How to write an ionic equation

An ionic equation describes the actual chemical reaction that occurs when we add drops of aqueous sodium chloride to a solution of silver nitrate, or when we place a piece of magnesium ribbon in a solution of zinc sulfate.

In both examples, some of the ions react to form a new product and some of the ions do not take part in the reaction at all.

Let’s look at the precipitation reaction between aqueous silver nitrate and aqueous sodium chloride.

A solution of silver nitrate is actually Ag⁺_(aq) and NO₃^–_(aq) and a solution of sodium chloride is made up of Na⁺_(aq) and Cl^–_(aq) (and water molecules). When we mix these two solutions together silver chloride precipitates out as an insoluble ionic solid, AgCl_(s).

The important thing here is to note that nothing happens to the Na⁺_(aq) and NO₃^–_(aq) ions. These ions were floating about in solution minding their own business before the solutions were mixed, and continue to do so after the solutions are mixed. They are completely unaware of the chemical reaction that has occurred, and hence they are called spectator ions or bystander ions.

When we write an ionic equation we take out the spectator ions so that the final equation simply describes the actual chemical reaction that occurred.

Ag⁺_(aq) + NO₃^–_(aq) + Na⁺_(aq) + Cl^–_(aq) ⇾ AgCl_(s) + NO₃^–_(aq) + Na⁺_(aq)

Ag⁺_(aq) + Cl^–_(aq) ⇾ AgCl_(s)

We can write ionic equations for many other types of chemical reaction. The key is to look at each species and decide whether anything has happened to it over the course of the reaction.

E.g. the acid-base reaction between calcium carbonate and nitric acid

The only spectator ion in this example is the NO₃^– ion, so the ionic equation for the reaction is:

CaCO₃^2-_(s) + 2H⁺_(aq) ⇾ Ca²⁺_(aq) + H₂O_(l) + CO_2(g)

E.g. the displacement reaction between zinc sulfate solution and magnesium

As you can see the reaction, the chemistry, is happening between magnesium atoms and zinc ion, the sulfate ions are the spectator ions.

You should know the charges on your ions … if you are not confident, then make a table and learn them! Here are just a few:

Simple ions  – charges can be worked out from their group in the Periodic table eg. potassium K⁺, barium Ba²⁺, aluminium Al³⁺, nitride N^3-, oxide O^2-, fluoride F^–

Compound ions – sulphate SO₄^2-, nitrate NO₃^–, hydroxide OH^–, phosphate PO₄^3-, ammonium NH₄⁺, carbonate CO₃^2-

Practice questions

Write ionic equations for the following reactions:

1. Solutions of barium chloride and lithium sulphate are mixed to give a precipitate of barium sulphate
2. Solutions of magnesium nitrate and potassium hydroxide are mixed to give a precipitate of magnesium hydroxide
3. Solutions of barium nitrate and sodium chromate (NaCrO₄) are mixed to give a precipitate of barium chromate.
4. Nitric acid is neutralised with sodium hydroxide to give sodium nitrate solution and water.
5. Magnesium ribbon is reacted with hydrochloric  acid to give magnesium chloride solution and hydrogen.
6. Powdered copper oxide is reacted with sulphuric acid to give copper sulphate solution and water.
7. Calcium carbonate chips are reacted with hydrochloric acid to give calcium chloride solution, carbon dioxide and water.

Answers