Indian Forest Service (Mains), Chemistry optional · 2016 · Paper I
Q1(c). Heat of reaction at 120°C: Kirchhoff's law
Question
The heat of the reaction
Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(l)
at 25°C was found to be -8.4 kcal. What will be the heat of the reaction at 120°C? The molar heat capacities at constant pressure and at 25°C for iron oxide, hydrogen, iron and water are 25.0 cal deg⁻¹ mol⁻¹, 6.9 cal deg⁻¹ mol⁻¹, 6.1 cal deg⁻¹ mol⁻¹ and 18.0 cal deg⁻¹ mol⁻¹ respectively.
Answer in brief
ΔH₃₉₃=-8400+20.5×95=-6452 cal ≈-6.45 kcal (-27.0 kJ).
Key points
- Kirchhoff's law. ((∂ΔH)/(∂ T))_P=ΔCₚ; with ΔCₚ constant over the range, ΔHT₂=ΔHT₁+ΔCₚ (T₂-T₁)
- ΔCₚ=Σ Cₚ(products)-Σ Cₚ(reactants)=[2(6.1)+3(18.0)]-[25.0+3(6.9)]
- ΔCₚ=66.2-45.7=+20.5 cal K⁻¹; T₂-T₁=393-298=95 K
- ΔH₃₉₃=-8400+(20.5)(95)=-8400+1947.5 cal
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.
All parts of Question 1
- 1(a)(i) Bond angle of F₂O smaller than that of H₂O5 marks
- 1(a)(ii) Residual entropy of CO and N₂O at 0 K5 marks
- 1(b)(i) Activation energy from rate constants at two temperatures5 marks
- 1(b)(ii) Ionic strength of 0.01 m H₂SO₄ + 0.02 m NaCl5 marks
- 1(c) Heat of reaction at 120°C: Kirchhoff's law10 marks
- 1(d)(i) Crystal systems with mutually perpendicular axes5 marks
- 1(d)(ii) Spacing of the (110) planes, axes mutually perpendicular5 marks