Question

Calculate the heat produce when 3.785 lit of octane (C₆ H₁₈) reacts with oxygen to form CO and water vapour at 25°C. The density of octane is 0.7025 g/ml. Heat of combustion of C₆H₁₈ is -1302.7 kcal/mol. 

ΔH°r CO₂ (g) = -94.05 kcal mol^-1; 

ΔH°r CO(g) = -26.41 kcal mol^-1

ΔH°r H₂O(l) = -68.32 kcal mol^-1; 

ΔH°r H₂O(g) = -57.79 kcal mol^-1

Answer 1

given C₈ H₁₈ + 12.5 O₂ → 8 CO₂ + 9H₂ O(l) 

∆H = –130.27 

∆H_comb = –n∆H_{f reactant} – n∆H_{f product}

–1302.7 = ∆H_f C₈ H₁₈+ 12.5 

× O₂ – 8 ×∆HCO₂ – 9 × ∆H_f H₂O

∴ ∆H_f C₈ H₁₈ = 13027+ 8×(–94.05)+9 × (–68.32) 

∴ ∆H for given conditions = ∆H_reac × 23.33

= –666.80 × 23.32 = – 15549 .7