Chemical equilibrium
Many reactions are reversible: the products can react to form the reactants again. A double arrow shows this.
N2 + 3 H2 ⇌ 2 NH3
At first the forward reaction is fast. As products build up, the reverse reaction speeds up. Equilibrium is reached when the forward and reverse rates are equal. From then on the amounts stay constant, although both reactions continue.
Le Chatelier’s principle: if a system at equilibrium is disturbed, it shifts in the direction that partly undoes the change.
| Change | Shift |
|---|---|
| Add a reactant | Toward products |
| Remove a product | Toward products |
| Add a product | Toward reactants |
| Raise the temperature | In the endothermic direction |
| Raise the pressure (gases) | Toward the side with fewer moles of gas |
A catalyst does not shift equilibrium. It only helps the system reach it faster.
Worked example: the pressure is raised on N2 + 3 H2 ⇌ 2 NH3
The left side has 1 + 3 = 4 mol of gas and the right side has 2 mol. Raising the pressure shifts the reaction toward NH3, the side with fewer moles of gas.
Common mistake
Thinking the reaction stops at equilibrium, or that the amounts of reactants and products are equal. The rates are equal. The amounts are constant but usually not equal.