels. (iii) Equilibrium is dynamic in nature i.e., there are two opposing processes taking place at equal rates. (iv) At equilibrium, the concentrations of the different substances become constant at constant temperature. (v) At equilibrium, the ratio of the species present in two different phases is constant at a constant temperature. For example, For CO2(g) CO2(aq.) Ratio of 2 2 [CO (aq)] [CO (g)] = Constant This constant is known as equilibrium constant. (vi) The magnitude of the equilibrium constant represents the extent to which the process proceeds before equilibrium is attained. EQUILIBRIUM IN CHEMICAL PROCESS Law of chemical equilibrium “The rate at which a substance reacts is proportional to its active mass and the rate of a chemical reaction is proportional to the product of the active masses of the reacting substances”. Active mass means molar concentration i.e., the number of gram moles of the solute per unit volume. It is expressed by enclosing the formula of the substance in square brackets. For example, [A] represents the active mass of the substance A. The active mass of solids is taken as one. Chemical Equilibrium At same given set of condition, the rate at which products are produced from reactants equals the rate at which reactants are produced from products is known as equilibrium of reaction. e.g., aA + bB cC + dD (i) aA + bB f k cC + dD ...(1) (ii) cC + dD b k  aA + bB ...(2) a b f f c d b b Rate of forward reaction (1) r k [A] [B] According to law of mass action Rate of backward reaction (2) r k [C] [D]         At equilibrium rf = rb  c d f a b b k [C] [D] k [A] [B]  c d f a b b k [C] [D] K k [A] [B]   where K = equilibrium constant, kf and kb are rate constants for forward and backward reaction
NEET Equilibrium 3 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456  [Pr oduct] K [Reactant]  e.g., H2(g) + I2(g) 2HI(g); 2 2 2 [HI] K [H ][I ]  Note : Solid and liquid (in excess) entities are not taken in this equation as their activity is taken as 1. Characteristics of equilibrium constant (i) The equilibrium constant does not depend upon initial concentration and has a definite value for every chemical reaction at a given temperature. Van't Hoff Equation It gives variation of K with temperature. log 2 2 1 1 1 2 K TT H K 2.303R T T          R is gas constant, H is enthalpy change, K1 and K2 are equilibrium constants at temperatures T1 and T2 respectively. (ii) If the reaction is reversed, the value of equilibrium constant is reciprocated. Example : For reaction, H2(g) + I2(g) 2HI(g) Kc = 50 If reaction is reversed 2HI(g) H2(g) + I2(g) Now, the value of the equilibrium constant will be 2 2 c 2 c 1 1 [H ][I ] K 0.02 K 50 [HI]     (iii) If the reaction is multiplied by a factor (n) then the value of equilibrium constant becomes K c = (Kc) n Example : A B 2C  ; Keq = Kc 2A 2B 4C  ; Keq = K c K c = (Kc) 2 or A B 2C  ; Keq = Kc 1 1 AB C 2 2  ; Keq = K c K c = (Kc) 1/2
4 Equilibrium NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 (iv) The pure solids and liquids are not considered for calculation of Kc because they have constant concentration, if one of the reactant or product is gas. (v) If any chemical reaction is completed in more than one step, then equilibrium constant for overall reaction is equal to the product of equilibrium constant of all steps. Example : Step 1 : N2 + O2 2NO ; K1 Step 2 : 2NO + O2 2NO2 ; K2 N2 + 2O2 2NO2 Overall reaction; K Here, K1 × K2 = K 2 2 2 2 2 2 2 2 2 2 22 [NO] [NO ] [NO ] K [N ][O ] [NO] [O ] [N ][O ]   HOMOGENEOUS EQUILIBRIA “When in an equilibrium reaction, all the reactants and the products are present in the same phase (i.e., gaseous or liquid) it is called a homogeneous equilibrium.” For example, N2(g) + 3H2(g) 2NH3(g) In the gaseous reaction, reactants and products are in the homogeneous phase. Similarly, for the reactions, CH3COOC2H5(aq) + H2O (l) CH3COOH (aq) + C2H5OH (aq) and, Fe3+(aq) + SCN–(aq) [Fe(SCN)]2+(aq) all the reactants and products are in same phase. We shall now consider them in equilibrium constant for some homogeneous reactions. HETEROGENEOUS EQUILIBRIA “When in an equilibrium reaction, the reactants and the products are present in two or more than two phases, it is called a heterogeneous equilibrium.” The equilibrium between water vapour and liquid water in a closed container is an example of heterogeneous equilibrium. H2O(l) H2O(g) Let us take thermal dissociation of calcium carbonate which is an interesting and important example of heterogeneous chemical equilibrium. CaCO3(s)  CaO(s) + CO2(g) On the basis of the stoichiometric equation, we can write, 2 c 3 [CaO(s)][CO (g)] K [CaCO (s)]  Since [CaCO3(s)] and [CaO(s)] are both constant, therefore modified equilibrium constant for the thermal decomposition of calcium carbonate will be K c = [CO2(g)] or Kp = pCO2