H^0 rxn[H^0 fSiCl 4 (g) 2 H^0 fCO(g)][H^0 fSiO 2 (s) 2 H^0 f C(graphite) 2 H^0 fCl 2 (g)]
[(657.0) 2(110.5)] [(910.9) 2(0) 2(0)]
32.9 kJ/mol
S^0 rxnS^0 SiCl 4 (g) 2 S^0 CO(g) [S^0 SiO 2 (s) 2 S^0 C(graphite) 2 S^0 Cl 2 (g)]
[330.6 2(197.6)][41.84 2(5.740) 2(223.0)]
226.5 J/molK0.2265 kJ/molKWhen G^0 0, neither the forward nor the reverse reaction is favored. Let’s find the temper-
ature at which G^0 0 and the system is at equilibrium.
0 G^0 H^0 TS^0
H^0 TS^0T145 KAt temperatures above 145 K, the TS^0 term would be greater (TS^0 would be more nega-
tive) than the H^0 term, which would make G^0 negative; so the reaction would be
product-favored above 145 K. At temperatures below 145 K, the TS^0 term would be smaller
than the H^0 term, which would make G^0 positive; so the reaction would be reactant-favored
below 145 K.
However, 145 K (128°C) is a very low temperature. For all practical purposes, the reac-
tion is product-favored at all but very low temperatures. In practice, it is carried out at 800°C
to 1000°C because of the greater reaction rate at these higher temperatures. This gives a useful
and economical rate of production of SiCl 4 , an important industrial chemical.
You should now work Exercises 104 and 108.
32.9 kJ/mol
0.2265 kJ/molKH^0
S^0Key Terms 635Key Terms
Absolute entropy (of a substance) The entropy of a substance
relative to its entropy in a perfectly ordered crystalline form at
0 K (where its entropy is zero).
Bomb calorimeter A device used to measure the heat transfer
between system and surroundings at constant volume.
Bond energy The amount of energy necessary to break one mole
of bonds in a gaseous substance, to form gaseous products at
the same temperature and pressure.
Calorimeter A device used to measure the heat transfer that
accompanies a physical or chemical change.
Endothermic process A process that absorbs heat.
Enthalpy change, H The quantity of heat transferred into or
out of a system as it undergoes a chemical or physical change
at constant temperature and pressure.
Entropy, S A thermodynamic state property that measures the
degree of disorder or randomness of a system.
Equilibrium A state of dynamic balance in which the rates of
forward and reverse processes (reactions) are equal; the state of
a system when neither the forward nor the reverse process is
thermodynamically favored.
Exothermic process A process that gives off (releases) heat.
First Law of Thermodynamics The total amount of energy in
the universe is constant (also known as the Law of Conserva-
tion of Energy); energy is neither created nor destroyed in
ordinary chemical reactions and physical changes.
Gibbs free energy, G The thermodynamic state function of
a system that indicates the amount of energy available for the
system to do useful work at constant Tand P.It is defined as
GHTS.
Gibbs free energy change, G The indicator of spontaneity of
a process at constant Tand P.GHTS.If Gis neg-
ative, the process is product-favored (spontaneous).
Hess’s Law of heat summation The enthalpy change for a reac-
tion is the same whether it occurs in one step or a series of
steps.
Internal energy, E All forms of energy associated with a specific
amount of a substance.
Mole of reaction (mol rxn) The amount of reaction that cor-
responds to the number of moles of each substance shown in
the balanced equation.