Effect of Change in Pressure

A change in pressure only affects the position of an equilibrium that involves gases, and only when there are different numbers of gas molecules on each side of the equation.By Le Chatelier's principle:• Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, which reduces the pressure.
• Decreasing the pressure shifts the equilibrium towards the side with more gas molecules.Count only the gaseous species, using the balancing numbers in the equation.Example 1. The Haber process:N₂(g) + 3H₂(g) ⇌ 2NH₃(g)There are 4 molecules of gas on the left and 2 on the right. Increasing the pressure shifts the equilibrium to the right, increasing the yield of ammonia.Example 2. Brown nitrogen dioxide and colourless dinitrogen tetroxide:2NO₂(g) ⇌ N₂O₄(g)If a syringe of the mixture is compressed, the colour first darkens because the NO₂ is more concentrated. It then becomes lighter as the equilibrium shifts to the right (1 molecule instead of 2).Example 3. H₂(g) + I₂(g) ⇌ 2HI(g) has 2 gas molecules on each side, so changing the pressure has no effect on the position of equilibrium.Solids and liquids are ignored when counting. Like concentration, a change in pressure does not change the value of K_c.