Changing the direction of a reversible reaction

A reversible reaction can proceed in either direction: reactants form products in the forward reaction, and the products can react to form the reactants again in the reverse reaction. Changing the conditions can favour one direction over the other.

Ammonium chloride provides an example. Its reversible reaction can be represented as:

The double arrow shows that the reaction can go in both directions. The symbols $$\ce{(s)}$$ and $$\ce{(g)}$$ indicate a solid and a gas, respectively.

When ammonium chloride is heated, it breaks down into ammonia and hydrogen chloride gases. Heating favours this forward direction, shown from left to right in the equation.

When the gases are cooled, ammonia and hydrogen chloride react together to form solid ammonium chloride again. Cooling favours this reverse direction, shown from right to left. Thus, changing the temperature can change the direction favoured in this reversible reaction.

Reversible reactions

A chemical reaction does not always proceed in only one direction. In a reversible reaction, reactants form products, but the products can also react to form the original reactants again.

A reversible reaction is represented by a double arrow:

In this general example, the forward reaction forms $$\ce{C}$$ and $$\ce{D}$$ from $$\ce{A}$$ and $$\ce{B}$$. The reverse reaction forms $$\ce{A}$$ and $$\ce{B}$$ from $$\ce{C}$$ and $$\ce{D}$$. The two directions of the arrow show that the reaction can happen either way.