The diagram below shows the energy profiles for a reaction carried out with and without a catalyst - Scottish Highers Chemistry - Question 4 - 2016

To determine the enthalpy change for the catalysed reaction, we observe the energy profiles shown in the diagram:

1. Identifying Reactants and Products: The enthalpy change ( ( \Delta H ) ) is calculated as the difference in potential energy between the products and reactants.

2. Energy Levels:

   • From the graph, locate the energy levels of the reactants and products for the catalysed reaction. This typically involves identifying the lowest and highest points on the energy axis.
   • For the catalysed reaction, the potential energy of the reactants is lower than that of the products, which may indicate an exothermic or endothermic reaction depending on the vertical placement of the curves.

3. Calculating Enthalpy Change: If we assume that the energy level of reactants is around 100 kJ mol⁻¹ and that for the products, after accounting for the catalyst, is around 50 kJ mol⁻¹, we find: $\Delta H = \text{Potential Energy of Products} - \text{Potential Energy of Reactants} = 50 - 100 = -50 \, \text{kJ mol}^{-1}$

4. Conclusion: The enthalpy change for the catalysed reaction is therefore ( -50 , \text{kJ mol}^{-1} ), corresponding to option B.