Predicting pressure effects from equilibrium equations

A change in pressure can change the position of a gaseous equilibrium. To predict the direction of the shift, use the balanced equation to compare the total numbers of gas molecules on the two sides.

Add the coefficients of the gaseous substances on each side. A substance with no written coefficient has a coefficient of 1. Count only substances marked (g); do not include solids or liquids.

When pressure is changed by changing the volume at constant temperature:

Increasing pressure favours the side with fewer gas molecules.
Decreasing pressure favours the side with more gas molecules.

If both sides have the same total number of gas molecules, changing the pressure does not shift the equilibrium.

Predicting a shift from an equation

Consider the following balanced equilibrium:

\ce{N2(g) + 3H2(g) <=> 2NH3(g)}

The left side has a total of $$$1 + 3 = 4$$$ gas molecules, while the right side has 2. The right side therefore has fewer gas molecules.

If pressure increases, the equilibrium shifts to the right, favouring the formation of more $\ce{NH3}$ . If pressure decreases, it shifts to the left, favouring $\ce{N2}$ and $\ce{H2}$ .

A shift describes a change in the equilibrium position; it does not mean that the reaction goes completely to one side.