Factors Affecting The Position Of Equilibrium (HT Only) (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride:

ICl (l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown            yellow

Predict the effect of a change in the concentration of ICl or Cl₂ on the position of the equilibrium.

Answer:

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

  • So, more of the yellow solid, ICl₃, is formed, which means that the reaction would be increasingly yellow!

  • This is because when the concentration of a reactant increases, the equilibrium moves to oppose the change and create more product from the excess reactant

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

  • So, less of the yellow solid, ICl₃, is formed, which means that the reaction would be increasingly brown!

  • This is because when the concentration of a reactant decreases, the equilibrium moves to oppose the change and create more reactant from the now in excess product

Changing the concentration of either the reactants or the products pushes the system away from equilibrium. The system responds to bring itself back to the equilibrium state by restoring the position of equilibrium. This means opposing the change.

You can think of this like a grumpy toddler trying to do exactly the opposite of what is done to them!

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

ICl (l) + Cl₂ (g) ⇌ ICl₃ (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

Remember all reversible reactions are exothermic in one direction and endothermic in the other.

Mark schemes don't usually give you a mark for stating that the system opposes the change but for the explanation of which reaction is favoured and why.

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

2NO₂ (g)   ⇌   N₂O₄ (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

Changes in pressure affect those systems that contain a gas only.

Use the balanced equation to determine which side has the most molecules of gas.