AIIMS2004Chemistry-Thermochemistry
AIIMS 2004 Chemistry Enthalpy of Combustion MCQ Question
Type: MCQ-numerical-Hard-Class 11
How much energy is released when 6 moles of octane is burnt in air? Given ΔHf° for CO₂(g), H₂O(g) and C₈H₁₈(g) respectively ar, -240 and +160 kJ/mol.
A
-6.2 kJ
B
-37.4 kJ
C
-35.5 kJ
D
-200.0 kJ
Correct Answer
Option B
Detailed Explanation
C + O2 → CO2; ΔHf° = – 490 kJ/mol × 8, H2 + 1/2 O2 → H2O; ΔHf° = –240 kJ/mol × 9, 8C + 18H → C8H18; ΔHf° = +160 kJ/mol, 8CO2 + 9H2O – 8C – 18H, ΔHf° = –3920 – 2160 – 160, C8H18 + 25/2 O2 → 8CO2 + 9H2O; ΔHf° = 6240 kJ/mol, ΔHf° for 6 moles of octane = 6240 × 6 = 37440 kJ/mol.
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