A reaction is exothermic by

CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics

Problem

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

. The uncatalysed pathway has a single activation barrier of

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

. A catalyst provides an alternative two-step pathway with activation barriers of

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

and

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

. Which statement is most consistent with the resulting energy-profile diagram? (a) The overall enthalpy change becomes

-60\,\text{kJ mol}^{-1}

because the

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

reduction in activation energy is released as additional heat. (b) The overall enthalpy change remains

-50\,\text{kJ mol}^{-1}

because a catalyst does not alter

\Delta H

. (c) The overall enthalpy change becomes

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

because the catalyst is regenerated, canceling out the energy difference. (d) The overall enthalpy change is

-10\,\text{kJ mol}^{-1}

, equal to the difference between the two activation barriers in the catalysed pathway.