Question

Which of the following statements is incorrect?

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

Answer 1

Answer: (B)

$\underset{\text{Yellow}}{Fe^{3+}_{(aq)}} + \underset{\text{Colourless}}{SCN^-_{(aq)}} \rightleftharpoons \underset{\text{Deep red}}{[Fe(SCN)]^{2+}_{(aq)}}$
When oxalic acid is added, it reacts with $Fe^{3+}$ ions to form stable complex ion $[Fe(C_2O_4)_3]^{3-}$, thus decreasing the cone, of free $Fe^{3+}_{(aq)}$. Now, according to Le Chatelier’s principle, the reaction will shift in backward direction to increase the cone, of free $Fe^{3+}_{(aq)}$. Thus, conc, of $[Fe(SCN)]^{2+}$ decreases, so the intensity of red colour decreases.