Single displacement reactions: Kicking out another element

In single displacement reactions, a more active element displaces (kicks out) another less active element from a compound. For example, if you put a piece of zinc metal into a copper (II) sulfate solution, the zinc displaces the copper, as this next equation shows

Zn_(s) + CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)

The notation (aq) indicates that the compound is dissolved in water — in an aqueous solution.  Because zinc replaces copper in this case, it’s said to be more active. If you instead place a piece of copper in a zinc sulfate solution, nothing will happen.

Using the activity series

Table below shows the activity series of some common metals. Notice that because zinc is more active in the table, it will replace copper, just as the earlier equation shows.

Writing ionic and net-ionic equations

Take a look at the reaction between zinc metal and a copper (II) sulfate solution. I’ve written this reaction as a molecular equation, showing all species in the neutral form.

Zn_(s) + CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)

However, these reactions normally occur in an aqueous (water) solution. When you dissolve the ionically bonded CuSO₄ in water, it breaks apart into ions (atoms or groups of atoms that have an electrical charge due to the loss or gain of electrons). The copper ion has a 2+ charge because it lost two electrons. It’s a cation, a positively charged ion. The sulfate ion has a 2– charge because it has two extra electrons. It’s an anion, a negatively charged ion.

Here’s an equation that shows the ions separately. Equations in this form are called ionic equations because they show the reaction and production of ions. Notice that the sulfate ion, SO₄^2–, doesn’t change in the reaction:

Zn_(s) + Cu²⁺ + SO₄^2– → Zn²⁺ + SO₄^2– + Cu_(s)

Ions that don’t change during the reaction and are found on both sides of the equation in an identical form are called spectator ions. Chemists often omit the spectator ions and write the equation showing only those chemical substances that are changed during the reaction. This is called the net-ionic equation:

Zn_(s) + Cu²⁺ → Zn²⁺ + Cu_(s)