Chemical Equilibrium - II Three Marks

1. Define / What is equilibrium constant?

At constant temperature the ratio of the rate of forward reaction to the rate of reverse reaction is a constant.

Or At any specific temperature the ratio kf / kr is a constant.

Or Equilibrium constant, Kc = kf / kr

Where,

kf = Rate of forward reaction.

kr = Rate of reverse reaction.

Or At constant temperature the ratio of product of initial concentrations of products to the product of initial concentrations of reactants under equilibrium conditions is a constant.

Or Equilibrium constant, Kc = Products concentrations / Reactants concentrations.

Or For a general equilibrium reaction, aA + bB ⇌ cC + dD,

the equilibrium constant, Keq is given by [C]c [D]d / [A]a [B]b.

2. Define / What is reaction quotient. How is it related to equilibrium constant?

The ratio of the product of initial concentrations of products to the product of initial concentrations of reactants under non-equilibrium conditions.

Reaction quotient Q is related to Equilibrium constant K as: When,

Q < K, more of product is formed. Forward reaction is favoured

Q > K, more of reactant is formed. Reverse reaction is favoured.

Q = K, equilibrium is attained.

These aspects will hold good only when Q and Kc are under the same conditions of temperature and pressure.

3. Dissociation equilibrium constant of HI is 2.06 x 10-2 at 458°C. At equilibrium, concentration of HI and I2 are 0.36 M and 0.15 M respectively. What is the equilibrium concentration of H2 at 458°C?

Dissociation equilibrium constant of HI = 2.06 x 10-2

Equilibrium concentration of HI = 0.36 M

Equilibrium concentration of I2 = 0.15 M

Equilibrium concentration of H2 = ?

2HI  ⇌  H2 + I2

       Kc = [H2] [I2] / [HI]2

2.06 x 10-2 = [H2] [0.15] / [0.36]2

     [H2] = 2.06 x 10-2 x (0.36)2 / 0.15

     [H2] = 1.78 x 10-2

4. Dissociation of PCl5 decreases in presence of increase in Cl2. Why?

PCl5(g)  ⇌ PCl3(g) + Cl2(g)

Kc = [PCl3] [Cl2] / [PCl5]

According to the above equation, increase in the concentration of Cl2 increases the value of Kc. But Kc is a constant. To maintain the constancy of Kc the concentration of PCl5 increases.

 In other words, dissociation of PCl5 decreases. Thus the reverse reaction is favoured.

5. For the reaction A + B⇌ 3C at 25°C, a 3 litre volume reaction vessel contains 1, 2 and 4 moles of A, B and C respectively at equilibrium, calculate the equilibrium constant Kc of the reaction at 25°C.

Kc = [C]3 / [A] [B]

      = [4 / 3]3 / [1 / 3] [2 / 3]

      = 32 / 3

      = 10.67 mol.dm-3

6. In the equilibrium H2 + I2 ⇌ 2HI, the number of moles of H2, I2 and HI are 1, 2, 3 moles respectively. Total pressure of the reaction mixture is 60 atm. Calculate the partial pressures of H2 and HI in the mixture.

Partial pressures of H2   = (n_H2 / n_H2 + n_I2 + n_HI) x Total pressure

 = (1 / 1 + 2 + 3) x 60

 = (1 / 6) x 60

Partial pressures of H2  = 10 atm

Partial pressures of HI   = (n_HI / n_H2 + n_I2 + n_HI) x Total pressure

 = (3 / 1 + 2 + 3) x 60

 = (3 / 6) x 60

Partial pressures of HI   = 30 atm

7. State Lechatlier’s principle.

If a system at equilibrium is subjected to a disturbance or stress, then the equilibrium shifts in the direction that tends to nullify the effect of the disturbance or stress.

8. The equilibrium constant Kc for A(g) ⇌ B(g) is 2.5 x 10-2. The rate constant of the forward reaction is 0.05 sec-1. Calculate the rate constant of the reverse reaction.           
            Equilibrium constant Kc = 2.5 x 10-2

Rate constant of the forward reaction kf = 0.05 sec-1

Rate constant of the reverse reaction kr = ?

Kc = kf / kr

kr = kf / Kc

kr = kf / Kc

     = 0.05 sec-1 / 2.5 x 10-2

     = 5 x 10-2 sec-1 / 2.5 x 10-2

kr  = 2 sec-1

9. What happens when Δng = 0, Δng = –ve, Δng = +ve in a gaseous reaction?

1. When Δng = 0,

the total number of moles of gaseous products are equal to the total number of moles of gaseous reactants.

Kp = Kc

2. When Δng = – ve,

the total number of moles of gaseous products are lesser than the total number of moles of gaseous reactants.

Kp <  Kc

3. When Δng = + ve,

the total number of moles of gaseous products are greater than the total number of moles of gaseous reactants.

Kp > Kc

10. What is the relationship between formation equilibrium constant and dissociation constant? Give one example.

The equilibrium constant of the dissociation equilibrium is the reciprocal of the equilibrium constant of the formation equilibrium reaction.

Example

2SO2(g) + O2(g) ⇌ 2SO3(g)

The formation equilibrium constant, Kc = [SO3]2 / [SO2]2[O2]

2SO3(g) ⇌ 2SO2(g) + O2(g)

The dissociation equilibrium constant, Kc’ = [SO2]2[O2] / [SO3]2 = 1 / Kc

11. Why is equilibrium reaction referred to as dynamic equilibrium?

Even at equilibrium the forward and reverse reactions take place endlessly and simultaneously with equal rates. Therefore equilibrium reaction is referred to as dynamic equilibrium.

12. Write the equlibrium constants Kc for the following reactions:

        i) H2O2(g) ⇌ H2O(g) + 1/2O2(g)        ii) CO(g)+H2O(g) ⇌ CO2(g)+H2(g).

            i) Kc = [H2O] [O2]1/2 / [H2O2]

ii) Kc = [CO2] [H2] / [CO] [H2O]

13. Give one gaseous equilibrium reaction as an example for the following: i) ∆ng = 0 ii) ∆ng = +ve iii) Δng = –ve

14. Explain the effect of temperature on the following equilibrium reaction by using Le Chatlier’s principle

N2O4(g) ⇌ 2NO2(g)   ΔH = + 59.0 kJ / mole

15. Calculate ∆ng for the following reactions

      a) N2(g) + 3H2(g) ⇌ 2NH3(g)     b) N2O4(g) ⇌ 2NO2(g)   c) PCl5(g) ⇌ PCl3(g) + Cl2(g)

16. Calculate ∆ng for the following reactions

      i) N2(g) + 3O2(g) ⇌ 2NO(g)       ii) N2O4(g) ⇌ 2NO2(g)   iii) PCl3(g) + Cl2(g) ⇌ PCl5(g)

17. Calculate ∆ng for the following reactions

      a) 2H2O(g) + 2Cl2(g) ⇌ 4HCl(g) + O2(g)        b) 3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g)

18. Write the equlibrium constants Kp for the following reactions:

        i) H2O2(g) ⇌ H2O(g) + 1/2O2(g)        ii) CO(g)+H2O(g) ⇌ CO2(g)+H2(g).
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