Thermochemistry Problems With Answers Page 3
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45.63 From the enthalpies of reaction
EQ 1 H
(g) + F
(g) 2 HF (g)
ΔH = -537 kJ
2
2
EQ 2 C (s) + 2 F
(g) CF
(g)
ΔH = -680 kJ
2
4
EQ 3 2 C (s) + 2 H
(g) C
H
(g)
ΔH = +52.3 kJ
2
2
4
calculate ΔH for the reaction of ethylene with F
2
C
H
(g) + 6 F
(g) 2 CF
(g) + 4 HF (g)
2
4
2
4
Identify a substance unique to only one DATA equation. Focus on handling that
substance in each case.
DOUBLE EQ 1 to get 4HF (double ΔH)
DOUBLE EQ 2 to get 2 CF
(double ΔH)
4
REVERSE EQ 3 to get C
H
on left side. (reverse sign ΔH)
2
4
(g) 4 HF (g)
2H
(g) + 2F
2 ΔH = 2 (-537 kJ) = -1074 kJ
2
2
2C (s) + 4 F
(g) 2CF
(g)
2 ΔH = 2 (-680 kJ) = -1360 kJ
2
4
C
H
(g) 2 C (s) + 2 H
(g)
- ΔH = - (+52.3 kJ) = -52.3 kJ
2
4
2
sum equations
2H
+ 2F
+ 2C + 4 F
+ C
H
2CF
+ 4 HF + 2 C + 2 H
2
2
2
2
4
4
2
combine F
2
C
H
+ 6 F
2CF
+ 4 HF
2
4
2
4
sum the adjusted ΔH values: -1074 kJ
-1360 kJ
- 52.3 kJ
sum = - 2486 kJ
(Prof H. says to use dp rules since we multiply by counting #s then add: so no dp for
this value here)
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