Equilibrium (Cambridge (CIE) O Level Chemistry): Revision Note

The Concept of Equilibrium

• We have already seen that a reversible reaction is one that occurs in both directions

• When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium

• Equilibrium is dynamic, i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate

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

• It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel

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

• An example of dynamic equilibrium is the reaction between H₂ and N₂ in the Haber process

• When only nitrogen and hydrogen are present at the beginning of the reaction, 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)

• Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

Diagram showing when the rates of forward and backward reactions become equal

• Equilibrium position refers to the relationship between the concentration of reactants and products at the equilibrium state

• When the position of equilibrium shifts to the left, it means the concentration of reactant increases

• When the position of equilibrium shifts to right, this means the concentration of product increases

Le Chatelier’s Principle

• 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 or 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 the 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 and 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 and there is an increase in the amount of reactants formed

The Effect of Temperature on Equilibrium

• Increase in temperature

  • The equilibrium moves in the endothermic direction to reverse the change

• Decrease in temperature

  • The equilibrium moves in the exothermic direction to reverse the change 

• Example:

  • Dark brown iodine monochloride reacts reversibly with chlorine to form yellow iodine trichloride:

ICl + Cl₂ ⇌ ICl₃

• When the equilibrium mixture is heated, it becomes darker brown in colour

• The darker brown colour means that more ICl is produced

• Therefore, increasing the temperature moves the equilibrium in the backward direction

• So, the backward reaction is endothermic

The Effect of Pressure on Equilibrium

• Increase in pressure

  • The equilibrium moves in the direction that produces the smaller number of gas molecules to decrease the pressure

• Decrease in pressure

  • The equilibrium moves in the direction that produces the greater number of gas molecules to increase the pressure 

• Example:

  • Brown nitrogen dioxide can form colourless dinitrogen tetroxide gas

2NO₂ (g) ⇌ N₂O₄ (g)

• To predict the effect of an increase in pressure on the position of equilibrium:

  • Number of molecules of gas on the left = 2

  • Number of molecules of gas on the right = 1

  • There are fewer gas molecules on the right

• Increasing the pressure causes the equilibrium to shift in the direction that produces the smaller number of molecules of gas

• So, the equilibrium shifts to the right

• The reaction mixture becomes paler as more colourless N₂O₄ is produced

The Effect of Concentration on Equilibrium

• Increase in concentration

  • The equilibrium moves to the right to reduce the effect of an increase in the concentration of a reactant

• Decrease in concentration

  • The equilibrium moves to the left to reduce the effect of a decrease in the concentration of a reactant (or an increase in the concentration of a product)

• Example:

  • Dark brown iodine monochloride reacts reversibly with chlorine to form yellow iodine trichloride:

ICl + Cl₂ ⇌ ICl₃

• An increase in the concentration of ICl or Cl₂ causes the equilibrium to shift to the right

  • So, more of the yellow ICl₃ product is formed

• A decrease in the concentration of ICl or Cl₂ causes the equilibrium to shift to the left

  • So, more of the dark brown ICl reactant is formed

The Effect of a Catalyst on Equilibrium

• 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 a catalyst on the time taken for equilibrium to be established