Redox equations for SQA Higher Chemistry

Redox equations

This page covers the following topics:

1. Redox half-equations
2. Redox equations
3. Oxidising and reducing agents

Reduction is the gain of electrons or the decrease of an oxidation number. Oxidation is the loss of electrons or the increase of the oxidation number. A particle gains electrons when it is reduced and a particle loses electrons when it is oxidised. Redox half-equations provide information about the change in particles and the loss or gain of electrons. Many half-equations can be balanced by adding electrons, water molecules and hydrogen ions.

Fe³⁺ + e⁻ → Fe²⁺ (reduction)
Mg → Mg²⁺ + 2e⁻ (oxidation)

Redox half-equations

Redox half-equations can be combined to construct full ionic equations by matching the number of electrons and removing duplicate particles from both sides of an equation. If the electrons do not cancel out after adding half-equations, they can be multiplied by suitable integers beforehand. Both sides of a half-equation can also be switched around to ensure that electrons appear on both sides of an ionic equation. In a redox equation a reducing agent is an electron donor, while an oxidising agent is an electron acceptor.

Redox equations

Strong reducing agents include elements with low electronegativity, especially in group 1, that lose electrons to form ions as well as compounds like carbon monoxide. Strong oxidising agents include elements with high electronegativity, especially in group 7, that gain electrons to form ions; elements like hydrogen peroxide; ions like acidified permanganate, dichromate ions. Oxidising agents are widely used for washing clothes, removing colours from hair and disinfecting surfaces.

Oxidising and reducing agents

1

Identify all particles that are reduced in the half-equations provided.

2Cl⁻ → Cl₂ + 2e⁻
Mn⁵⁺ + 3e⁻ → Mn²⁺

A particle gains electrons when it is reduced. In these half-equations Mn⁵⁺ gains electrons.

Mn⁵⁺

Identify all particles that are reduced in the half-equations provided.

2Cl⁻ → Cl₂ + 2e⁻
Mn⁵⁺ + 3e⁻ → Mn²⁺

2

Which of the following half-equations could be combined together to create an ionic equation that corresponds to the reaction presented in the image?

A) Na → Na⁺ + e⁻
B) Cl₂ + 2e⁻ → 2Cl⁻
C) Br₂ + 2e⁻ → 2Br⁻

In the reaction bromine and chlorine change their oxidation numbers. Redox half-equations can be combined to construct full ionic equations by matching the number of electrons and removing duplicate particles from both sides of an equation. Both sides of a half-equation can also be switched around to ensure that electrons appear on both sides of an ionic equation.

B, C

Which of the following half-equations could be combined together to create an ionic equation that corresponds to the reaction presented in the image?

A) Na → Na⁺ + e⁻
B) Cl₂ + 2e⁻ → 2Cl⁻
C) Br₂ + 2e⁻ → 2Br⁻

3

Describe how does the oxidising ability of elements change going towards the right of the periodic table.

Strong reducing agents include elements with low electronegativity, strong oxidising agents include elements with high electronegativity. Since the electronegativity generally increases going towards the right of the periodic table, the oxidising ability of elements increases.

Going towards the right of the periodic table, the oxidising ability of elements increases.

Describe how does the oxidising ability of elements change going towards the right of the periodic table.

4

Which of the following elements are strong reducing agents?

sulphur, sodium, lithium, bromine, iodine

Strong reducing agents include elements with low electronegativity, especially in group 1, that lose electrons to form ions.

sodium, lithium

Which of the following elements are strong reducing agents?

sulphur, sodium, lithium, bromine, iodine

5

Balance the half-equation provided.

MnO₄⁻ + H⁺ + e⁻ → Mn²⁺ + H₂O

MnO₄⁻ + H⁺ + e⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ + e⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Balance the half-equation provided.

MnO₄⁻ + H⁺ + e⁻ → Mn²⁺ + H₂O

End of page