Half Equations and Ionic Equations

Half Equations

Half equations help us understand how atoms, molecules or ions gain or lose electrons during chemical reactions. This is because a half equation shows the movement of electrons during a chemical reaction. Writing a half equation is quite similar to writing a balanced equation.

Let’s look at an example in which oxide ions are converted to oxygen molecules.

StepActionHalf equation
1Write down the reactant and the product.O²⁻ → O2
2Balance the number of atoms on both sides of the equation.2O²⁻ → O2
3Write the total charge next to each substance in the equation.2O²⁻ (-4) → O2 (0)
4Balance the charge by adding electrons to ensure the charge is equal on both sides of the equation.2O²⁻ → O2 + 4e

In this equation, the oxide ions (O²⁻) lose four electrons and are converted into oxygen molecules (O2). The half equation for this reaction is:

2O²⁻O2 + 4e

Ionic Equations

Ionic equations help us understand what happens to ions in a chemical reaction. The formulae of ions can be used to deduce ionic compounds.

The list below shows some common cations (positively charged ions).

  • Sodium, Na⁺
  • Potassium, K⁺
  • Silver(I), Ag⁺
  • Calcium, Ca²⁺
  • Copper(II), Cu²⁺
  • Iron(II), Fe²⁺
  • Magnesium, Mg²⁺
  • Aluminium, Al³⁺
  • Iron(III), Fe³⁺

Let’s look at some common anions (negatively charged ions):

  • Chloride, Cl⁻
  • Bromide, Br⁻
  • Iodide, I⁻
  • Oxide, O²

We can use ionic equations to show what happens to ions during a neutralisation reaction. For example, let’s look at the reaction between hydrochloric acid and sodium hydroxide:

HCl (aq) + NaOH (aq)  → NaCl (aq) + H2O (l)

The ionic equation for this reaction is:

H (aq) + OH (aq)H2O (l)

In this ionic equation, the hydrogen ion (H⁺) and the hydroxide ion (OH⁻) react to form water (H2O). The sodium and chloride ions are present on both sides of the equation and are not included in the ionic equation because they remain unchanged. These ions are known as spectator ions, as they do not take part in the reaction.

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