Equilibrium – Where? vs. Why?

The value of K is an indication of WHERE the equilibrium rests.

We haven’t addressed WHY the equilibrium exists where it does.

ΔG

A ball rolling down two different hills.

Potential energy “drives” the ball to the point of minimum potential energy.

In the same way, free energy (ΔG) of a chemical reaction decreases until it reaches a minimum value.“Chemical Potential”

A ball rolling down two different hills.

The ball will come to rest at the position of lowest potential energy.

A reaction will proceed to the point where free energy is lowest.

This drawing is called a free-energy curve.

DG vs DGo

×DG applies only when the reactants and products are in their standard states.

– Their normal state at that temperature– Partial pressure of gas = 1 atm– Concentration = 1 M

• DG calculated for given (current) conditions.

• Only one value for DG for a given reaction.• Many possible values for DG.

N2O4 2NO2

ΔG

ΔG = ΔG + RT ln Q

R = 8.31 J/mol.K

Consider the reaction at 298 K:

2H2S(g) + SO2(g) 3S(s, rhombic) + 2H2O(g)

DGorxn = -102 kJ

Calculate DGrxn under these conditions:

PH2S = 2.00 atm

PSO2 = 1.50 atm

PH2O = 0.100 atm

ΔG

ΔG = ΔG + RT ln Q

At equilibrium, ΔG = 0

ΔG = -RT ln K

and Q = K

Calculate Kp for the reaction

2H2O(l) 2H2(g) + O2(g)

Given that ΔG[H2O(l)] = -237.1 kJ/mol