I Can-do Chemistry

With

Miss Williams

Today’s Topic:

Redox Reactions

Objectives:

• Investigate the action of common oxidising and reducing substances in everyday activities;
• Define oxidation and reduction;
• Deduce oxidation number from formulae;
• Identify oxidation and reduction reactions including reactions at electrodes;
• Distinguish between oxidising and reducing agents.

Oxidation-Reduction Reactions in Terms of Electrons

• In oxidation-reduction reactions, electrons move between atoms.
• One element loses electrons and another gains them.
• A substance will not lose electrons unless there is another substance available to gain the electrons.
• Oxidation-reductions reactions are known as redox reactions.

• All ionic bonding is a transfer of electrons from metals to non-metals.
• Reactions forming ionic bonds are redox reactions
• Oxidation is the loss of electrons by an element in its free state, or an element in a compound.
• Reduction is the gain of electrons by an element in its free state, or an element in a compound.

Examples of Redox Reactions

• Burning of magnesium in oxygen

2 Mg₍ₛ₎ + O_{2 (g)} → 2 MgO ₍ₛ₎

• Mg is oxidised and O₂ is reduced
• The half equations would be:

oxidation half reaction: 2 Mg₍ₛ₎ → 2 Mg²⁺₍ₛ₎ + 4 e⁻

reduction half reaction: O_{2 (g)} + 4 e⁻ → 2 O²⁻₍ₛ₎

• Bubbling chlorine gas through potassium bromide solution

Cl_{2 (g)} + 2KBr_(aq) → Br₂₍ₗ₎ + 2 KCl_(aq)

• Br⁻ is oxidised and Cl₂ is reduced
• The half equations would be:

oxidation half reaction: 2 Br⁻_(aq) → Br₂₍ₗ₎ + 2 e⁻

reduction half reaction: Cl_{2 (g)} + 2 e⁻ → 2 Cl⁻_(aq)

Oxidation Number or Oxidation State

• This indicates the number of electrons lost, gained or shared as a result of chemical bonding.
• Oxidation number can be positive, negative or zero.

Rules for Finding Oxidation Numbers

Rule 1: The oxidation number of each atom of an element in its free, uncombined state is zero.

In Mg, oxidation number of the Mg atom = 0

In H₂, oxidation number of each H atom = 0

Rule 2: The oxidation number of each monoatomic ion in an ionic compound is the same as the charge on the ion.

Rule 3: The oxidation number of hydrogen in a compound or polyatomic ion is always +1, except in metal hydrides where it is -1.

Rule 4: The oxidation number of oxygen in a compound in an ionic compound is the same as the charge on the ion

Rule 5: With the exception of hydrogen and oxygen, the oxidation numbers of elements in covalent compounds and polyatomic ions may vary. The oxidation number may appear in the name of the compound or ion:

In sulphur (VI) oxide (SO₃), oxidation number of sulphur = +6

In the sulphate (IV) ion (SO₃²⁻), oxidation number of sulphur =

+4

​

Rule 6: The sum of the oxidation numbers of all the atoms or ions in a compound is zero.

In MgCl₂, the sum of the oxidation numbers of all the ions is

zero:

(oxidation number of Mg) + 2(oxidation number of Cl) = 0

(+2) + 2(-1) = 0

(+2) + (-2) = 0

Rule 7: The sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the charge on the ion.

In the OH⁻ ion, the sum of the oxidation numbers of the two

elements is -1.

(oxidation number of O) + (oxidation number of H) = -1

(-2) + 2(+1) = -1

​

Oxidation State and Naming Compounds

• Some elements can exist with different oxidation states.
• When naming a compound that contain these elements, the oxidation state should be included in their name.

Oxidation-Reduction Reactions in Terms of Oxidation Number

• Oxidation is the increase in oxidation number of an element in its free state, or an element in a compound.
• Reduction is the decrease in oxidation number of an element in its free state, or an element in a compound.

​

• Oxidation numbers can be used to identify redox reactions by:
• Writing a balanced chemical equation for the reaction
• Writing the oxidation number of each element below it in brackets
• Decide which element shows an increase in oxidation number
• Decide which element shows a decrease in oxidation number.

​

Oxidising and Reducing Agents

• For redox reactions to occur, there needs to be a transfer of electrons, which results in a change in oxidation numbers.

2 Mg₍ₛ₎ + O_{2 (g)} → 2 MgO ₍ₛ₎

Mg has brought about the reduction of oxygen because the oxidation number of each oxygen atom has decreased from 0-2.

Because magnesium brought about the decrease in oxidation number of oxygen it is said to be a reducing agent.

• At the same time, oxygen has brought about the oxidation of magnesium because the oxidation number of each magnesium atom has increased from 0 to +2.
• Because magnesium brought about the increase in oxidation number of magnesium it is said to be the oxidising agent.

• Electrochemistry

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