Equilibrium (Cambridge O Level Chemistry)

• 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 e.g. 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:

3H₂ (g) + N₂ (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

• 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

Table showing the Effects of Temperature on Equilibrium

• Example: Iodine monochloride reacts reversibly with chlorine to form iodine trichloride

  ICl             +              Cl₂             ⇌             ICl₃ 
  dark brown                                                    yellow
   
• When the equilibrium mixture is heated, it becomes dark brown in colour.  You can use this observation to deduce whether the backward reaction is exothermic or endothermic
• Equilibrium has shifted to the left as the colour dark brown means that more ICI is produced
• Increasing temperature moves the equilibrium in the endothermic direction
• So the backward reaction is endothermic

The Effect of Pressure on Equilibrium 

Table showing the Effects of Pressure on Equilibrium

Example: Nitrogen dioxide can form dinitrogen tetroxide, a colourless gas

• 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

• An increase in pressure will cause equilibrium to shift in the direction that produces the smaller number of molecules of gas
• So equilibrium shifts to the right
• The reaction mixture becomes paler as more colourless N₂O₄ is produced

The Effect of Concentration on Equilibrium

 Table showing the Effects of Concentration on Equilibrium

 Example: Iodine monochloride reacts reversibly with chlorine to form iodine trichloride

• Predict the effect of an increase in concentration on the position of equilibrium:
  • An increase in the concentration of ICl or Cl₂ causes the equilibrium to shift to the right so more of the yellow 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 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