The Position of Equilibrium (Edexcel IGCSE Chemistry (Modular))

Revision Note

Test yourself

The position of equilibrium

  • The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction

  • This balance is framed in an important concept known as Le Chatelier's Principle,

    • Named after Henri Le Chatelier, who was a French military engineer in the 19th century

  • This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change

  • The principle is used to predict changes to the position of equilibrium when there are changes in:

    • Temperature

    • Pressure 

  • Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions

  • For example, if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules

  • The position of equilibrium is said to shift to the right when the forward reaction is favoured

    • This means that there is an increase in the amount of products formed

  • The position of equilibrium is said to shift to the left when the reverse reaction is favoured

    • So, there is an increase in the amount of reactants formed

Temperature

  • Le Chatelier's Principle can be used to predict the effect of changes in temperature on systems in equilibrium

  • To make this prediction it is necessary to know whether the reaction is exothermic or endothermic

  • If the temperature of the reaction increases:

    • The equilibrium will shift in the direction of the endothermic reaction

  • If the temperature of a reaction decreases:

    • The equilibrium will shift in the direction of the exothermic reaction

Worked Example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride and the forward reaction is exothermic:

ICl (l) + Cl2 (g) ⇌ ICl(s)

dark brown            yellow

What colour will the mixture turn when heated? Explain your answer.

Answer:

  • The system will oppose the increase in temperature

    • Increasing the temperature of an equilibrium reaction favours the endothermic reaction

  • If the forward reaction is exothermic, then the backward reaction must be endothermic

  • Therefore, the equilibrium will move to the left and produce more reactants

  • This means that the colour of the mixture will become increasingly brown as the temperature increases

Pressure

  • We can predict the effect of changes in pressure on systems in equilibrium

  • Changes in pressure only affects gases

  • In gaseous reactions:

    • An increase in pressure will favour the reaction that produces the least number of molecules

    • A decrease in pressure will favour the reaction that produces the greatest number of molecules

  • If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed

    • Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged

Worked Example

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:

2NO2 (g)   ⇌   N2O(g)

dark brown     colourless

What will the colour change be if the pressure is increased? Explain your answer.

Answer:

  • The number of gas molecules produced by the forward reaction = 1

  • The number of gas molecules produced by the reverse reaction = 2

  • An increase in the pressure will favour the reaction that produces the least number of molecules

    • This is the forward reaction

  • So, the equilibrium shifts to the right

    • This means that the mixture will appear increasingly colourless as the concentration of N2O4 increases

Examiner Tip

When the conditions at equilibrium are changed, the system always responds by doing the opposite.

The effect of catalysts on equilibrium

The effect of a catalyst on equilibrium position

  • The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached

  • This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)

  • As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

Diagram showing the effect of catalyst on equilibrium position

Graph to show a catalyst increasing the rate of the forward and reverse reaction

A catalyst increases the rate of the forwards and reverse reaction but does not alter the position of equilibrium

Last updated:

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.