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3.17 - 3.22 Reversible reactions and equilibria

A question of balance

 Many chemical reactions can be reversed.  

When this happens we can describe a forward reaction and a back reaction.

Sometimes the forward and back reaction take place at equal rates. When this happens an Equilibrium is established

 Students should:

  • 3.17 know that some reactions are reversible and this is indicated by the symbol ⇌ in equations

 Students should:

  • 3.18 describe reversible reactions such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride

This classic demonstration is used to demonstrate diffusion rates

in this reaction the two reactants are ammonia and hydrogen chloride

  1. write down the formula of each of these compounds
  2. work out their relative formula masses
  3. predict the name of the white solid formed
  4. predict the equation for this reaction
  5. why does the white solid form to the right hand end of the tube?
  6. what does this tell you about how the rate of diffusion depends upon the relative formula mass of a gas?
  1. Hydrogen chloride (HCl) and  ammonia (NH3 )
  2. HCl is 36.5 and ammonia NH3 is 17 
  3. HCl + NH3  -> NH4Cl
  4. ammonium chloride NH4Cl
  5. the white solid forms towards the right of the tube because the hydrogen chloride diffuses more slowly than the ammonia.
  6. the lower the relative formula mass of a compound the faster it diffuses.

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In this video you can see the same process happening in close up. Hydrogen chloride is combining with ammonia gas to make the white solid called ammonium chloride.

  1. What is the name of the process which causes the hydrogen chloride and ammonia gases to mix in the air?
  2. What feature of the nitrogen atom in an ammonia molecule enables it to bond with a hydrogen chloride molecule?
  3. What is the name of the product formed in this reaction?
  4. What does the product look like?
  5. This reaction is known as a neutralisation reaction. Explain why this is.
  1. diffusion causes the two gases to mix
  2. nitrogen has a non bonding electron pair
  3. ammonium chloride is the white solid produced 
  4. Ammonium chloride is a white powder (smoke) 
  5. Hydrogen chloride (HCl ) is an acid and ammonia (NH3 ) is a base. When an acid reacts with a base the reaction is called neutralisation. A salt is produced.
  1. What is the name of the name of the colourless gas with a pungent odour that is given off at first?
  2. What  is the name of the gas which turns damp red litmus paper blue?
  3. What happens in t he cooler part of the test tube?



  1. hydrogen chloride
  2. ammonia
  3. white solid (ammonium chloride) is deposited

This video shows and describes the principles of dynamic equilibrium. These are:

  • reversible chemical reactions are only in equilibrium if: the temperature is constant and the reaction is taking place in a closed system.
  • a closed system cannot exchange matter with its surrounding but can exchange energy.
  1. How does the number of molecules of hydrogen change during the reaction? 
  2. How does the number of molecules of iodine change during the reaction?
  3. How does the number of molecules of hydrogen iodide change during the reaction
  4. what happens to the numbers eventually . Explain why
  1. What can you  say about the rate of the forward and the rate of the back reaction in a dynamic equilibrium?
  2. What do you observe when looking at a system in a state of dynamic equilibrium
  3. What sort of a system is required for a dynamic equilibrium to become established?
  4. What sort of change is established in a  closed flask containing liquid bromine.
  5. Why is dynamic equilibrium called dynamic?

  1. What effect does increasing the concentration of A have on the equilibrium position ? 
  2. What effect doe decreasing the concentration of C have on the equilibrium position?
  3. What effect does an increase in pressure have on an equilibrium involving gases
  4. How does a temperature increase affect an equilibrium ?

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