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06/10/2020 11:54 am
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How much energy is released when 6 moles of octane is burnt in air?
Given ΔH°f for CO2 (g), H2O (g) and C8H18 (l) respectively are -490, -240 and +160 J/mol.
(a) - 6.2 kJ
(b) - 37.4 kJ
(c) - 35.5 kJ
(d) - 20.0 kJ
1 Answer
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06/10/2020 12:08 pm
(c) - 35.5 kJ
Explanation:
C + O2 = CO2 ΔH = -490 kJ/mol (i)
H2 + 1/2O2 = H2O ΔH = -240 kJ/mol (ii)
8C + 9H2 = C8H18 ΔH = +160 kJ/mol (iii)
2C8H18 + 25O2 = 16CO2 + 18H2O
The required reaction can be obtained by 2 × (iii) – 16 (i) – 18(ii)
16C + 18H2 - 16C - 16O2 - 18H2 - 9O2
= 2C8H18 + 16CO2 - 18H2O
or -25O2 = 2C8H18 - 16CO2 - 18H2O
2C8H18 + 25O2 = 16CO2 + 18H2O
[ΔH = 2 x 160 - 16 x -490 - 18 x -240]
ΔH = – 11840 J = – 11.840 kJ for 2 moles of octane. Energy released for 6 moles of octane
= –11.840 × 3
= –35.5 kJ