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The equilibrium constant

At equilibrium the reaction goes equally fast both ways, and the concentrations stay put. The equilibrium constant K is a ratio of product to reactant, each raised to the coefficient from the balanced equation. Pure solids and the solvent water are left out. A large K means that the equilibrium lies towards the products. K changes only when the temperature changes. A catalyst makes the equilibrium arrive sooner, but it does not change K.

K=[HI]2[H2][I2]K = \frac{[\mathrm{HI}]^2}{[\mathrm{H}_2][\mathrm{I}_2]}for H2 + I2 in equilibrium with 2 HI

Symbols

KKequilibrium constant
[,][,]concentration at equilibriummol/L

Example

At equilibrium [H2] = 0.20 mol/L, [I2] = 0.40 mol/L and [HI] = 0.80 mol/L.

K=0.802/(0.20⋅0.40)=8K = 0.80^2 / (0.20 \cdot 0.40) = 8.

The coefficient becomes the exponent. If you forget the square on HI, K is wrong.
Practise chemical equilibrium for free →

Reaction quotient →

Part of Chemistry 2: Chemical equilibrium.