An ionic equation describes the actual chemical reaction that occurs when two solutions are mixed and a precipitate forms (insoluble salt), ignoring any spectator ions – there is a nice video of the formation of silver chloride here.
So, 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 off chemical reaction, such as the reaction between calcium carbonate and nitric acid. The key is to look at each species and decide whether anything has happened to it over the course of the reaction …
The only spectator ion in this example is the NO3– ion, so the ionic equation for the reaction is:
CaCO32-(s) + 2H+(aq) ⇾ Ca2+(aq) + H2O(l) + CO2(g)
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 N3-, oxide O2-, fluoride F–
Compound ions – sulphate SO42-, nitrate NO3–, hydroxide OH–, phosphate PO43-, ammonium NH4+, carbonate CO32-
Practice questions
Write ionic equations for the following reactions:
- Solutions of barium chloride and lithium sulphate are mixed to give a precipitate of barium sulphate
- Solutions of magnesium nitrate and potassium hydroxide are mixed to give a precipitate of magnesium hydroxide
- Solutions of barium nitrate and sodium chromate (NaCrO4) are mixed to give a precipitate of barium chromate.
- Nitric acid is neutralised with sodium hydroxide to give sodium nitrate solution and water.
- Magnesium ribbon is reacted with hydrochloric acid to give magnesium chloride solution and hydrogen.
- Powdered copper oxide is reacted with sulphuric acid to give copper sulphate solution and water.
- Calcium carbonate chips are reacted with hydrochloric acid to give calcium chloride solution, carbon dioxide and water.
Answers
