Equilibrium ( $K_c$ )

The mathematical link between how far a reaction goes (Equilibrium) and the available energy (Thermodynamics).

\Delta G^\theta = -RT \ln K

R

(Gas Constant)

8.31 $J K^{-1} mol^{-1}$

K

(Equilibrium Constant)

Ratio of Products/Reactants

\Delta G < 0

The reaction wants to make products. Therefore, $K$ will be Large ( $K > 1$ ). Equilibrium lies to the right.

\Delta G > 0

The reaction barely happens. Therefore, $K$ will be Small ( $0 < K < 1$ ). Equilibrium lies to the left (mostly reactants).

\Delta G = 0

There is no net drive either way. $K = 1$ .

Putting it into Practice

Paper 2 Style

Calculate the Equilibrium Constant ( $K$ ) for a reaction at 298 K where standard Free Energy change $\Delta G^\theta = -30 \: kJ \: mol^{-1}$ .

Use $R = 8.31 \: J K^{-1} mol^{-1}$ .

[1 Marks]

If a reaction has a very large positive $\Delta G^\theta$ (e.g., +200 kJ), what can you deduce about the value of the Equilibrium Constant $K$ ?