Nitric acid, HNO₃, can be made by reacting nitrogen dioxide with water. The enthalpy change for the reaction can be measured indirectly using a Hess' cycle. ΔH₁ 3NO₂(g) + H₂O(l) → 2HNO₃(l) + NO(g)

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [M1] (enthalpy / energy change) when one mole of a compound/substance is formed
2. [M2] from its elements in their standard states
3. [M1] use of correct stoichiometry in calculation
4. [M2] correct signs associated with the appropriate ∆Hfvalues/terms used for the calculation of ∆Hreaction
5. [M3] ∆Hreaction = −(102 – 286) + (-346 + 91.1) = −70.9 kJ mol⁻¹
6. [M1] nitrogen has a triple bond
7. [M2] EITHER high energy is needed to break the bond OR at normal temperatures there is not enough energy to break the bond / to overcome the activation energy
8. [1] lightning
9. [M1] define homogeneous (homogeneous catalyst is) in the same phase / state as the reactants
10. [M2] and M3 Define catalyst increase the rate AND lowers the activation energy AND without being chemically altered at the end of the reaction / are regenerated at the end of the reaction
11. [M1] NO₂ + SO₂ → NO + SO₃
12. [M2] NO + ¹/₂ O₂ → NO₂