Equilibrium Eng. PDF

Equilibrium 2. Equilibrium is a Dynamic System, i.e., at equilibrium, the reaction does not stop but both forward and backward reactions proceed with the s...

Equilibrium 2. Equilibrium is a Dynamic System, i.e., at equilibrium, the reaction does not stop but both forward and backward reactions proceed with the same rate. 3. Equilibrium can be attains from either of the direction. Irreversible Reactions 4. Change in temperature, pressure or concentration can change the state of The reactions which proceed only in forward direction i.e., in equilibrium. which products do not react under similar conditions to give back 5. The presence of catalyst does not disturb the state of equilibrium because it reactants are called Irreversible Reactions. increases the rate of forward as well as backward reaction to the same NaOH + HCl ⎯→ NaCl + H2O extent. 6. At equilibrium, the free energy change (∆G) for any reaction is zero Reversible Reactions The reactions which proceeds in both forward and backward Active Mass direction under similar conditions but do not complete in any direction Concentration of a substance in mole per litre is known as Active are called Reversible Reactions. Mass. Its unit is mole per litre. It is usually represented by a square N2 + 3H2 ⇋ 2NH3 bracket ‘[ ]’. Mole of Substance W×1000 Characteristics of Reversible Reactions Active Mass of a Substance = = Volume (in litre) M× V 1. These reactions can be started from any direction. 2. After initiation, these reactions proceed in both forward and backward direction and thus they do not proceed to completion. Law of Mass Action According to this law, At constant temperature, the rate at which 3. Sign of reversibility ^⇋’ is used in these reactions. a substance reacts is proportional to its active mass and the rate of a 4. Reversible reactions always attain the state of equilibrium. reaction is directly proportional to the product of the active masses of the reactants. Chemical Equilibrium A state of a reversible reaction in which both forward and For example, for a reaction, A + B ⎯→ Product, backward reactions proceed with the same rate is known as the According to law of mass action, Chemical Equilibrium. or Rate at which A reacts α [A] A state of a reversible reaction in which the concentrations of the Rate at which B reacts α [B] and the rate of reaction α [A][B] reactant and the product do not change with the passage of time is known as the Chemical Equilibrium. Law of Mass Action and Chemical Equilibrium Let us consider a reversible reaction, Characteristics of Equilibrium State A + B ⇋ C + D 1. At equilibrium, the rate of both forward and backward reaction becomes equal. For this reaction, according to law of mass action, Rate of forward reaction, (rf) α [A][B]  or rf = Kf[A][B] ………..(i) Differences between Equilibrium Constant and Rate Constant and the rate of backward reaction, rb α [C][D] 1. Rate constant is equal to the rate of reaction at unit concentration of reactants whereas equilibrium constant is equal to the ratio of rate constant or rb = Kb [C][D] ………..(ii) of forward and backward reactions. 2. Rate constant increases in the presence of positive catalyst whereas At equilibrium, rf = rb equilibrium constant remains unchanged in the presence of a catalyst. ∴ From eq. (i) and (ii), Kf[A][B] = Kb[C][D] 3. Rate constant increases with the increase in temperature whereas equilibrium constant may increase (for endothermic reactions) or decrease Kf [C] [D] [C] [D] (for exothermic reactions) with increase in temperature. or Kb = [A] [B] or K = c [A] [B] Relationship between the equilibrium constants of forward and The above equation is known as equilibrium equation. Here Kc is a constant which is known as Equilibrium Constant in terms of backward reaction concentration. Its value is constant at a given temperature and does not Let equilibrium constant for a reaction, H2 + I2 ⇋ 2HI, is K1 and for the depend on the concentration of reactants and products or on pressure, reaction, 2HI ⇋ H2 + I2 equilibrium constant = K2, then volume, catalyst, etc. 1 K1 = K2 Equilibrium Constant for Gaseous Reactions (Kp and Kc) Let us consider a gaseous reversible reaction, Equation of a Reaction and Equilibrium Constant aA(g) + bB(g) ⇋ cC(g) + dD(g) Let for a reaction, N2 + 2O2 ⇋ 2NO2, equilibrium constant is K1 and for For this reaction, a reaction, ½N2 + O2 ⇋ NO2, equilibrium constant is K2, then K1 = K22 [C]c [D]d Equilibrium constant in terms of concentration, K c = Combination of Chemical Equilibria [A]a [B]b Let for a reaction, N2 + O2 ⇋ 2NO, equilibrium constant = K1 pCc pDd for a reaction, 2NO + O2 ⇋ 2NO2, equilibrium constant = K2 and equilibrium constant in terms of partial pressure, K p = p A a p Bb and for a reaction, N2 + 2O2 ⇋ 2NO2, equilibrium constant = K3, here pA, pB, pC and pD are the partial pressure of A, B, C and D respectively at then K3 = K1 x K2 equilibrium. For a reaction, KP = KC x (RT) ∆n Reaction Quotient, Q The ratio of the concentration of the products to the reactants at ∆n = (no. of molecules of gaseous products) – (no. of molecules of gaseous any stage of a reversible reaction is known as Reaction Quotient. reactants) Let us consider a reaction, aA(g) + bB(g) ⇋ cC(g) + dD(g) Note [C]c [D]d When ∆n = 0, then KP = KC when ∆n > 0, then KP > KC when ∆n < 0, then KP > KC For this reaction, at any stage the reaction quotient Q = [A]a [B]b  Now, by comparing reaction quotient with equilibrium constant, 2KClO3) ⎯→ 2KCl + 3O2 we can predict the direction of a reaction. 2FeSO4 ⎯→ Fe2O3 + SO2 + SO3 i. When Q = Kc, then the reaction is in equilibrium. ii. When Q > KC,then the net reaction will proceed in forward direction. Thermal Dissociation iii. When Q < Kc, the net reaction will proceed in backward direction. A reversible reaction in which substance breaks up into two or more simple molecules on heating is known as Thermal Dissociation. For example : PCl5 ⇋ PCl3 + Cl2 Degree of Dissociation, 𝛂 A fraction of total molecules which dissociates at equilibrium is known as Degree of Dissociation of that substance. No. of dissociated molecules Types of Chemical Equilibrium α = Total no. of molecules 1. Homogeneous Equilibrium The equilibrium in which all the substances are present in the Some example of reversible reactions same phase is known as Homogeneous equilibrium. Gaseous equilibrium and the equilibrium formed by miscible liquids are homogeneous. For 1. Formation of Hydrogen Iodide example : H2 + I2 ⇋ 2HI N2(g) + 3H2(g) ⇋ 2NH3(g) Initial moles a b 0 H2(g) + I2(g) ⇋ 2HI(g) Moles at equilibrium (a-x) (b-x) 2x Conc. at equilibrium ( a-x ) ( b-x ) ( 2x ) 2. Heterogeneous Equilibrium V V V The equilibrium in which the substances involved are present in [HI]2 4x 2 KP = K C different phases is called Heterogeneous Equilibrium. For example :  KC = = [H 2 ] [I 2 ] (a-x) (b-x) CaCO3(s) ⇋ CaO(s) + CO2(g) AgCl(s) + (aq) ⇋ Ag+(aq) + Cl-(aq) 2. Dissociation of Phosphorous Penta Chloride Note Concentration of pure solids and pure liquids are not shown in the PCl5 ⇋ PCl3 + Cl2 equilibrium expression of heterogeneous equilibria as they remain constant. Initial moles a 0 0 Moles at equilibrium (a-x) x x Thermal Decomposition Conc. at equilibrium ( a-x ) x x An irreversible reaction in which a substance breaks up into two V V V or more simple molecules on heating is known as Thermal [PCl 5 ] x2 KP = KC x RT  KC = = Decomposition. For example: [PCl 3 ] [Cl 2 ] (a-x)V  Some substances like KNO3, NH4Cl, etc absorb heat when they 3. Formation of Ammonia dissolve in water. So, according to Le Chatelier’s Principle, their solubility N2 + 3H2 ⇋ 2NH3 in water increases with increase in temperature. Initial moles a b 0 KNO3(s) + aq ⇋ KNO3(aq) - 83.4 Kcal Moles at equilibrium (a-x) (b-3x) 2x Conc. at equilibrium ( a-x ) ( b-3x ) ( 2x ) On the other hand, some solids like NaOH, CaO, etc evolve heat V V V on dissolution. So their solubility decreases with increase in 2 2 2 temperature. [NH 3 ] 4x V KP = KC x (RT)-2  KC = = [N 2 ] [H 2 ]3 (a-x) (b-3x)3 2. Effect of Pressure on the Solubility of Gases When a gas dissolves in liquid, its volume decreases. So, on Factors Affecting Equilibria : Le Chatelier’s Principle increasing the pressure of gas ⇋ solution system, the solubility of gas in This principle is given by French Scientist Le Chatellier. According liquid increases. to this principle, If a system in equilibrium is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in the 3. Melting of Ice direction that tends to undo the effect of the change. At one atm. pressure and 00C, there exist following equilibrium Following conclusion can be drawn from this principle : - between water and ice :- Ice ⇋ Water + 80 cal/g Factor Change Direction in which Equilibrium Shifts 1. i. Increase of pressure Towards lesser number of gaseous moles. Melting of ice is an endothermic reaction in which volume ii. Decrease of pressure Towards larger number of gaseous moles. decreases. Change in temperature or pressure of this system have the 2. i. Increase in the conc. of reactants Forward direction. following effects :- ii. Increase in the conc. of products Backward direction. Effect of Temperature 3. i. Increase in temperature Endothermic reaction in forward while It is an endothermic reaction. So, when temperature increases, Exothermic reaction in backward direction. equilibrium will shift in the forward direction, i.e., more ice will melt. ii. Decrease in temperature Endothermic reaction in backward while Exothermic reaction in forward direction. Effect of Pressure 4. i. Addition of inert gas at. No Effect. In this reaction, there is a decrease in the volume in the forward constant volume direction. So, when the pressure of this system increases, equilibrium will ii. Addition of inert gas at. Towards larger number of gaseous moles. shift in the forward direction, i.e, more ice will melt. constant pressure 5. Addition of catalyst No Effect. Application of Le Chatelier’s Principle on Physical Equilibria 1. Effect of Temperature on the Solubility of Solids