Redox 2 - Oxidation Numbers

This lesson covers: 

  1. Understanding what oxidation numbers represent and some key rules for determining them
  2. Calculating oxidation numbers for atoms in compounds, ions, and uncombined elements
  3. Using oxidation numbers to identify oxidation and reduction in redox reactions
  4. Examples of common oxidising and reducing agents: MnO4- and I-
  5. Observing oxidisation and reduction through colour changes and oxidation number shifts

What are oxidation numbers?


Every atom and ion has an oxidation number that indicates:

  • The number of electrons it has gained or lost when bonded with other atoms
  • Whether it has been oxidised or reduced


Rules for oxidation numbers:

  • Uncombined elements have an oxidation number of 0
  • Elements combined with identical atoms, like O2 and H2, also have an oxidation number of 0
  • The oxidation number of a monatomic ion is equal to its charge
  • Oxygen atoms typically have an oxidation number of -2, except in peroxides where it's -1
  • Hydrogen atoms typically have an oxidation number of +1, but it's -1 in metal hydrides

For compounds and polyatomic ions, the sum of oxidation numbers equals the overall charge.

Worked example: Calculating oxidation numbers of compounds.


Calculate the oxidation number of the carbonate ion CO32-. Carbon has an oxidation number of +4 and oxygen has an oxidation number of -2.


Step 1: Identify the oxidation numbers of each element.

Oxidation number of carbon = +4

Oxidation number of oxygen = -2

Oxidation number of O3 = 3 x -2 = -6


Step 2: Calculate the overall oxidation number

Overall oxidation number = -6 + 4 = -2

Using oxidation numbers to identify redox reactions


Oxidation numbers help identify what is oxidised and reduced:

  • Oxidation - Results in a increase in oxidation number.
  • Reduction - Results in a decrease in oxidation number.


Oxidising and reducing agents

  • Oxidising agents - Compounds that promote the reduction of other substances while they themselves get oxidised.
  • Reducing agents - Compounds that promote the oxidation of other substances while they themselves get reduced.

Examples of oxidising and reducing agents


Potassium manganate(VII) (KMnO4)

  • Potassium manganate is a strong oxidising agent.
  • When it reacts, the solution will change from purple to colourless.


Potassium iodide (KI)

  • Potassium iodide is a reducing agent.
  • When it reacts, the solution will change from colourless to brown.


The colour changes provide visual evidence of the oxidation and reduction taking place.

Identifying oxidation and reduction


To identify what is oxidised or reduced:

  1. Write out the chemical equation for the reaction
  2. Assign oxidation numbers for the reactants
  3. Assign oxidation numbers for the products
  4. Compare if the oxidation numbers for each element increase or decrease


Increases in oxidation numbers indicate oxidation, while decreases indicate reduction.

Worked Example: Oxidation of magnesium


When magnesium reacts with Oxygen it is oxidised. State the oxidation numbers of magnesium and oxygen and hence determine the change in oxidation number for magnesium.


Step 1: Write out the chemical equation for the reaction

2Mg + O2 ➔ 2MgO


Step 2: Assign oxidation numbers to reactants

Mg has an oxidation number of 0

O2 has an oxidation number of 0


Step 3: Assign oxidation numbers to products

Mg has an oxidation number of +2

O has an oxidation number of -2

MgO has an oxidation number of 0


Step 4: Compare oxidation numbers

The oxidation number of magnesium has increased from 0 to +2. This shows it has been oxidised.

Worked example: Reaction between zinc (Zn) and hydrochloric acid (HCl)


When zinc reacts with hydrochloric acid it forms hydrogen and zinc chloride. State the oxidation numbers of zinc, hydrogen and chlorine and hence determine if zinc is oxidised or reduced in this reaction.


Step 1: write out the chemical equation for the reaction

Zn + 2HCl ➔ H2 + ZnCl2


Step 2: Assign oxidation numbers to the reactants

Zn has an oxidation number of 0

H in HCl has an oxidation number of +1

Cl in HCl has an oxidation number of -1


Step 3: Assign oxidation numbers to the products

H2 has an oxidation number of 0

Zn in ZnCl2 has an oxidation number of +2

Cl has an oxidation number of -1


Step 4: Compare oxidation numbers

The oxidation number of Zn has increased from 0 to +2. This means Zn has been oxidised.

The oxidation number of H has decreased from +1 to 0. This means H has been reduced.


Zinc is the reducing agent. 

Hydrochloric acid is the oxidising agent.

What do oxidation numbers represent?

The number of protons in an atom

The number of neutrons in an atom

The number of electrons gained or lost by an atom

The overall charge of an atom

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Which of the following statements about oxidation numbers is true?

Uncombined elements always have an oxidation number of 0

Hydrogen atoms always have an oxidation number of +1

The sum of oxidation numbers in a compound is equal to the overall charge of the compound

Oxygen atoms always have an oxidation number of -1

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Which of the following is an example of an oxidising agent?

CO3

H2

I-

MnO4-

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What happens to the oxidation number during reduction?

It depends on the specific reaction

It decreases

It remains the same

It increases

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What is the purpose of observing colour changes in redox reactions?

To determine the overall charge of the compound

To visually confirm the occurrence of oxidation and reduction

To identify the oxidising agent

To identify the reducing agent

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