5 Steps to a 5 AP Chemistry

c. CaSO 4 H 2 OCO 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 ΔHand positive ΔS), 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 Tis the Kelvin temperature

Like most thermodynamic functions, only the change in Gibbs free energy can be meas-

ured, so the relationship becomes:

ΔG=ΔH−TΔS

ΔGis the best indicator chemists have as to whether or not a reaction is spontaneous:

 If ΔG>0, the reaction is not spontaneous; energy must be supplied to cause the reac-

tion to occur.

 If ΔG<0, the reaction is spontaneous.

 If ΔG=0, the reaction is at equilibrium.

If there is a ΔGassociated with a reaction and that reaction is then reversed, the sign of

ΔGchanges.

Just like with the enthalpy and entropy, the standard Gibbs free energy change, (ΔG°),

is calculated:

ΔG°=ΣΔGf°products −ΣΔGf°reactants

ΔGf°of an element in its standard state is zero.

ΔG°for a reaction may also be calculated by using the standard enthalpy and standard

entropy of reaction:

ΔG°=ΔH°rxn−TΔS°rxn

Calculate ΔG°for:

(If you do not have a table of ΔG°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

Thermodynamics  129

KEY IDEA