Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 33

Determining DG°
?G° indicates whether or not a chemical reaction or physical process will proceed spontaneously as written under standard conditions.
In later chapters, ?G ° will be necessary for calculations involving chemical equilibrium [? Chapters 15–17] and electrochemistry
[? Chapter 18]. Using tabulated ?G f° values, we can calculate the standard free energy change (?G °) using Equation 14.11:
?G° = ?n ?G f° (products) – ?? ?G f° (reactants)
m
rxn

Look up ?G° values
f
for reactants and
products
4HBr(g)

+

Sum all ?G°
f
values for
products

O2(g)

–53.2

Sum all ?G°
f
values for
reactants

2Br2(l)

0

+

0

Subtract reactant
sum from product
sum

2H2O(l)
–237.2
H2O(l)

?G° (kJ/mol)
f

H2O(g)

–237.2

–228.6

Br2(l)

0

O2(g)

0

HBr(g)

?G° =
rxn

–

[2(0) + 2(–237.2)]

[4(–53.2) + 0]

=

-53.2

–261.6 kJ/mol

Alternatively, ?G° can be calculated using Equation 14.10 (?G° = ?H° – T ?S° ), absolute temperature (T ), and ?H°/?S° values,
rxn
rxn
rxn
which themselves typically are calculated from tabulated data. (Remember that the units of ?S° must be converted from J/K·mol to
kJ/K·mol prior to using Equation 14.10.)

4HBr(g)
–36.2

+

O2(g)

2Br2(l)

0

0

+

2H2O(l)
–285.8

?H° (kJ/mol)
f

S° (J/K•mol)
f

H2O(l)

–

[4(–36.2) + 0]

=

–241.8
0

188.7
152.3

0

205.0

HBr(g)

[2(–0) + 2(–285.8)]

69.9

H2O(g)
Br2(l)
O2(g)

?H° =
rxn

–258.8

–36.2

198.48

– 426.8 kJ/mol

602

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