Position of equilibrium

Photo of tightrope walkers in a circus

Aims of this page

After studying this page, you should be able to:

  • recall that imposing a change on a chemical reaction can change the position of equilibrium
  • describe and explain how a change in conditions affect the position of equilibrium.

What is position of equilibrium?

The position of equilibrium (also called equilibrium position) describes the relative concentrations of reactants and products at equilibrium.
Position of equilbriumEquilibrium mixture
Lies to the leftReactant concentrations higher
Lies to the rightProduct concentrations higher

You cannot tell the position of equilibrium by looking at the balanced symbol equation for the reaction. You need to know the concentrations of reactants and products at equilibrium.

The position of equilibrium can be changed by changing the reaction conditions.

Le Chatelier’s Principle provides a way to predict what happens:

  • A change in a condition affecting a dynamic equilibrium causes the position of equilibrium to move to counteract that change.

This lets you know what will happen, but it doesn’t fully explain why it will happen. However, the detailed explanations are more complicated than you need for your GCSE (and probably for your A Level too).

Henry Louis Le Chatelier (1850 – 1936)

Changing a concentration

The position of equilibrium will move in the direction away from an added substance (and towards any substance that is removed).

The reaction between iron(III) ions and thiocyanate ions is easy to do in the school lab:

Fe3+(aq) + SCN–(aq) ⇌ FeSCN2+(aq)
iron(III) ion thiocyanate ion thiocyanatoiron(III) ion
pale yellow colourless blood red
Aqueous thiocyanatoiron(III) complexes
A mixture containing amounts of Fe3+ and SCN– is pale orange-brown. The table shows what happens when you change the concentration of these two ions.
ActionChangeResult
Add more Fe3+Fe3+ concentration ⇧Turns red
Add more SCN–SCN– concentration ⇧Turns red
Add NH4Cl powderFe3+ concentration ⇩Turns yellow

Explanation

In terms of Le Chatelier’s principle:

  • adding Fe3+ or SCN– causes the position of equilibrium to move to the right to use up these ions
  • removing Fe3+ causes the position of equilibrium to move to the left to produce more of these ions.

Changing the temperature

The position of equilibrium will move in the direction of:

  • the exothermic reaction if the temperature is decreased
  • the endothermic reaction if the temperature is increased.

It may help if you only learn one of these ideas (exothermic or endothermic) because the other is just the other way round. 

This reaction is easy to do in the school lab:

\(\ce{$\underset{\large\textsf{blue}}{\ce{Cu^2+(aq)}}$ + $\underset{\large\textsf{colourless}}{\ce{4Cl-(aq)}}$ <=>[\large\textsf{endothermic}][\large\textsf{exothermic}] $\underset{\large\textsf{green}}{\ce{[CuCl4]^2-(aq)}}$}\)

The forward reaction is endothermic (and so the reverse reaction is exothermic). You can change the temperature of the reaction mixture by heating the test tube in boiling water or by cooling it in iced water.

Cu2+(aq) in the presence of Cl–(aq) ions. The mixture is blue in low concentrations of Cl– ions and green in high concentrations of Cl– ions.
ActionChangeResult
Put in boiling waterTemperature ⇧Turns green
Put in iced waterTemperature ⇩Turns blue

Explanation

In terms of Le Chatelier’s principle:

  • increasing the temperature causes the position of equilibrium to move to the right (in the direction of the endothermic reaction)
  • this causes more [CuCl4]2– to form, so the mixture changes from blue to green.
When the temperature is decreased, the position of equilibrium moves to the right. More Cu2+(aq) forms, so the mixture changes from green to blue.

Changing the pressure

If the pressure is increased, the position of equilibrium moves in the direction of the fewest molecules of reacting gas.

You need to look at the balanced chemical equation to work out which side has the fewest molecules of gas. For example, look at this reaction:

2NO2(g) ⇌ N2O4(g)
nitrogen dioxide dinitrogen tetroxide
red-brown colourless
A brown gas in a transparent bottle
Nitrogen dioxide is a red-brown gas

There are 2 molecules of gas on the left-hand side of the equation, and only 1 molecule of gas on the right-hand side. This means that:

  • increasing the pressure causes the position of equilibrium to move to the right (in the direction of the fewest molecules of gas)
  • this causes the reaction mixture to become paler.
When the pressure is decreased, the position of equilibrium moves to the left. More red-brown NO2 gas forms, so the mixture becomes darker.

This video shows what happens.

The syringe contains nitrogen dioxide. It is sealed at the end so that the pressure increases when the plunger is pushed down. The bottle on the right contains nitrogen dioxide at atmospheric pressure.

Explanation

There are four main events in the video:

  1. The plunger is rapidly pushed in, which reduces the volume and increases the pressure
  2. The colour darkens because the concentration of NO2 increases
  3. The colour lightens slightly as the equilibrium shifts and some NO2 is converted to N2O4
  4.  The plunger is released, the colour lightens noticeably, and then darkens slightly as the equilibrium shifts again.

Adding a catalyst

A catalyst is a substance that increases the rate of a reaction, without being used up or changed chemically by the end of the reaction.

A catalyst reduces the time taken to reach equilibrium, but it does not alter the position of equilibrium. Think of someone on a treadmill. They adjust their running speed to match the speed of the belt to stay in the same position.

Explanation

A catalyst increases the rate of the forward reaction and reverse reaction by the same ratio.

Summary

ChangePosition of equilibrium
Add more reactantMoves to the right
Add more productMoves to the left
Increase the temperatureMoves in the direction of the endothermic reaction
Increase the pressureMoves in the direction of the fewest molecules of reacting gas
Add a catalystNo change
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Position of equilibrium

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