UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper I
Q1(g). Heat of combustion of naphthalene from bomb calorimetry
Question
0.500 g of benzoic acid was burnt under oxygen. The combustion produced a temperature rise of 1.236 K. The same calorimetric set-up was used to burn 0.300 g of naphthalene and the resulting temperature rise was 1.128 K. The heat of combustion of benzoic acid, ΔcU298 = −3227 kJ mol⁻¹. What is the heat of combustion of naphthalene?
Answer in brief
ΔcU(naphthalene) = −5.15 × 10³ kJ mol⁻¹.
Key points
- A bomb calorimeter works at constant volume, so the heat released equals ΔU; benzoic acid calibrates the set-up.
- n(C₇H₆O₂) = 0.500/122.12 = 4.094 × 10⁻³ mol; heat released = 13.21 kJ; calorimeter constant C = 13.21/1.236 = 10.69 kJ K⁻¹.
- Heat released by naphthalene = 10.69 × 1.128 = 12.06 kJ; n(C₁₀H₈) = 0.300/128.17 = 2.341 × 10⁻³ mol.
- ΔcU = −12.06/2.341 × 10⁻³ = −5152 kJ mol⁻¹; with Δng = −2, ΔcH = ΔcU − 2RT = −5157 kJ mol⁻¹.
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.
Coversbomb calorimetercalorimeter constantheat of combustionbenzoic acid calibrationΔU and ΔHnaphthalene
All parts of Question 1
- 1(a) Probability of the 1s electron at a₀ and at 10a₀5 marks
- 1(b) Born–Haber cycle for NaCl at 298 K5 marks
- 1(c) Raising the conductivity of Ge and Si by trace elements5 marks
- 1(d) Heat conduction between two copper sheets through air5 marks
- 1(e) Superheating of liquids and the Kelvin equation5 marks
- 1(f) Ascending order of dipole moment: NH₃, NF₃, H₂O5 marks
- 1(g) Heat of combustion of naphthalene from bomb calorimetry5 marks
- 1(h) Sealed sample: ethane, neon or a mixture5 marks
- 1(i) Time to plate a gold layer5 marks
- 1(j) Maximum work of a steam turbine5 marks