6.4 - Redox

Oxidation and reduction as oxygen transfer

• Oxidation is the gain of oxygen by a substance.
• Reduction is the loss of oxygen from a substance.

Examples

• When magnesium burns in oxygen to form magnesium oxide, the magnesium gains oxygen, so it has been oxidised:
  • 2Mg + O₂ → 2MgO
• When copper(II) oxide reacts with carbon, the copper(II) oxide loses oxygen and is reduced, while the carbon gains oxygen and is oxidised:
  • 2CuO + C → 2Cu + CO₂
• A reaction where oxidation and reduction happen together is a redox reaction.

Following hydrogen instead

• Losing hydrogen is oxidation, gaining hydrogen is reduction.
• When hydrogen and chlorine react, the chlorine gains hydrogen, so it has been reduced:
  • H₂ + Cl₂ → 2HCl

Oxidising and reducing agents

• Oxidising agent - A substance that oxidises another substance, and is itself reduced.
• Reducing agent - A substance that reduces another substance, and is itself oxidised.
• In summary:
  • The substance that is reduced is the oxidising agent.
  • The substance that is oxidised is the reducing agent.

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Worked example - Copper(II) oxide and carbon

2CuO + C → 2Cu + CO₂

• Copper(II) oxide loses oxygen, so it is reduced; the carbon took its oxygen away, so carbon is the reducing agent.
• Carbon gains oxygen, so it is oxidised; the copper(II) oxide supplied that oxygen, so copper(II) oxide is the oxidising agent.

Testing for oxidising and reducing agents

• Acidified aqueous potassium manganate(VII) is an oxidising agent (starts purple). If it turns colourless, it has been reduced, showing that the tested substance is a reducing agent.
• Aqueous potassium iodide is a reducing agent (starts colourless). If it turns brown, iodine has formed and the iodide has been oxidised, showing that the tested substance is an oxidising agent.
• A colour change in either test confirms that a redox reaction has occurred.

The Roman numeral in a name

• A Roman numeral in a compound's name gives the oxidation number of the element it follows. For a simple metal ion this is the same as the charge on the ion, so iron is +3 in iron(III) oxide and +2 in iron(II) oxide.
• Oxidation is an increase in oxidation number, and reduction is a decrease in it. An iron atom (0) that becomes an Fe²⁺ ion (+2) has been oxidised, and an Fe³⁺ ion that becomes an Fe²⁺ ion has been reduced.

Oxidation and reduction as electron transfer

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• Remember: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
• Electrons are transferred from one substance to another, so oxidation and reduction always occur together in a redox reaction.

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Worked example - Sodium and chlorine

2Na + Cl₂ → 2NaCl

• Each sodium atom starts neutral and becomes Na⁺ (loses one electron), so sodium is oxidised.
• Each chlorine atom starts neutral and becomes Cl^- (gains one electron), so chlorine is reduced.

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Identifying oxidation and reduction from charges

• An atom that becomes a positive ion loses electrons: oxidised.
• An atom that becomes a negative ion gains electrons: reduced.
• A positive ion that becomes an atom gains electrons: reduced.