C(s) + 2H₂(g) → CH₄(g); ΔH = −74.8 kJ mol⁻¹
Which of the following diagrams gives an accurate representation of the above reaction?
[R → reactants; P → products]

CONCEPT:

Enthalpy Change (ΔH) and Energy Diagram

• The enthalpy change (ΔH) of a reaction represents the difference in energy between the products and reactants.
• When ΔH is negative, the reaction is exothermic, meaning energy is released and the products have lower energy than the reactants.
• Energy diagrams for exothermic reactions show the reactants starting at a higher energy level and the products at a lower energy level, with an energy "drop" between them.

EXPLANATION:

• In the given reaction:

  C(s) + 2H₂(g) → CH₄(g); ΔH = -74.8 kJ mol^-1

• ΔH is negative, indicating that the reaction is exothermic.
• In an energy diagram for this reaction:
  • The reactants (C and H₂) start at a higher energy level.
  • The products (CH₄) are at a lower energy level, showing that energy is released during the reaction.
  • The difference in energy between the reactants and products corresponds to the magnitude of ΔH (74.8 kJ mol^-1).

Therefore, the correct energy diagram will show the reactants at a higher energy level than the products, with an energy drop of 74.8 kJ mol^-1.