Unit 6 · Lesson 6.2.5
The Effect of Changing Concentration
Predict and explain how adding or removing a reactant or product shifts the position of equilibrium (HT only).
The idea (HT only)
Disturbing the balance
- Disturbing the balance A system at equilibrium has constant concentrations because forward and reverse rates are equal.
- Changing the concentration of any substance upsets this balance.
Opposing the change
- Opposing the change The system moves to reduce the effect of what you have done, until the rates match again.
Increasing a reactant concentration (HT only)
1Add more reactant
- Add more reactant The concentration of a reactant on the left goes up.
2Forward rate rises
- Forward rate rises More collisions between reactant particles mean the forward reaction is faster than the reverse.
3Reactant is used up
- Reactant is used up Extra product forms, so the position moves to the right.
4New equilibrium
- New equilibrium The rates are equal again with more product present than before.
Summary of concentration changes (HT only)
| Change | Effect on rates | Shift in position |
|---|---|---|
| Increase concentration of reactant | Forward reaction speeds up first | Right, towards products |
| Decrease concentration of reactant | Reverse reaction is faster | Left, towards reactants |
| Increase concentration of product | Reverse reaction speeds up first | Left, towards reactants |
| Decrease concentration of product | Forward reaction is faster | Right, towards products |
Worked idea: A + B ⇌ C + D
1
Start with the mixture at equilibrium, with fixed amounts of A, B, C and D.
2
Remove D as it forms, for example by distilling it off.
3
The concentration of D falls, so the reverse reaction slows while the forward reaction carries on at nearly the same rate.
4
The forward reaction now wins, making more C and D and using up A and B.
5
Continuously removing a product pulls the reaction towards completion and raises the yield (HT only).
Why it matters
Industrial use
Recycling reactants
- Recycling reactants Unreacted starting materials are returned to the reactor so the reactant concentration stays high.
Removing product
- Removing product Products are taken out as they form, for example ammonia is liquefied and removed in the Haber process.
Yield
- Yield Both actions keep the position to the right and improve the percentage yield.
Key terms
| Term | Meaning |
|---|---|
| Equilibrium | The state where forward and reverse reactions have equal rates. |
| Concentration | The amount of a substance in a given volume. |
| Shift to the right | More products are formed as the position of equilibrium moves towards them. |
| Shift to the left | More reactants are formed as the position moves towards them. |
| Yield | The quantity of product actually obtained. |
Avoid these mistakes
1
Saying the equilibrium constant or the rates stay different forever; a new equilibrium has equal rates again.
2
Writing that adding reactant moves the position left because 'there is too much on that side'.
3
Forgetting that removing a product moves the position towards the products.
4
Mixing up the shift (a change in position) with the rate (how fast the reaction goes).