Question: 65. \mathrm{C}(\mathrm{s})+2 \mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{CH}_{4}(\mathrm{~g}) ; \Delta \mathrm{H}=-74.8 \mathrm{~kJ} \mathrm{~mol}^{-1} Which of the following diagrams gives an accurate representation of the above reaction? [ \mathrm{R} \rightarrow reactants; \mathrm{P} \rightarrow products]
Answer: Option (1)
Explanation:
The given reaction has enthalpy change \Delta \mathrm{H} = -74.8 \mathrm{~kJ~mol}^{-1}.
A negative value of \Delta \mathrm{H} indicates that the reaction is exothermic.
In an exothermic reaction, the products have lower energy than the reactants.
Thus, in the energy profile diagram:
• Reactants should be at a higher energy level.
• Products should be at a lower energy level.
• The difference in energy between reactants and products should be 74.8 \mathrm{~kJ~mol}^{-1}.
Option (1) correctly shows:
• Reactants at a higher energy level.
• Products at a lower energy level.
• A downward energy change of 74.8 \mathrm{~kJ~mol}^{-1}, representing heat released.
The other diagrams either show products at higher energy or represent the enthalpy change incorrectly.
Therefore, the correct diagram representing the reaction is Option (1).