Question

C(s) + 2H$_2$(g) $\rightarrow$ CH$_4$(g); $\Delta H = -74.8 \, \text{kJ mol}^{-1}$
Which of the following diagrams gives an accurate representation of the above reaction?

• A.
• B.
• C.
• D.

Hint:

For exothermic reactions, the products are lower in energy than the reactants, and the enthalpy change (\(\Delta H\)) is negative. The energy profile typically shows a downward slope from reactants to products.

Answer 1

The Correct Option is C

Given the reaction: \[ \text{C(s)} + 2\text{H}_2\text{(g)} \rightarrow \text{CH}_4\text{(g)} \quad (\Delta H = -74.8 \, \text{kJ mol}^{-1}) \] The negative value of \(\Delta H\) indicates that the reaction is exothermic. In an exothermic reaction, the energy of the products is lower than that of the reactants. This is represented in the energy profile diagram by a downward curve from reactants to products, with a peak corresponding to the activation energy (the energy required to reach the transition state).
- The correct diagram will show: - Reactants at a higher energy level than the products. - A peak at the transition state with an energy difference (activation energy). - The energy difference between the reactants and products corresponds to the given \(\Delta H = -74.8 \, \text{kJ mol}^{-1}\).

Thus, diagram (3) correctly represents the energy profile for this exothermic reaction, with the products at a lower energy than the reactants and a transition state with a well-defined peak.

Therefore, the correct answer is (3).