Advanced Chemical Principles and Molecular Dynamics · 6th Year

Active learning ideas

Oxidation Numbers

Mastering oxidation numbers is the key for your students to unlock the world of redox reactions, a cornerstone of the Leaving Cert Chemistry course.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Inorganic Chemistry - Oxidation and Reduction

• 15–25 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Collaborative Problem-Solving
• 20 min · Small Groups

Oxidation Number Whiteboard Race

In small groups, students race to correctly calculate and display the oxidation number of a specific element in a series of compounds or ions presented by the teacher. Points are awarded for the first team with the correct answer, encouraging both speed and accuracy.

Analyse the oxidation number of manganese in the permanganate ion, MnO4-.

Facilitation Tip

Include a mix of simple compounds, polyatomic ions, and common exceptions like H2O2 to test all the rules.

What to look for

Use an exit ticket with three problems: assign oxidation numbers in a simple compound (e.g., MgCl2), a polyatomic ion (e.g., SO4^2-), and an exception (e.g., NaH).

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 02

• Collaborative Problem-Solving
• 25 min · Pairs

Redox Reaction Card Sort

Students work in pairs with a set of cards, each showing a chemical reaction. They must calculate the oxidation numbers for each element to determine if it is a redox reaction and then sort the cards into 'Redox' and 'Not Redox' piles.

Explain how to use oxidation numbers to determine if a reaction is a redox reaction.

Facilitation Tip

Encourage students to circle the elements that change oxidation number and label them as oxidised or reduced.

What to look for

A section on a class test requires students to assign oxidation numbers to all elements in a given reaction, identify it as redox or not, and name the oxidising and reducing agents.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 03

• Collaborative Problem-Solving
• 15 min · Small Groups

The Permanganate Puzzle

Present the permanganate ion (MnO4-) and ask groups to use the rules they know to work backwards and deduce the oxidation number of manganese. This inquiry-based activity promotes problem-solving and reinforces the rule for polyatomic ions.

Justify the oxidation number assigned to oxygen in hydrogen peroxide.

Facilitation Tip

Provide hints by first establishing the known oxidation number of oxygen and the overall charge of the ion.

What to look for

Provide a worksheet of practice problems with a detailed answer key, allowing students to check their work and identify areas where they need more practice.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

A few notes on teaching this unit

Begin with the foundational rules and build complexity step-by-step. Use numerous worked examples on the board, focusing on polyatomic ions and exceptions like peroxides to build confidence. Emphasise that it is a logical, hierarchical process, not just random guessing.

After this topic, students will be able to confidently assign oxidation numbers and use them to analyse any chemical reaction for electron transfer.

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Watch Out for These Misconceptions

• Oxidation number is the same as the real charge on an ion.

  The oxidation number is a hypothetical charge assigned to an atom assuming all its bonds are 100% ionic. While it matches the charge for simple ions (e.g., Na+ is +1), in covalent molecules like H2O, it's a formal way of tracking electrons, not a real charge.

• The oxidation number of an element is always the same in every compound.

  Many elements, especially transition metals and non-metals like nitrogen and sulfur, can have variable oxidation numbers depending on the compound. For example, manganese can be +2, +4, or +7 in different species.

• The sum of oxidation numbers in a polyatomic ion is always zero.

  The sum of the oxidation numbers of all atoms in a polyatomic ion must equal the overall charge of the ion. It only equals zero for neutral compounds.

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Methods used in this brief

• Collaborative Problem-Solving