Chemistry Reference
In-Depth Information
oxygen to yield carbon monoxide and that of carbon monoxide plus oxygen to
yield carbon dioxide:
Desired:
C(s)
O
2
(g) £ CO
2
(g)
H
(kJ)
1
2
O
2
(g) £ CO(g)
Given:
C(s)
110
1
2
O
2
(g)£ CO
2
(g)
CO(g)
283
Adding the two chemical equations given:
C(s)
CO(g)
O
2
(g) £ CO
2
(g)
CO(g)
Eliminating CO from both sides results in the desired equation:
C(s)
O
2
(g) £ CO
2
(g)
Therefore, adding these two
¢
H
values will give the
¢
H
desired:
¢
H
(
110 kJ)
(
283 kJ)
393 kJ
Note that we have not used enthalpies of formation explicitly in this process.
Enthalpy changes of
any
type of reaction will do.
EXAMPLE 14.19
Calculate the enthalpy change of the general reaction
Z
Q £ X
2 R
from the following data:
H
(kJ)
27.1
Z
Q £ E
2 D
E
2 D £ X
2 R
13.7
Solution
Adding the two equations will result in the elimination of D and E, and yield
the desired equation. We then merely add the corresponding
¢
H
values:
¢
H
(27.1 kJ)
(
13.7 kJ)
13.4 kJ
Practice Problem 14.19
Calculate the enthalpy change of the gen-
eral reaction
Z
Q £ X
2 R
from the following data:
H
(kJ)
Z
Q £ E
2 D
41.7
E
2 D £ J
A
17.7
J
A £ X
2 R
10.6
Using Hess's law is straightforward if the equations given are in a form in
which simply adding them yields the equation desired. More often, however, to
get the desired equation, we must multiply or divide a given equation by a small
