Effect of Concentration on Equilibrium
Simple Explanation
Adding more of a substance to an equilibrium system shifts the equilibrium away from that substance (using some of it up); removing a substance shifts the equilibrium toward replacing it.
Why Do We Need It?
This is one of the most direct, controllable ways to push a reaction toward more product — industrially, reactants are often continuously added (or products continuously removed) to keep shifting equilibrium toward higher yield.
Worked Example
Predict a shift from adding a reactant
For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), extra N₂ is added to the system. Which direction does the equilibrium shift?
Why Does This Work?
Adding more of a substance makes the reaction quotient no longer match the equilibrium constant — specifically, it makes the forward reaction rate briefly exceed the reverse rate (or vice versa when removing a substance), so the system shifts in that direction until the rates rebalance at a new equilibrium.
Real-Life Example
The industrial Haber process
Ammonia is manufactured industrially from nitrogen and hydrogen gas.
Continuously removing ammonia as it forms (by cooling and condensing it out of the gas mixture) keeps shifting the N₂ + 3H₂ ⇌ 2NH₃ equilibrium forward, converting far more of the nitrogen and hydrogen into ammonia than would otherwise be possible.
Practice
For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), NH₃ is continuously removed from the system as it forms. Which direction does equilibrium shift?
MediumCommon mistake
Assuming adding a reactant shifts equilibrium toward that SAME substance — actually, adding a substance shifts the equilibrium AWAY from it (consuming some of what was added), not toward it.
Quick Review
- Adding a substance shifts equilibrium away from it (consumes some of it).
- Removing a substance shifts equilibrium toward it (replaces some of it).
- Continuously removing product is a common industrial trick to boost yield.