128 ❯ STEP 4. Review the Knowledge You Need to Score High
c. CaSO 4 H 2 O CO 2 CaCO 3 H 2 SO 4
[107 + 188.7 + 213.6] - [92.9 + 157] J/mol K
= 259 J/mol K
One of the goals of chemists is to be able to predict whether or not a reaction will be spon-
taneous. Some general guidelines for a spontaneous reaction have already been presented
(negative DH and positive DS), but neither is a reliable predictor by itself. Temperature
also plays a part. A thermodynamic factor that takes into account the entropy, enthalpy,
and temperature of the reaction should be the best indicator of spontaneity. This factor is
called the Gibbs free energy.Gibbs Free Energy
The Gibbs free energy (G) is a thermodynamic function that combines the enthalpy,
entropy, and temperature:
G = H - TS, where T is the Kelvin temperature
Like most thermodynamic functions, only the change in Gibbs free energy can be mea-
sured, so the relationship becomes:
DG = DH - T DS
DG is the best indicator chemists have as to whether or not a reaction is spontaneous:● (^) If DG > 0, the reaction is not spontaneous; energy must be supplied to cause the reaction
to occur.
● (^) If DG < 0, the reaction is spontaneous.
● (^) If DG = 0, the reaction is at equilibrium.
If there is a DG associated with a reaction and that reaction is then reversed, the sign of
DG changes.
Just like with the enthalpy and entropy, the standard Gibbs free energy change, DG°,
is calculated:
DG° = S DGf° products - S DGf° reactants
DGf° of an element in its standard state is zero.
DG° for a reaction may also be calculated by using the standard enthalpy and standard
entropy of reaction:
DG° = DH°rxn - T DS°rxn
Calculate DG° for:
(If you do not have a table of DG° values, just set up the problems.)
a. 2 NH 4 Cl(s) + CaO(s) → CaCl 2 (s) + H 2 O(l) + 2 NH 3 (g)
b. C 2 H 4 (g) + H 2 O(g) → C 2 H 5 OH(l)
c. Ca(s) + 2 H 2 SO 4 (l) → CaSO 4 (s) + SO 2 (g) + 2 H 2 O(l)
Answers:
a. CaCl 2 (s) H 2 O(l) NH 3 (g) NH 4 Cl(s) CaO(s)
-750.2 -237.2 -16.6 -203.9 -604.2 kJ/mol
KEY IDEA