Question

Which of the following statements is incorrect?

• A. The intensity of red colour increases when oxalic acid is added to a solution containing iron $\left(III\right)$ nitrate and potassium thiocyanate.
• B. Equilibrium constant for a reaction with negative $\Delta H$ value decreases as the temperature increases.
• C. On addition of catalyst the equilibrium constant value is not affected.
• D. In equilibrium mixture of ice and water kept in perfectly insulated flask mass of ice and water does not change with time.

Answer 1

The correct option is A The intensity of red colour increases when oxalic acid is added to a solution containing iron $\left(III\right)$ nitrate and potassium thiocyanate.

Option (A) It is correct statement because at equilibrium, concentration of both the reactants and product remains constant i.e., they do not change with time.

So, there is no change of mass of ice and water, as the rates of transfer of molecules from ice to water and of reverse transfer from water to ice are equal.

Option (B) It is incorrect statement because according to Le chatelier’s principle, ‘The concentration stress of an added reactants/ products is relieved by net reaction in the direction that consumes the added substance.

Equilibrium between iron $\left(III\right)$ nitrate and thiocyanate ion is given as-

$F{e}^{3+}\left(aq\right)+SC{N}^{-}\left(aq\right)\rightleftharpoons \left[Fe\right(SCN\left){]}^{2+}\right(aq)$

The red colour is due to the formation of $\left[Fe\right(SCN){]}^{2+}$ ion.

On adding oxalic acid $\left(H_2{C}_2{O}_4\right)$ it consumes $F{e}^{3+}$ ions as it forms more stable complex $\left[Fe\right(C_2{O}_4{)}_3{]}^{3-}.$

Thus, decreasing the concentration of $F{e}^{3+}$ ions in the solution.

According to $Le$ chatelier’s principle, the concentration stress of removed $F{e}^{3+}$ is relieved by dissociation of $\left[Fe\right(SCN){]}^{2+}$ to replenish the $F{e}^{3+}$ ions and because of this, the concentration of $\left[Fe\right(SCN){]}^{2+}$ ion decreases. Hence, the intensity of red colour decreases.

Therefore, option $\left(B\right)$ is correct.

Option (C) is correct statement because the catalyst does not affect the equilibrium composition and it does not appear in the balanced chemical equation or in equilibrium constant expression.

Option (D) is correct statement because $\Delta H$ is negative. it means it is an exothermic reaction and releases heat.

Reactant $\rightleftharpoons$ product + heat

So, as temperature increases equilibrium shifts to the opposite side according to the $Le$ chatelier’s principle.

Thus, concentration of reactant increases and concentration of product decreases.

$K=\frac{\left[product\right]}{\left[reactant\right]}$

As [reactant] increases, the value of $K$ decreases.