**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.

**ΔG ^{0} = – 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.

**ΔG ^{0} = – RT 1n (K)**

**ΔG ^{0} = – (8.314 J K^{-1} mol^{-1}) (298 K) ln (1.8 x 10^{-5})**

**ΔG ^{0} = – (8.314 J K^{-1} mol^{-1}) (298 K) (- 10.925) = + 2.7068 x 10^{4} J mol^{-1}**

**ΔG ^{0} = + 27.1 K J mol^{-1}**

Note that we normally quote results in multiples of 10^{3} or 10^{3} 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 ΔG^{0} with ≈ 0.14 K J mo1^{-5} hence the above final result.

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