Consider the reversible aqueous reaction carried out at 298 K: Fe^ 3+ (aq) + SCN^ -…

CHEMISTRY · Multi-part · A Level · Chemistry · Kinetics · Kinetics

Problem

Consider the reversible aqueous reaction carried out at 298 K: Fe^{3+} (aq) + SCN^{-} (aq)

\rightleftharpoons

FeSCN^{2+} (aq)

\Delta H < 0

In a separate calorimetric study the following activation energies were measured for the uncatalyzed reaction:

E_a(\mathrm{fwd}) = 54\,\text{kJ}\,\text{mol}^{-1}

E_a(\mathrm{rev}) = 67\,\text{kJ}\,\text{mol}^{-1}

When Ag^{+} ions are added as a homogeneous catalyst, each activation energy decreases by

25\,\text{kJ}\,\text{mol}^{-1}

. (a) Using the given data, calculate the enthalpy change

\Delta H

for the reaction, and confirm whether the sign reported above is consistent with the activation energies. (b) Compute the new values of

E_a(\mathrm{fwd})

and

E_a(\mathrm{rev})

in the presence of

Ag^{+}

. Explain why the enthalpy change

\Delta H

remains unaffected by the catalyst. (c) Assuming the pre-exponential factors remain the same, determine by what factor the forward rate constant will increase at 298 K when

Ag^{+}

is present. Use the Arrhenius equation and state any approximations clearly. (d) Discuss thoroughly whether the equilibrium constant for formation of

FeSCN^{2+}

will change upon addition of

Ag^{+}

. Support your answer with thermodynamic reasoning.

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