A-Level Chemistry May/June 2024 Q3(a): Write an equation to show the reaction for the standard enthalpy change of formation of…

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  The definition of standard enthalpy change of formation (ΔH_f°) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

  • Compound: 1 mole of water, H₂O.
  • Elements: Hydrogen and oxygen.
  • Standard states (at 298 K, 1 atm): Hydrogen is H₂(g), oxygen is O₂(g), and water is H₂O(l).

  To form one mole of H₂O, we need one mole of H₂ and half a mole of O₂.

  Final Equation: H₂(g) + ½O₂(g) → H₂O(l)

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  How the marks are awarded

  • M1 — The first mark is awarded for the correct species and balancing to form exactly one mole of the product, H₂O. This requires the equation H₂ + ½O₂ → H₂O.
  • M2 — The second mark is awarded for providing the correct state symbols for all three species based on their standard states: H₂(g), O₂(g), and H₂O(l).

  Common mistakes

  • Writing '2H₂(g) + O₂(g) → 2H₂O(l)'. This equation is balanced but forms two moles of water, which does not match the definition of standard enthalpy of formation.
  • Using the incorrect state symbol for water, such as H₂O(g). The standard state of water at 298 K and 1 atm is liquid (l).
  • Forgetting to include state symbols entirely, which results in the loss of the second mark.
  • Using atoms instead of molecules for the elements, for example '2H(g) + O(g) → H₂O(l)'. The definition requires elements in their standard states, which are H₂ and O₂.

  Examiner tip: Always learn the precise definitions of key chemical terms, as questions on standard enthalpy changes test your understanding of every component, including 'one mole of product' and 'standard states'.

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