Question

1) At equilibrium, there is no overall change in the concentrations of reactants and products. Why, then, is this state described as dynamic? (3 marks) 2) Increasing temperature tends to increase the solubility of a solid in a liquid, but it tends to decrease the solubility of gases. Explain why. (3 marks)

Answer 1

(1) -

As we know that Equilibrium is that state of reaction at which rate of formation of products becomes equal to the rate of formation of reactants and therefore there is no overall change occur in concentration of reactants and products. Chemical reaction at equilibrium seems to be static but it is not static in nature , it is dynamic in nature because there is net formation of products is equal to the net formation of reactants. e.g.

A + B   $\rightleftharpoons$ C + D

reactants products

therefore according to the rate law , rate of reaction is directly proportional to the molar concentration of reactants

rate of forward reaction (r_f)  $\alpha$ [A][B]

rate of forward reaction (r_f) = K_f[A][B] ----------------------------------------------------------------(1)

where ​​​​​​​ K_f = rate constant of forward reaction

similarly

rate of backward reaction (r_b)  $\alpha$ [C][D]

rate of backward reaction (r_b) = ​​​​​​​ K_b[C][D] ----------------------------------------------------------------(2)

where ​​​​​​​ K_b = rate constant of backward reaction

At equilibrium

rate of forward reaction (r_f) = rate of backward reaction (r_b)

from equation no. 1 and 2 we get

K_f[A][B] = K_b[C][D]

K_f / K_b = [C][D] / [A][B]

equilibrium constant (K_eq) = K_f / K_b = [C][D] / [A][B]

equilibrium constant (K_eq) =  [C][D] / [A][B]

(2)-

The solubility of solid in liquid is increased on rising the temperature of solution of solid in liquid because higher temperature provides the energy required to breakdown of solid solute into its constituents which are soluble in liquid solvent i.e high temperature helps in dissociation of solid solute into its constituents by breaking the intermolecular forces exist between in solid solute. therefore on rising temperature , the dissociation of solid solute into its constituents increased which result increased the solubility of solid in liquid i.e increased solubility of solution of solid in liquid.

The solubility of gases in liquid is decreased on rising the temperature of solution of gases in liquid because higher temperature increased the kinetic energy of gases molecules in liquid due to which weak Vander waals forces exist between the gases molecules and liquid molecules decreased which ultimately decreased the concentration of gases in liquid because gases molecules escaped form the liquid solution due to high kinetic energy which is increased by rising the temperature. As we know that solubility of gases in liquid is an exothermic reaction therefore on rising the temperature the solubility of  gases in liquid is decreased.