The Effect of Changing Concentration

Section 1 of 6The 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.

Section 2 of 6Increasing 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.

Section 3 of 6Worked 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).

Section 4 of 6Why 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.

Section 5 of 6Key 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.

Section 6 of 6Exam technique

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).