Equilibrium (Cambridge (CIE) IGCSE Chemistry)
Revision Note
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 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:
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 ⇌ ICl3
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 ⇌ ICl3
dark brown yellow
Explain what happens when:
The concentration of ICl3 increases
Some Cl2 is removed
Answers:
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
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
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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