Ionic Half Equations

Section 1 of 7Oxidation and reduction in terms of electrons (HT only)

Definitions

• Definitions Oxidation is the loss of electrons and reduction is the gain of electrons.
  • Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain.
• Electrochemistry is about moving electrons, which is why these definitions are used instead of 'gain of oxygen'.

Where each happens

• Where each happens Reduction takes place at the cathode (RED CAT) because positive ions arrive there and gain electrons.
• Oxidation takes place at the anode (AN OX) because negative ions arrive there and lose electrons.

Half equations

• Half equations A half equation shows only one of these changes.
  • Both the atoms and the charges must balance, with electrons written as e^-.

Section 2 of 7Writing a half equation

1

Decide which electrode you are at and what ion arrives there.

2

Write the ion on the left and the product atom or molecule on the right.

3

Balance the atoms, remembering that H, O, Cl, Br and I leave as diatomic molecules.

4

Add electrons to the correct side so that the total charge is the same on both sides: electrons on the left at the cathode, on the right at the anode.

5

Check both atoms and charges once more.

Section 3 of 7Reactions at the cathode (HT only)

Ion	Half equation	Type
Lithium ion	Li+ + e- → Li	Reduction
Copper(II) ion	Cu2+ + 2e- → Cu	Reduction
Aluminium ion	Al3+ + 3e- → Al	Reduction
Hydrogen ion	2H+ + 2e- → H2	Reduction

Section 4 of 7Reactions at the anode (HT only)

Ion	Half equation	Type
Chloride ion	2Cl- → Cl2 + 2e-	Oxidation
Bromide ion	2Br- → Br2 + 2e-	Oxidation
Hydroxide ion	4OH- → O2 + 2H2O + 4e-	Oxidation

Section 5 of 7Whole-electrolysis examples (HT only)

Link the electrolyte to its half equations

Molten lead(II) bromide

• Molten lead(II) bromide Cathode: Pb²⁺ + 2e^- → Pb. Anode: 2Br^- → Br₂ + 2e^-.

Concentrated aqueous sodium chloride

• Concentrated aqueous sodium chloride Cathode: 2H⁺ + 2e^- → H₂. Anode: 2Cl^- → Cl₂ + 2e^-.

Dilute sulfuric acid

• Dilute sulfuric acid Cathode: 2H⁺ + 2e^- → H₂. Anode: 4OH^- → O₂ + 2H₂O + 4e^-.

Balancing electrons

• Balancing electrons The number of electrons gained at the cathode equals the number lost at the anode,
  • so for the same current twice as much hydrogen forms as oxygen.

Section 6 of 7Key terms

Term	Meaning
Oxidation	A reaction in which a species loses electrons.
Reduction	A reaction in which a species gains electrons.
Half equation	An equation showing the loss or gain of electrons by one species at one electrode.
Electron (e-)	A negatively charged particle transferred in oxidation and reduction.
Diatomic	Made of two atoms joined together, for example Cl2.

Section 7 of 7Exam technique

1

Putting electrons on the wrong sideelectrons are gained (left side) at the cathode and lost (right side) at the anode.

2

Writing chlorine as Cl instead of Cl₂, which leaves atoms unbalanced.

3

Balancing the atoms but not the charge; the total charge on each side of a half equation must be equal.

4

Mixing up oxidation and reduction; use OIL RIG and RED CAT to check the electrode.