Looking for classes? Ksquare Career Institute, Bengaluru →
$\text{C}(s) + 2\text{H}_2(g) \rightarrow \text{CH}_4(g); \Delta H = -74.8\text{ kJ mol}^{-1}$. Which of the following diagrams gives an accurate representation of the above reaction? [R $\rightarrow$ reactants; P $\rightarrow$ products]
A
Energy profile where Products (P) lie $74.8\text{ kJ mol}^{-1}$ below Reactants (R) with downward enthalpy arrow. 
B
Energy profile where Products (P) lie above Reactants (R). 
C
Energy profile showing an endothermic plateau. 
D
Energy profile with no activation energy barrier. 
Explanation
Since $\Delta H = -74.8\text{ kJ mol}^{-1}$ is exothermic, products have lower potential energy than reactants by $74.8\text{ kJ mol}^{-1}$.
Detailed Solution
The formation of methane from carbon and hydrogen is exothermic ($\Delta H = -74.8\text{ kJ mol}^{-1}$). For an exothermic reaction, the enthalpy of products is lower than the enthalpy of reactants: $H_P < H_R$, with $\Delta H = H_P - H_R = -74.8\text{ kJ mol}^{-1}$. Thus, the potential energy profile shows an initial activation barrier followed by a product level lying $74.8\text{ kJ mol}^{-1}$ below the reactant level, represented accurately by diagram (1).