Indian Forest Service (Mains), Chemistry optional · 2021 · Paper I
Q1(b). Changing the enthalpy reference
Question
Does the enthalpy of formation of compounds containing a certain element change, if the enthalpy of formation of the element under standard state conditions is set equal to 100 kJ mol⁻¹ rather than zero? If it changes, how will it change for the compound AₙBₘ, if the formation enthalpy of element A is set equal to 100 kJ mol⁻¹?
Answer in brief
The tabulated enthalpy assigned to one mole of AₙBₘ shifts by +100n kJ mol⁻¹ when A's reference value is raised consistently; the measurable enthalpy of its formation reaction is unchanged.
Key points
- Enthalpy has an arbitrary reference zero; let the new reference add cA = 100 kJ per mole of A atoms and leave B unchanged.
- Then H′(AₙBₘ) = H(AₙBₘ) + ncA and H′(A) = H(A) + cA.
- For nA + mB → AₙBₘ, Δ_rH′ = {H(AₙBₘ) + ncA} − n{H(A) + cA} − mH(B) = Δ_rH.
- The expected change in the tabulated reference value is +100n; a consistent change of zero cannot change a calorimetric heat of reaction.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversstandard enthalpy of formationreference stateenthalpy of element zeroHess's law
All parts of Question 1
- 1(a) Frenkel disorder in silver bromide8 marks
- 1(b) Changing the enthalpy reference8 marks
- 1(c) C–F and C–I potential-energy curves8 marks
- 1(d) Desilverisation of lead8 marks
- 1(e) Equilibrium constant of a Zn–Ag cell8 marks