Reversible Reactions

When products can react back

In many familiar reactions, the reaction continues until a reactant is used up. For example, a fuel burns to form products that do not simply turn back into the original fuel and oxygen under the same circumstances.

Some reactions are different: their products can react together to produce the original reactants. These are reversible reactions. They can proceed in either direction rather than having only one possible chemical change.

Representing both directions

A general reversible reaction is written as

$A + B \rightleftharpoons C + D$

The two opposing half-arrows show that two reactions are possible:

• the forward reaction goes from left to right, so $A$ and $B$ form $C$ and $D$;
• the reverse reaction goes from right to left, so $C$ and $D$ react to reform $A$ and $B$.

The words forward and reverse depend only on how the equation has been written. They do not mean that one direction is more important or must be faster.

A reversible arrow also does not mean that the substances have stopped reacting, that equal amounts must be present, or that the two directions necessarily have equal rates. It tells you only that chemical change can occur in both directions.

Changing the direction by changing the conditions

The direction in which a reversible reaction proceeds can be changed by changing its conditions, such as whether the reaction mixture is heated or cooled. A useful example involves ammonium chloride:

$\mathrm{NH_4Cl(s) \rightleftharpoons NH_3(g) + HCl(g)}$

When solid ammonium chloride is heated, it forms ammonia and hydrogen chloride gases. This is the forward reaction as the equation is written. In a cooler part of the apparatus, the two gases react together and reform solid ammonium chloride: the reverse reaction.

The same substances therefore take different roles depending on the direction. Ammonia and hydrogen chloride are products of the forward reaction but reactants in the reverse reaction. Ammonium chloride is the reactant in the forward reaction and the product in the reverse reaction.

This example makes the central idea visible: changing the conditions can make one direction occur, while a different set of conditions can make the products change back into the original reactant.

Core idea

In a reversible reaction, the products can react to reform the original reactants.

$A + B \rightleftharpoons C + D$

Read the equation both ways

• Forward: $A+B \rightarrow C+D$
• Reverse: $C+D \rightarrow A+B$

Forward means left to right; reverse means right to left.

Changing conditions

Changing the conditions can change the reaction’s direction. For ammonium chloride:

$\mathrm{NH_4Cl(s) \rightleftharpoons NH_3(g) + HCl(g)}$

Heating forms ammonia and hydrogen chloride; cooling allows them to recombine and reform ammonium chloride.

Essential distinction

Reversible means that both directions are possible. It does not, by itself, mean equal rates or equal amounts of reactants and products.