Equilibrium (Cambridge (CIE) IGCSE Chemistry)

Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

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The concept of equilibrium

Extended tier only

  • Reversible reactions occur in both the forward and backward directions

  • A reversible reaction can reach equilibrium in a closed system

    • This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter

Equilibrium in open & closed systems

Equilibrium can only be reached in a closed vessel which prevents reactants or products from escaping the system

  • At equilibrium:

    • The rate of the forward reaction is equal to the rate of the reverse reaction

    • The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)

  • Equilibrium is dynamic

    • This means that the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate

  • An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process:

    • At the start of the reaction, only nitrogen and hydrogen are present

      • This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest

    • As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease

      • So, the rate of the forward reaction will decrease

    • However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase

      • Ammonia will decompose to reform hydrogen and nitrogen

    • In a closed system, the two reactions are interlinked and none of the gases can escape

    • So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

Graph showing the rates of forward and backward reactions as a reaction proceeds to equilibrium

At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction

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Le Chatelier’s principle

Extended tier only

  • 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,

  • 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 

    • Concentration

  • 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

How temperature affects equilibrium

  • We can 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 is raised:

    • The yield from the endothermic reaction increases

    • The yield from the exothermic reaction decreases

  • If the temperature is lowered:

    • The yield from the endothermic reaction decreases

    • The yield from the exothermic reaction increases

  • When a change in temperature is made to a system, the system will oppose the change

    • E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature

    • It will do this by favouring the endothermic reaction

The effects of temperature on equilibrium

Change

How the equilibrium shifts  

Increase in temperature

Equilibrium moves in the endothermic direction to reverse the change

Decrease in temperature

Equilibrium moves in the exothermic direction to reverse the change 

Worked Example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride.

The forward reaction is exothermic:

ICl             +              Cl2             ⇌             ICl

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 of the reactants

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

How pressure affects equilibrium

  • Changes in pressure only affect reactions where the reactants or products are gases

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

    • To make this prediction, the balanced symbol equation must be known

The effects of pressure on equilibrium

Change

How the equilibrium shifts  

Increase in pressure

Equilibrium shifts in the direction that produces the least number of molecules

Decrease in pressure

Equilibrium shifts in the direction that produces the greatest number of molecules

Worked Example

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

2NO2 (g)            ⇌              N2O4 (g) 
brown gas                    colourless gas

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 pressure will cause equilibrium to shift in the direction that produces the least number of molecules of gas

    • This is the forward reaction

  • So, the equilibrium shifts to the right

  • This means that:

    • The mixture will become increasingly colourless

    • The concentration of N2O4 will increase 

How concentration affects equilibrium

  • The effect of changing concentration can be thought of as a balance, with the reactants on the left and the products on the right

    • If the concentration of a reactant increases, then the equilbrium shifts to the right to balance this balance

 The effects of concentration on equilibrium

Change

How the equilibrium shifts  

Increase in concentration of a reactant

Equilibrium shifts to the right

Decrease in concentration of a reactant 

Equilibrium shifts to the left 

Increase in concentration of a product

Equilibrium shifts to the left 

Decrease in concentration of a product

Equilibrium shifts to the right

Worked Example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride

ICl             +              Cl2             ⇌             ICl

dark brown                                                    yellow

Explain what happens when:

  1. The concentration of ICl3 increases  

  2. Some Cl2 is removed

Answers:

  1. The concentration of ICl3 increases  

    • There are more molecules of ICl3 on the products side

    • So, the position of the equilibrium moves to the left

    • This produces more ICl and Cl2  

      • So, the reaction mixture gets darker / turns dark brown

  2. Some Cl2 is removed

    • There are less molecules of Cl2 on the reactants side

    • So, the position of the equilibrium moves to the left

    • This produces more Cl2 (and ICl) 

      • So, the reaction mixture gets darker / turns dark brown

How catalysts affects equilibrium

  • The presence of a catalyst:

    • Does not affect the position of equilibrium

    • Increases 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 equilibrium concentration of reactants and products is the same as it would be without the catalyst

    Showing how a catalyst affects rate for a reversible reaction

Diagram showing the effect of a catalyst on the time taken for equilibrium to be established

Examiner Tips and Tricks

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

For example if the concentration is increased the system tries to reduce it by changing the direction of the reaction or if the temperature is increased the system will try to reduce the temperature by absorbing the extra heat.

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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.