Chemical Work at Constant Pressure

The ability of a chemical reaction to perform work at a constant pressure is given by the Gibbs energy AG° where K is the equilibrium constant.

ΔG0 = – RT 1n (K)

Calculate ΔG° for a reaction which has an equilibrium constant K= 1.8 x 10-5 at a temperature T = 298 K and R = 8.314 J K-1 mo1-1 is the gas constant.

 

ΔG0 = – RT 1n (K)

ΔG0 = – (8.314 J K-1 mol-1) (298 K) ln (1.8 x 10-5)

ΔG0 = – (8.314 J K-1 mol-1) (298 K) (- 10.925) = + 2.7068 x 104 J mol-1

ΔG0 = + 27.1 K J mol-1

 

Note that we normally quote results in multiples of 103 or 103 and use the appropriate multiple symbol, here it is kilo with the symbol k. The equilibrium constant K is quoted to two significant figures and a variation in K of ± 0.1 x 10-5 gives ΔG0 with ≈ 0.14 K J mo1-5 hence the above final result.