Oxidation and reduction in terms of electrons (HT only)
Emmanuel Opoku
Teacher
Contents
What Is Redox?
Many reactions involve electron transfer - one species loses electrons while another gains electrons.
These reactions are called redox reactions.
Definition:
Oxidation = loss of electrons
Reduction = gain of electrons
Remember: OIL RIG
Term | Meaning | Electron Change |
|---|---|---|
Oxidation | Is Loss | Loses electrons becomes more positive |
Reduction | Is Gain | Gains electrons becomes more negative |
Mnemonic:
OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons)
What Are Ionic Equations?
An ionic equation shows only the particles that take part in a chemical reaction.
It removes the spectator ions — ions that do not change during the reaction.
Definition:
An ionic equation includes only the ions and atoms that are involved in the chemical change, excluding spectators.
What Are Spectator Ions?
Spectator ions are ions that:
Appear on both sides of a reaction equation
Do not gain or lose electrons
Example:
In the reaction below, the sulfate ions are spectators - they don’t take part in electron transfer.
Spectator ion:
Reactive ions:
No answer provided.
Writing Ionic Equations Step-by-Step
Follow this method to go from a full balanced equation to a simplified ionic equation:
Example 1 (using aqueous ionic compounds)
Step 1: Write the full balanced equation
Step 2: Split any aqueous ionic compounds (or acids) into ions
Step 3: Cancel out spectator ions (ions that appear on both sides)
appears on both sides → cancel it.
Step 4: Write the ionic equation
This shows the actual chemical change - zinc atoms are oxidised, and copper ions are reduced.
Oxidation and Reduction in Ionic Equations
Species | Change | Process | Half Equation |
Loses 2 electrons | Oxidation | ||
Gains 2 electrons | Reduction |
Key idea:
The number of electrons lost in oxidation equals the number gained in reduction.
Zinc is oxidised
Copper is reduced
No answer provided.
Example 2 (using an acid)
Step 1: Split any aqueous ionic compounds (or acids) into ions
Step 2: Cancel out spectator ions (ions that appear on both sides)
Chloride ions appear unchanged on both sides so we can cancel it from both sides.
Step 3: Write the ionic equation
Step 4: Identify oxidation and reduction
Species | Change | Process | Half Equation |
Lose 2 electrons | Oxidation | ||
gains one electron (Each H+ ions) | Reduction |
Summary:
Magnesium loses electrons (oxidised), hydrogen ions gain electrons (reduced).
No answer provided.
Tips for Writing Ionic Equations
Only split aqueous ionic compounds - not solids, gases, or liquids (or covalent substances).
Remember to split acids (they always realase H⁺ when aqueous) - i.e. HCl, HNO₃, H₂SO₄, H₃PO₄
Cancel any ions that appear unchanged on both sides.
Always check charge balance and atom balance after simplifying.
Use state symbols:
(s) = solid
(aq) = aqueous
(l) = liquid
(g) = gas
No answer provided.
Example 3 (insoluble ionic compounds)
Step 1: Split aqueous compounds into ions
Step 2: Remove spectator ions
is unchanged → cancel it.
Step 3: Ionic equation
Here, (solid) is not split because it’s insoluble, seen by the and is covalent in nature and therefore is not split up.
Key idea:
The hydrogen ions react with oxide ions to form water, a covalent molecule.
No answer provided.
Why Use Half Equations?
Half equations help us to:
Show electron transfer clearly.
Identify oxidation and reduction in redox reactions.
Ensure the number of electrons lost equals those gained.
They are essential when writing ionic equations for redox reactions - for example in displacement or metal-acid reactions.
Writing Half Equations Step-by-Step
Step 1 - Identify What’s Oxidised and What’s Reduced
Look at which species loses electrons and which gains them.
Step 2 - Write Each Change Separately
Write what happens to each species as a separate half equation.
Step 3 - Add or Remove Electrons to Balance Charges
Add electrons to balance the charges on both sides of each half equation.
Step 4 - Combine Both Equations
Make sure the number of electrons lost = electrons gained, then combine both equations to show the overall redox reaction.
Example 1: Metal Displacement
Step 1: Identify oxidation and reduction
Zinc loses electrons → oxidised
Copper(II) ions gain electrons → reduced
Step 2: Write half equations
Oxidation:
Reduction:
Step 3: Combine
Balanced redox equation - 2 electrons lost by are gained by
No answer provided.
Example 2: Redox with a Non-Metal
Step 1: Identify oxidation and reduction
Bromide ions lose electrons → oxidised
Chlorine gains electrons → reduced
Step 2: Write half equations
Oxidation:
Reduction:
The 2 electrons lost by bromide are gained by chlorine.
Here’s a Check Your Understanding section built directly from your Oxidation and Reduction (in terms of electrons) and Ionic Equations (HT only) passage.
It follows your MES assessment tone - progressive questioning (Recall → Apply → Challenge) and aligns with higher-tier expectations.
No answer provided.
Check Your Understanding (HT Only)
Recall
a) Define oxidation and reduction in terms of electrons.
Answer
a) Oxidation loss of electrons; Reduction gain of electrons.
b) What does the acronym OIL RIG stand for?
Answer
b) OIL RIG → Oxidation Is Loss, Reduction Is Gain (of electrons).
c) In a redox reaction, what happens to the number of electrons lost and gained?
Answer
c) They are equal: total electrons lost = total electrons gained.
d) What is a spectator ion?
Answer
d) A spectator ion appears unchanged on both sides of the equation (no electron gain/loss, no oxidation-state change).
e) Why do spectator ions not appear in ionic equations?
Answer
e) Because they don’t change, we omit them to show only the reacting particles in the ionic equation.
Apply
f) Write the ionic equation for the reaction between zinc and copper(II) sulfate.
Answer
f) Ionic equation for with
g) For the equation identify:
The substance oxidised
The substance reduced
Answer
g) For
Oxidised: (loses electrons)
Reduced: (gains electrons)
h) Write half equations for the oxidation and reduction processes in (g).
Answer
h) Half-equations:
Oxidation:
Reduction:
i) Write the ionic equation for the reaction between magnesium and hydrochloric acid, and identify the spectator ions removed.
Answer
i) Mg with (and spectators):
Full:
Split & cancel:
Ionic:
Spectator ions removed: (two chloride ions).
j) For the reaction below:
Why is not written as ions?
Why is water written as a molecule?
Answer
j)
Why not as ions? is a solid and insoluble; solids are not split into ions.
Why water as a molecule? is covalent and not ionised in the equation’s context, so it’s written as a molecule.
Challenge
k) Explain how you would determine whether a reaction is a redox reaction or a neutralisation reaction from its ionic equation.
Answer
k) Use the ionic equation:
If it includes electrons or shows oxidation-state changes, it’s redox.
If it simplifies to with no electrons, it’s neutralisation (acid–base), not redox.
l) Compare the electron transfer and nature of products in the two ionic equations below:
1.
2.
Answer
l) Compare:
: Redox. loses (oxidation); gains (reduction).
: Neutralisation, no electron transfer shown; ions combine to form covalently bonded water.
m) In the reaction :
Write both half-equations.
Identify which species is oxidised and which is reduced.
Answer
m)
Oxidation :
Reduction :
Oxidised: (loses ). Reduced: (gains ).
n) Why is water never shown as ions in ionic equations, even though it contains hydrogen and oxygen?
Answer
n) is written as a molecular liquid: it’s covalent and not represented as ions in these equations (water is the product/solvent, not a freely dissociated ionic species in this context).