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

The provided reaction, C(s) + 2H₂(g) → CH₄(g) (ΔH = -74.8 kJ mol⁻¹), is exothermic, as indicated by the negative ΔH. Exothermic reactions release energy, meaning the products possess less energy than the reactants. An energy profile diagram for such a reaction depicts reactants at a higher energy level than products, with an upward curve to a transition state (representing activation energy) followed by a downward curve to the products. The activation energy is the energy barrier to overcome, and the difference in energy between reactants and products is equal to the enthalpy change (ΔH). - A correct diagram will illustrate: - Reactants situated at a higher energy level compared to the products. - A peak at the transition state, signifying the activation energy. - An energy difference between reactants and products that matches the given ΔH = -74.8 kJ mol⁻¹. Diagram (3) accurately depicts this exothermic reaction's energy profile, showing products at a lower energy state than reactants and a distinct peak for the transition state. Consequently, option (3) is the correct answer.