Chemical Equilibrium

Question. Write down the relationship of different types of equilibrium constant i.e. Kc and Kp for the following general reactions. 

Answer. For a more general reaction If the reactants, A, B, and the products C, D are ideal gases, then the molar concentrations of each gas are proportional to its partial pressure. When concentrations are expressed in terms of partial pressure, the expression of Kp is, 

Here are partial pressures of A, B, C, D respectively at equilibrium position. If number of moles of reactants and products are same then, their values of Kp and Kc remains same, otherwise following relation is given as.

                                                                 

where  '' is the difference of number of moles of gaseous products and number of moles of reactant, R is gas constant, T is absolute temperature. 

Question. Decide the comparative magnitudes of Kc and Kp for the following reversible reactions. 

                              i) ammonia synthesis         ii) Dissociation of PCl5

 Answer. For synthesis of NH3, i.e.

 For dissociation of PCl5, i.e.

Question. Write down Kc for the following reversible reactions. Suppose that volume of reaction mixture in all the cases is ‘V’ dm3 at equilibrium stage.

 Answer. 

Question. How do you explain that some of the reactions mentioned above are affected by change of volume at equilibrium stage. 

Answer. In above reactions only dissociation of PCl5 and synthesis of NH3 is affected by the change in volume. If we increase the pressure or decrease the volume of both reaction reaction mixture, reactions will mover in move in forward direction to release the stress. 

Question. Explain the following two applications of equilibrium constant. Give examples i) Direction of reaction ii) Extent of reaction 

Answer. (i) As we know that, 

                                                               

 Any moment during the chemical reaction, there are three possibilities. (i) The product/Reactant ratio is less than Kc i.e. 

The reaction will mover in forward direction to make the ratio equal to Kc. (ii) The product/Reactant ratio is greater than Kc i.e.

 The reaction will mover in reverse direction to make the ratio equal to Kc--. (iii) The product/Reactant ratio is equal to Kc i.e.

 The reaction is already at equilibrium position. 

(ii) Prediction of Extent of reaction: The extent of reaction in forward or reverse direction can be calculated from the values. 

(i) If Kc value is vary large, it indicates that reaction is almost complete. 

(ii) If Kc value is very small, it shows that the reactants are present in very large quantity of reactant in equilibrium mixture. 

 (iii) If value of Kc is small, it indicates the reactants and products are present in reasonable quantities in equilibrium mixture.

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