Half Equations and Ionic Equations

Section 1 of 8Half equations

What they show

• What they show A half equation shows what happens to one species, with electrons written as e⁻.
• Two half equations can be added together, with the electrons cancelling, to give the full ionic equation for a redox reaction.

Losing electrons

• Losing electrons Metal atoms form positive ions: Na → Na⁺ + e⁻. Electrons appear on the product side.

Gaining electrons

• Gaining electrons Non-metal atoms form negative ions: Cl₂ + 2e⁻ → 2Cl⁻.
  • Electrons appear on the reactant side.

Balanced twice

• Balanced twice The number of atoms and the total charge must be equal on both sides.

Section 2 of 8Writing a half equation

1

Write the species before and after the change, such as Mg and Mg²⁺.

2

Balance the atoms, adding a coefficient where needed, as in Cl₂ → 2Cl⁻.

3

Work out the total charge on each side.

4

Add electrons to the side with the more positive charge so that the charges match.

5

Check that atoms and charge balance: Mg → Mg²⁺ + 2e⁻ is neutral on the left and 2+ plus 2− on the right.

Section 3 of 8Writing an ionic equation

1

Write the balanced symbol equation with state symbols.

2

Split every aqueous ionic compound into its ions. Leave solids, liquids and gases whole.

3

Cross out the ions that appear unchanged on both sides, the spectator ions.

4

Write what is left as the ionic equation.

5

Check that atoms and charges balance.

Section 4 of 8Worked example: silver nitrate and sodium chloride

Stage	Equation
Full equation	AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Split into ions	Ag+ + NO3− + Na+ + Cl− → AgCl(s) + Na+ + NO3−
Remove spectators	Na+ and NO3−
Ionic equation	Ag+(aq) + Cl−(aq) → AgCl(s)

Section 5 of 8More ionic equations

Reaction	Ionic equation	Observation
Neutralisation	H+(aq) + OH−(aq) → H2O(l)	Solution becomes neutral
Barium sulfate precipitate	Ba2+(aq) + SO42−(aq) → BaSO4(s)	White precipitate forms
Copper hydroxide precipitate	Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)	Blue precipitate forms

Section 6 of 8Electrolysis half equations

Electrodes

Cathode

• Cathode Reduction: Cu²⁺ + 2e⁻ → Cu, or 2H⁺ + 2e⁻ → H₂.
• Positive ions are attracted to the negative cathode, where they gain electrons.

Anode

• Anode Oxidation: 2Cl⁻ → Cl₂ + 2e⁻, or 4OH⁻ → O₂ + 2H₂O + 4e⁻.
• Negative ions are attracted to the positive anode, where they lose electrons.

Electron balance

• Electron balance The electrons gained at the cathode equal those lost at the anode, which is why the overall equation has no electrons in it.

Section 7 of 8Key terms

Term	Meaning
Half equation	An equation showing the loss or gain of electrons by one species.
Ionic equation	An equation showing only the ions or particles that take part in a reaction.
Spectator ion	An ion that is present but unchanged during the reaction.
Precipitate	An insoluble solid formed in a reaction between solutions.
Electron	A negatively charged particle, written e−.
Aqueous	Dissolved in water, shown by the state symbol (aq).

Section 8 of 8Exam technique

1

Splitting a solid or gas into ions.

2

Not balancing the charge, only the atoms.

3

Putting electrons on the wrong sideelectrons are lost on the product side for positive ions.

4

Leaving spectator ions in the final ionic equation.

5

Forgetting that diatomic elements like Cl₂ take 2 electrons.