Enthalpy cycle identifies formation enthalpy via Hess's law

Steps:

• Identify the path in the cycle where X represents the direct formation of water from elements.
• Apply Hess's law: total enthalpy change is path-independent.
• Trace reactants (H₂ + ½O₂) to product (H₂O) for the formation reaction.
• Confirm X matches ΔH_f for H₂O(l) or (g).

Why D is correct:

• Enthalpy of formation is defined as the enthalpy change when 1 mole of compound forms from elements in standard states, matching X's path.

Why the others are wrong:

• A: Combustion of H₂ forms water but involves excess oxygen, not formation.
• B: Involves methane, absent from water's cycle.
• C: Multiplies formation by 2, but X shows single mole formation.

Not enough information: Enthalpy cycle diagram not provided.

Final answer: D