Example 1
Predict the effect of compression on N2+3H2 ⇌ 2NH3 at constant temperature.
There are four gas moles on the left and two on the right. Compression favours NH3 formation, the direction reducing gas-particle amount.
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Year 12 · SACE Stage 2
Dynamic equilibrium and shifts
Apply equilibrium rules to concentration, pressure and temperature changes.
Method and worked examples
In a closed system at fixed conditions, a reversible reaction can reach dynamic equilibrium. Forward and reverse reactions continue at equal rates, making macroscopic concentrations constant. Reactant and product concentrations need not be equal. Changing a condition can temporarily make the two rates unequal, after which the system approaches a new equilibrium.
Adding a reactant or removing a product generally favours forward reaction. Compressing a gaseous equilibrium favours the side with fewer gas moles; equal gas coefficients give no such shift. Raising temperature favours the endothermic direction. Only temperature changes the equilibrium constant for the same reaction. A catalyst speeds both directions and attainment of equilibrium without changing its position.
Example 1
Example 2
For exothermic 2NO2 ⇌ N2O4, predict the equilibrium effect of heating.
Heat favours the reverse, endothermic direction. More NO2 and less N2O4 are present at the new equilibrium; the forward Kc decreases.
Watch out: The forward and reverse rates are equal; concentrations are constant but can have very different values.