Enthalpy Calculation Worksheet
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1Enthalpy Calculation worksheet.
Determine the ∆H
for the combustion of propane.
rxn
à 3CO
C
H
+ 5 O
+ 4H
O
3
8
2
2
2
According to Hess, you can do it by any path you would like. We will use two: ∆H
o
and bond energies.
f
• For ∆H
use Appendix K and ∆H
= Σ n ∆H
- Σ n ∆H
o
o
o
o
f
rxn
f
prod
f
reac
• For BE use the tables below and ∆H
= Σ BE reactants - Σ BE products
o
rxn
o
Path one.
H
The path is through the elements in their standard states (C
, O
, H
)
f
s
2(g)
2(g)
∆H
o
C
H
+ 5 O
-----à 3CO
+ 4H
O
= ?????
3
8
2
2
2
rxn
Path two. BE. The path is through breaking all bonds to form gas phase atoms (C
, H
, O
).
g
g
g
What about Entropy? Note that just because we know ∆H, we can’t say for sure whether the reaction is
spontaneous. For that we need to look at the value of S. Can you make a good guess as to whether entropy
increases or decreases in the combustion reaction?
What about Free Energy? With this information, what can be said about ∆G and the spontaneity of the
reaction?
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