Chapter 15 · Chemical Equilibrium
15.8The Effect of a Concentration Change on Equilibrium
9 min · two checks
Predict
For N₂ + 3 H₂ ⇌ 2 NH₃, what is the shift if H₂ is added?
The idea
- Predict the direction after adding or removing a species.
- State what happens to K.
Adding a reactant or removing a product drives the forward reaction. Adding a product or removing a reactant drives the reverse. Think of Q. Stuffing more reactant into the denominator makes Q < K, so the reaction runs forward until Q climbs back to K. The amounts of other substances move too: adding H₂ consumes some N₂ and produces some NH₃.
Do this only for species that appear in the K expression. Adding more solid copper to a reaction where copper is a pure solid does not change Q. In solution, diluting the whole mixture changes every concentration and can shift an equilibrium if the number of dissolved particles differs on the two sides. That effect is easier to see as a volume change, which is next.
Keep these
- Add reactant or remove product → forward shift.
- Add product or remove reactant → reverse shift.
- K is unchanged at constant temperature.