Chemical Reactions And Equations Worksheet With Answer Key - The University Of Sydney Page 18
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40CHEM1101
2010-N-11
November 2010
Marks
• The heat of combustion of acetylene, C
–1
H
(g), is –1301 kJ mol
. What is the heat of
2
2
4
formation of acetylene gas?
Data: Δ
Δ
–1
–1
H° of CO
(g) = –393.5 kJ mol
H° of H
O(l) = –285.8 kJ mol
f
2
f
2
The heat of combustion corresponds to the reaction below in which one mol of
C
H
(g) is burnt:
2
2
C
H
(g) + 5/2 O
(g)
2CO
(g) + H
O(l)
2
2
2
2
2
Using Δ
H° = ΣmΔ
H°(products) - ΣnΔ
H°(reactants), this becomes:
rxn
f
f
Δ
H° = [2 × Δ
H° (CO
(g) + Δ
H° (H
O(l)] – [Δ
H° (C
H
(g)]
combustion
f
2
f
2
f
2
2
or
[Δ
H° (C
H
(g)] = [2 × Δ
H° (CO
(g) + Δ
H° (H
O(l)] - Δ
H°
f
2
2
f
2
f
2
combustion
-1
= [(2 × -395.5) + (--285.8) – (-1301)] kJ mol
-1
= +228 kJ mol
-1
Δ
H° = +228 kJ mol
f
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