Chemistry 4.17 - Changing equilibrium

Predicting a change in equilibrium position

Higher tier only

Chemistry 4.17 is Higher-only. Recall that at dynamic equilibrium in a closed system both reactions continue at equal rates and each concentration remains constant. The reversible arrow by itself does not mean equilibrium has been reached.

The position of equilibrium describes the relative amounts of reactants and products in an equilibrium mixture. If the position is to the right, the mixture contains proportionally more products; if it is to the left, it contains proportionally more reactants. It does not mean that only one side is present.

Le Chatelier's principle provides a prediction rule:

Use this four-step route:

1. Name exactly what was increased, decreased, added or removed.
2. Identify the reaction direction that counteracts that change.
3. State whether equilibrium shifts left or right.
4. State which requested substance has a greater or smaller equilibrium amount.

Concentration

At constant temperature, changing the concentration of a dissolved or gaseous reactant/product disturbs the equal rates. The system then shifts in the direction that uses up some added substance or replaces some removed substance. It counteracts the change partly; it does not necessarily restore the original concentration. Treat one change at a time, with other conditions fixed.

For example, removing ammonia from the Haber equilibrium lowers the ammonia concentration. The equilibrium shifts right to replace some of the removed ammonia.

Pressure

Pressure predictions here apply when a gaseous equilibrium is compressed or expanded at constant temperature. Increasing pressure by adding an unreactive gas at fixed volume is a different change and does not increase the concentrations of the reacting gases. Count the equation coefficients for gases on each side; do not count atoms within a molecule.

• Increasing pressure favours the side with fewer gas particles.
• Decreasing pressure favours the side with more gas particles.
• If the gas-particle totals are equal, changing pressure does not move the equilibrium position.
• Solids and liquids are not counted in this GCSE pressure comparison.

Temperature

For a temperature prediction, first identify which direction is exothermic and which is endothermic.

• Increasing temperature favours the endothermic direction, which absorbs energy.
• Decreasing temperature favours the exothermic direction, which releases energy.

Temperature does not simply "speed up the forward reaction" and leave the reverse reaction unchanged. Both rates change, and a new equilibrium is reached with a different composition.

What a catalyst does not change

A catalyst speeds up both forward and reverse reactions. It allows equilibrium to be reached sooner, but it does not move the equilibrium position and does not change the equilibrium yield.

Worked example: three disturbances

For this supplied example the forward reaction is endothermic:

$\ce{N2O4(g) <=> 2NO2(g)}$

1. Some $\ce{NO2}$ is removed. The system counteracts the lower product concentration by making more $\ce{NO2}$. The equilibrium shifts right, towards products.

2. Pressure is decreased. The left side has one gas molecule in the equation ratio; the right side has two. Lower pressure favours the side with more gas particles, so equilibrium shifts right.

3. Temperature is increased. The forward direction is stated to be endothermic. Increasing temperature favours that endothermic direction, so equilibrium shifts right.