Question

Calculate the enthalpy of combustion of ethylene (gas) to form CO₂ (gas) and H₂O (gas) at 298 K and 1 atmospheric pressure. The enthalpies of formation of CO₂ , H₂O and C₂H₄ are –393.5, –241.8, + 52.3 kJ per mole respectively.

Answer 1

We are given :

(i) C (s) + O₂ (g)→ CO₂ (g), ΔH°₁ = – 393.5 kJ mol^–1

(ii) H₂(s) +1/2 O₂ (g) →H₂O(g) ΔH°₂ = – 241.8 kJ mol^–1

(iii) 2C (s) + 2H₂ (g) → C₂H₄ (g),ΔH°₃ = + 52.3 kJ mol^–1

We aim at : C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O(g)

ΔH = – ΔH₃ + 2ΔH₁ + 2ΔH₂ 

= –1322.9 kJ mol^–1