Reversible and Irreversible Reactions
Melting an ice cube is reversible but it is not a chemical reaction.

Reversible and Irreversible Reactions

There are many types of different chemical reactions: neutralisation, decomposition, combustion and oxidation are four of the common ones. This GCSE Chemistry quiz is all about reversible and irreversible chemical reactions. When a reaction is described as being reversible, it can go both forwards and backwards at the same time. These type of reactions occur when the products are able to react with each other or if the product is decomposed by the conditions used for the reaction. A good example is the decomposition of ammonium chloride. The products are ammonia and hydrogen chloride. Ammonia is a base, hydrogen chloride is an acid and they react together to form ammonium chloride.

NH4Cl ⇌ NH3 + HCl

When you see a reaction written down using a double arrow between the reactants and products, it means that it is reversible. A single arrow indicates an irreversible reaction.

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When talking about reversible reactions, you will hear the phrase dynamic equilibrium used. This describes the situation in a closed system when the forward and reverse reactions are taking place at exactly the same rate - the concentration of products and reactants remain the same. In a closed system, no products or reactants can escape or be lost from the reaction vessel - it is fully sealed. At equilibrium, the reactions are both still happening but everything is balanced. This balance can be changed either by altering the conditions or by adding/removing a reactant or product.

In any reversible reaction it is the forward portion that we want, so we must change the conditions to be as favourable as possible. You studied this in detail when you were learning about the Haber process. If the forward reaction is exothermic for example, the reverse reaction will be endothermic by exactly the same amount. Increasing the temperature means that there is more heat available which helps the reverse reaction (remember that endothermic reactions absorb energy (usually heat) from the surroundings, so if there is more heat energy available it can be more readily absorbed). We say that the equilibrium shifts to the left. In other words, the backwards reaction is favoured and less product is made. Cool down the system and the opposite will be true as it will be easier for the forward exothermic reaction to lose energy to the surroundings.

Try this quiz and see how well you understand reversible and irreversible chemical reactions.

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  1. Pick the correct combination for how we represent reversible and irreversible reactions.
    The top or the bottom arrow of the double ended arrow can be bigger or smaller to indicate in which direction the reaction is dominant
  2. Which of the following reactions is reversible?
    The Haber process produces ammonia from nitrogen and hydrogen. Ammonia decomposes to form nitrogen and hydrogen
  3. Which of the following reactions is irreversible?
    When sodium reacts with water, sodium hydroxide is formed and you cannot directly reverse this to get water and sodium metal
  4. If a reversible reaction is exothermic in the forward direction, it will be...
    The reverse reaction is a 'mirror image' of the forward reaction
  5. A reversible reaction is...
    The reactants react to produce the products, which can then break down or react together to form the reactants
  6. Why is a reaction described as irreversible?
    It may be possible to get back to the original reactants by carrying out a series of other chemical reactions but the key feature of a reversible reaction is that the products are converted directly back into the reactants
  7. Which of the following is reversible?
    The water can be frozen again to form ice. This is a physical change, all the others are irreversible chemical changes
  8. Which of the following equations is CORRECT?
    This is the word equation for the thermal decomposition of limestone (calcium carbonate).
  9. Which of the following equations is NOT correct?
    You should recognise this from the introduction to this quiz, it needs a reversible reaction arrow between the reactants and products
  10. When a reversible reaction reaches equilibrium...
    The forward and reverse reactions are happening at the same time and at the same rate, so the concentrations of reactants and products do not change

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