Redox

What is oxidation?

Loss of electrons.

What is reduction?

Gain of electrons.

What mnemonic can help remember oxidation and reduction?

OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons).

What is an oxidising agent?

A substance that accepts electrons and causes another substance to be oxidised. The oxidising agent is itself reduced.

What is a reducing agent?

A substance that donates electrons and causes another substance to be reduced. The reducing agent is itself oxidised.

What happens to oxidation and reduction in a redox reaction?

They occur simultaneously because electrons lost by one species are gained by another.

What is a redox reaction?

A reaction involving oxidation and reduction occurring together, with transfer of electrons between species.

What is a half-equation?

An equation representing either the oxidation or reduction part of a redox reaction, including the electrons transferred.

What happens when magnesium reacts with Cu²⁺?

Magnesium loses electrons and is oxidised: Mg → Mg²⁺ + 2e⁻. Cu²⁺ gains electrons and is reduced: Cu²⁺ + 2e⁻ → Cu.

Which species is the reducing agent in Mg + Cu²⁺ → Mg²⁺ + Cu?

Mg, because it donates electrons and is oxidised.

Which species is the oxidising agent in Mg + Cu²⁺ → Mg²⁺ + Cu?

Cu²⁺, because it accepts electrons and is reduced.

What is an oxidation state?

A number assigned to an element that represents the apparent charge of the atom based on electron transfer in a compound or ion.

What is the oxidation state of an uncombined element?

0.

Give examples of elements with oxidation state 0.

Na(s), Mg(s), H₂(g), O₂(g), Cl₂(g).

What is the oxidation state of a simple monatomic ion?

Equal to its ionic charge.

What is the oxidation state of Na⁺?

+1.

What is the oxidation state of Mg²⁺?

+2.

What is the oxidation state of Cl⁻?

−1.

What is the sum of oxidation states in a neutral compound?

0.

What is the sum of oxidation states in a polyatomic ion?

Equal to the overall charge of the ion.

What is the usual oxidation state of a Group 1 metal in compounds?

+1.

What is the usual oxidation state of a Group 2 metal in compounds?

+2.

What is the usual oxidation state of fluorine?

−1.

What is the usual oxidation state of oxygen?

−2.

What is the oxidation state of oxygen in a peroxide such as H₂O₂?

−1.

What is the oxidation state of oxygen in OF₂?

+2 because fluorine is more electronegative and has an oxidation state of −1.

What is the usual oxidation state of hydrogen?

+1.

What is the oxidation state of hydrogen in a metal hydride such as NaH?

−1.

What is the oxidation state of chlorine in most simple compounds?

−1.

Can chlorine have positive oxidation states?

Yes, particularly in compounds with oxygen or fluorine.

How do you work out an unknown oxidation state in a neutral compound?

Use the known oxidation states and make the sum equal to zero.

How do you work out an unknown oxidation state in an ion?

Use the known oxidation states and make their sum equal to the overall charge of the ion.

What is the oxidation state of sulfur in SO₄²⁻?

Oxygen is −2, so four oxygens give −8. The overall charge is −2, so sulfur must be +6: +6 − 8 = −2.

What is the oxidation state of nitrogen in NO₃⁻?

Oxygen contributes −6 overall. The ion has charge −1, so nitrogen must be +5: +5 − 6 = −1.

What is the oxidation state of sulfur in SO₂?

Oxygen is −2 each, giving −4 overall. Since SO₂ is neutral, sulfur is +4.

What is the oxidation state of sulfur in SO₃?

Oxygen contributes −6, so sulfur must be +6.

What is the oxidation state of iron in Fe₂O₃?

Oxygen contributes −6 overall. Therefore the two Fe atoms contribute +6, so each Fe is +3.

What is the oxidation state of copper in CuSO₄?

Sulfate is SO₄²⁻, so Cu must be +2 to give an overall charge of zero.

What is the oxidation state of oxygen in H₂O₂?

Hydrogen is +1 each, giving +2 overall. Therefore the two oxygen atoms must contribute −2 overall, so each oxygen is −1.

How can you identify which species has been oxidised using oxidation states?

Its oxidation state becomes more positive/increases.

How can you identify which species has been reduced using oxidation states?

Its oxidation state becomes more negative/decreases.

What happens to the oxidation state when a species loses electrons?

It increases.

What happens to the oxidation state when a species gains electrons?

It decreases.

If an element changes from +2 to +3, has it been oxidised or reduced?

Oxidised.

If an element changes from +5 to +3, has it been oxidised or reduced?

Reduced.

Why does an increase in oxidation state indicate oxidation?

The species has effectively lost electrons.

Why does a decrease in oxidation state indicate reduction?

The species has effectively gained electrons.

What is the general form of an oxidation half-equation?

Reactant → oxidised product + electrons.

What is the general form of a reduction half-equation?

Reactant + electrons → reduced product.

Write the oxidation half-equation for Mg becoming Mg²⁺.

Mg → Mg²⁺ + 2e⁻.

Write the reduction half-equation for Cu²⁺ becoming Cu.

Cu²⁺ + 2e⁻ → Cu.

Write the oxidation half-equation for Na becoming Na⁺.

Na → Na⁺ + e⁻.

Write the reduction half-equation for Cl₂ becoming Cl⁻.

Cl₂ + 2e⁻ → 2Cl⁻.

Why must electrons be included in half-equations?

They show the transfer of electrons during oxidation or reduction and allow the two half-equations to be combined correctly.

What are the steps for combining two half-equations?

Write the oxidation and reduction half-equations.
Balance atoms other than H and O where necessary.
Balance the electrons so the number lost equals the number gained.
Add the half-equations together.
Cancel electrons and any other identical species appearing on both sides.
Check that atoms and overall charge are balanced.

Why must the number of electrons lost equal the number gained?

Electrons cannot be created or destroyed; they are transferred from the reducing agent to the oxidising agent.
Example

Combine Mg → Mg²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.

Mg + Cu²⁺ → Mg²⁺ + Cu.

Why can the electrons be cancelled in the overall equation Mg + Cu²⁺ → Mg²⁺ + Cu?

Two electrons are lost by Mg and exactly two are gained by Cu²⁺, so the electrons cancel when the half-equations are added.

What should you check after combining half-equations?

Both atoms and overall charge should be balanced.

How can you identify the oxidising agent in a redox reaction?

It is the species that gains electrons and is reduced.

How can you identify the reducing agent?

It is the species that loses electrons and is oxidised.

In a redox reaction, which direction do electrons transfer?

From the reducing agent to the oxidising agent.

If substance X is oxidised, what must it be?

The reducing agent.

If substance Y is reduced, what must it be?

The oxidising agent.

What must I be able to do for AQA Redox?

Define oxidation as electron loss.
Define reduction as electron gain.
Identify oxidising and reducing agents.
Apply oxidation-state rules.
Calculate unknown oxidation states in compounds and ions.
Identify which element has been oxidised/reduced from oxidation-state changes.
Write oxidation half-equations.
Write reduction half-equations.
Combine half-equations into an overall redox equation.
Check that atoms and charge balance