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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.

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.

Pick the correct combination for how we represent reversible and irreversible reactions.
reversible ⇌
irreversible →
reversible ←
irreversible →
reversible ⇌
irreversible ←
irreversible ⇌
reversible →
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
Which of the following reactions is reversible?
The Haber process
Boiling an egg
The Haber process produces ammonia from nitrogen and hydrogen. Ammonia decomposes to form nitrogen and hydrogen
Which of the following reactions is irreversible?
Reaction between anhydrous copper sulfate and water
Decomposition of ammonium chloride
Reaction between sodium and water
Reaction between hydrogen and nitrogen
When sodium reacts with water, sodium hydroxide is formed and you cannot directly reverse this to get water and sodium metal
If a reversible reaction is exothermic in the forward direction, it will be...
exothermic in the reverse direction
endothermic in the forward direction
endothermic in the reverse direction
not involve energy in the reverse direction
The reverse reaction is a 'mirror image' of the forward reaction
A reversible reaction is...
a reaction that can proceed in both directions
a reaction that can only go in one direction
a reaction that can only go backwards
a reaction that does not happen
The reactants react to produce the products, which can then break down or react together to form the reactants
Why is a reaction described as irreversible?
The products cannot be turned directly back into the reactants
You need to apply heat to make the reaction work
The products can be turned back into the reactants
The reactants cannot be turned into the products
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
Which of the following is reversible?
Frying a potato
Melting an ice cube
Making toast
Baking a cake
The water can be frozen again to form ice. This is a physical change, all the others are irreversible chemical changes
Which of the following equations is CORRECT?
Sugar ← water + carbon
Anhydrous copper sulfate → copper sulfate + water
Sodium hydrogencarbonate ⇌ sodium carbonate + carbon dioxide + water
Calcium carbonate → calcium oxide + carbon dioxide
This is the word equation for the thermal decomposition of limestone (calcium carbonate).
Which of the following equations is NOT correct?
CuCO3 → CuO + CO2
2Mg + O2 → 2MgO
NH4Cl → NH3 + HCl
CH4 + 2O2 → CO2 + 2H2O
You should recognise this from the introduction to this quiz, it needs a reversible reaction arrow between the reactants and products
When a reversible reaction reaches equilibrium...
the reverse reaction is happening more quickly than the forward reaction
the forward reaction is happening more quickly than the reverse reaction
the forward reaction is happening at the same rate as the reverse reaction
the reverse reaction is happening slower than the forward reaction
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
You can find more about this topic by visiting BBC Bitesize - Reversible reactions

Author:  Kate Gardiner

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