University12 min

Equilibrium constant, reaction quotient and van 't Hoff

Write K with activities, go from ΔrG° to K without a units slip, compare Q with K to predict the direction, and track K with temperature via van 't Hoff.

The essentials

1

K is written with activities

K=∏iaiνiK = \prod_i a_i^{\nu_i} with νi>0\nu_i > 0 for products, <0< 0 for reactants. Gas: ai=PiP∘a_i = \dfrac{P_i}{P^\circ} (P∘=100 kPaP^\circ = 100\ \text{kPa}); solute: ai=[i]c∘a_i = \dfrac{[i]}{c^\circ} (c∘=1 mol/Lc^\circ = 1\ \text{mol/L}); **pure solid or liquid, solvent: ai=1a_i = 1**. KK is dimensionless and depends only on TT.
2

The reaction quotient

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3

The link with standard Gibbs energy

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4

Van 't Hoff equation

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5

Special cases to recognise

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