General Chemistry Principles, Patterns, and Applications, 2011

B Substitute the appropriate values into Equation 18.26 to obtain ΔG° for the reaction.
C Assuming that ΔH and ΔS are independent of temperature, substitute values intoEquation 18.23 to obtain
ΔG for the reaction at 300°C.
olution:
a. A To calculate ΔG° for the reaction using Equation 18.26, we must know the temperature as well
as the values of ΔS° and ΔH°. At standard conditions, the temperature is 25°C, or 298 K. We can calculate
ΔS° for the reaction from the absolute molar entropy values given for the reactants and the products
using the “products minus reactants” rule:
We can also calculate ΔH° for the reaction using the “products minus reactants” rule. The value
of
zero for both N 2 and H 2:
B Inserting the appropriate values into Equation 18.26,
b.
c. C To calculate ΔG for this reaction at 300°C, we assume that ΔH and ΔSare independent of
temperature (i.e., ΔH 300°C = H° and ΔS 300°C = ΔS°) and insert the appropriate temperature (573 K)
into Equation 18.23
:
DS rxn = 2S°(NH 3) - [S°(N2)
+3S°(H 2) =
/(mol × K)]
/(mol × K) +
] [ 2mol NH 3´192.8 J
- {[1mol N2 ´191.6 J
] [ 3mol H 2 ´130.7 J / (mol × K)]}
= -198.1 J / K (per mole of N2)
DH f (NH 3
) is given, and DH f is
DH rxn = 2DH f (NH 3) -
] [ 2
[DH f (N2) + 3DH f (H 2) =
´(-45.9 kJ / mol) ]-[(1´ 0 kJ / mol)
+(3´ 0 kJ / mol)] = -91.8 kJ(per mole ofN2)
DG rxn = DH° - T DS° = (-91.8 kJ)
-(298 K)(-198.1 J / K)(1 kJ /1000 J)
= -32.7 kJ (per mole of N2)
c. DG300°C = DH 300°C - (573 K)(DS300°C)
= DH° - (573 K)DS° = (-91.8 kJ) - (573 K)(-198.1 J / K) mole of N2)
(1 kJ /1000 J) = 21.7 kJ (per mole of N2)
In this example, changing the temperature has a major effect on the thermodynamic spontaneity of the
reaction. Under standard conditions, the reaction of nitrogen and hydrogen gas to produce ammonia is
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