Writing Ionic Equations
Ionic equations (also called half equations) show how atoms and ions gain and lose electrons.
Consider the combustion of magnesium oxide. 2Mg + O2 → 2MgO
Let us consider what is happening to the magnesium separately. Magnesium oxide is an ionic compound
consisting of magnesium ions (Mg2+) and oxide ions (O2-). Therefore, the reaction for magnesium involves
turning magnesium metal into magnesium ions. Mg → Mg2+
Now let us consider what is happening to the oxygen separately. The reaction for oxygen involves turning
one lot of elemental oxygen (O2) into two lots of oxide ions (2O2-). Therefore, we can write: O2 → 2O2However, the above ionic equations are not balanced in terms of charge. Forming ions involves atoms
gaining or losing electrons.
Metals like to lose electrons and form positive ions. Mg → Mg2+ + 2eNon-metals like to gain electrons and form negative ions. O2 + 4e- → 2O2-
To write ionic (a.k.a. half) equations:
① Write equations for what you start and finish with.
One lot of magnesium metal forms one lot of magnesium ions: Mg → Mg 2+
One lot of elemental oxygen forms two lots of oxide ions: O2 → 2O2-
② Add electrons to the left or right of the equation to balance the charge.
(You can only add electrons to the left or right. You can’t subtract electrons or add or subtract protons.)
Try Mg + 2e- → Mg2+(wrong -2 charge → +2 charge)
Try O2 + 4e- → 2O2- (right -4 charge → -4 charge)
Try Mg → Mg2+ + 2e- (right 0 charge → 0 charge)
Try O2 → 2O2- + 4e- (wrong 0 charge → -8 charge)
③ Multiply the equations by a multiplier if necessary to make the number of electrons in each equation
the same then add them to check that they add up to give the overall reaction.
2 x (Mg → Mg2+ + 2e-)
+
2O2 + 4e → 2O
------------------------------2Mg + O2 + 4e- → 2Mg2+ + 2O2- + 4e- Cancelling electrons left and right gives: 2Mg + O2 → 2Mg2+ + 2O22Mg + O2 → 2Mg2+ + 2O2- is the same as 2Mg + O2 → 2MgO, which is the equation for the overall reaction.
Net Ionic Equations
A net ionic equation is not the same as an ionic equation (half equation), so the term net ionic equation is
somewhat confusing. Net ionic equations are equations that show soluble ions reacting and omit the ions
which are not involved in the reaction (spectator ions).
For example, consider the reaction between calcium carbonate and hydrochloric acid:
① Write a balanced chemical equation.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
② Write soluble ionic substances as separate ions. Write insoluble ionic and non-ionic substances using
their chemical formula.
CaCO3(s) + 2H+(aq) + 2Cl-(aq) → Ca2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g)
③ Cancel out any soluble ions that appear on both the left and right of the equation (spectator ions).
CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
Ionic Equations (Half Equations) Questions
1. Write an ionic equation for the formation of sodium ions (Na +) from sodium metal (Na).
2. Write an ionic equation for the formation of calcium ions (Ca2+) from calcium metal (Ca).
3. Write an ionic equation for the formation of hydrogen gas (H2) from hydrogen ions (H+).
4. Write an ionic equation for formation of copper (II) ions (Cu2+) from copper (I) ions (Cu+).
5. Write an ionic equation for the formation of iron metal (Fe) from iron (III) ions (Fe 3+).
6. Write separate ionic equations to describe the combustion of lithium metal (Li).
7. Write separate ionic equations to describe the combustion of aluminium metal (Al).
8. Write separate ionic equations to describe the reaction between aluminium metal (Al) and chlorine
gas (Cl2).
9. Write separate ionic equations to describe the reaction between zinc metal (Zn) and hydrochloric
acid (HCl).
10. Write separate ionic equations to describe the reaction between magnesium metal (Mg) and nitric
acid (HNO3).