C(s)+2H2(g)CH4(g); δ H=-74.8 kJ mol^-1. Which of the following diagrams gives an accurate representation of the above reaction? […

Question

C(s)+2H2(g)CH4(g); δ H=-74.8 kJ mol^-1.

Which of the following diagrams gives an accurate representation of the above reaction? [R reactants; P products]

1. A. Reactants (R) at higher energy, products (P) lower by 74.8 kJ, with an activation-energy hump (exothermic) (Correct answer)
2. B. Products (P) at higher energy, reactants (R) lower by 74.8 kJ (endothermic)
3. C. Reactants (R) higher, products (P) lower but no activation barrier
4. D. Products (P) higher than reactants by 74.8 kJ with a barrier (endothermic)

Correct Answer

Option A — Reactants (R) at higher energy, products (P) lower by 74.8 kJ, with an activation-energy hump (exothermic)

Detailed Solution & Explanation

The correct answer is Option 1.

Key Points

• $\Delta H=-74.8$ kJ mol$^{-1}$ is negative, so the reaction is exothermic: products have lower energy than reactants.
• The energy profile shows reactants (R) rising over an activation-energy barrier and then falling to products (P), which lie 74.8 kJ below the reactants.
• Only Option 1 depicts this exothermic profile correctly.