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OCR GCSE Combined Science A · J250
OCR J250 · C5.2a Check the specification (PDF) (opens in a new tab)
In many reactions, reactants are used up to make products, and the reaction stops when at least one reactant runs out. However, some reactions are : their products can react to form the original reactants again.
The forward reaction forms the products shown on the right of an equation. The reverse reaction, also called the backward reaction, reforms the reactants shown on the left. These are chemical changes in opposite directions, not simply a change in the physical state of a substance.
A reversible reaction is represented by two opposing half-headed arrows, , instead of a single arrow:
Read this as “A and B can react to form C and D, and C and D can react to reform A and B”. The upper arrow points to the right and represents the forward reaction; the lower arrow points to the left and represents the reverse reaction.
Changing the reaction conditions can reverse some chemical reactions. Ammonium chloride provides a clear example of reversal by changing temperature.
When solid ammonium chloride is heated, it decomposes into two gases: ammonia and hydrogen chloride.
When these hot gases cool, ammonia and hydrogen chloride react together to form solid ammonium chloride again:
Heating and cooling therefore produce opposite chemical changes. The solid has not merely evaporated and condensed: it has formed two different substances, which then react to remake the original compound.
Both directions can be combined into one balanced equation:
The state symbols show that ammonium chloride is a solid, , while ammonia and hydrogen chloride are gases, . Each side contains one nitrogen atom, four hydrogen atoms and one chlorine atom, so atoms are conserved in either direction.
Reversibility does not mean that every reaction can be undone just by cooling it. The appropriate change in conditions depends on the reaction. For ammonium chloride, the important change is temperature: heating produces the gases, while cooling allows them to recombine.
The energy changes are also opposite. Decomposing ammonium chloride takes in energy and is endothermic; the reverse reaction releases energy and is exothermic.
Use for a reversible reaction, rather than a single forward arrow.
When explaining how a reaction is reversed, name the change in conditions and the substances formed: cooling ammonia and hydrogen chloride gases forms solid ammonium chloride.
The reverse reaction reforms the original reactants; it does not simply stop the forward reaction.
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Reversible reaction
A chemical reaction in which the products can react to form the original reactants.
Forward reaction
The direction of a reversible reaction that converts the substances written on the left of the equation into those written on the right.
Reverse reaction
The direction of a reversible reaction that converts the substances written on the right of the equation back into those written on the left.
Reaction conditions
The physical circumstances in which a reaction takes place, such as temperature.
Put your knowledge into practice — try past paper questions for Combined Science A
Reversible reaction
A chemical reaction in which the products can react to form the original reactants.
Forward reaction
The direction of a reversible reaction that converts the substances written on the left of the equation into those written on the right.
Reverse reaction
The direction of a reversible reaction that converts the substances written on the right of the equation back into those written on the left.
Reaction conditions
The physical circumstances in which a reaction takes place, such as temperature.