Oxidation and reductionIB MYP Chemistry: Revision notes
Section 1
Oxidation and reduction with oxygen
Oxidation is the gain of oxygen. Reduction is the loss of oxygen.
Example: 2Mg + O₂ → 2MgO. Magnesium gains oxygen, so it is oxidised.
Example: CuO + H₂ → Cu + H₂O. Copper(II) oxide loses oxygen, so it is reduced. Hydrogen gains oxygen, so it is oxidised.
Oxidation and reduction always happen together in one reaction, so we call it a redox reaction.
Look for oxygen being added or removed first. It is the quickest way to spot redox.
Section 2
Oxidation and reduction with electrons
A better definition uses electrons. Remember OIL RIG:
- Oxidation Is Loss of electrons
- Reduction Is Gain of electrons
This lets us describe reactions with no oxygen, such as a metal reacting with an ion in solution. Atoms that lose electrons form positive ions, and ions that gain electrons form atoms.
Do not mix them up: in terms of electrons, oxidation is loss and reduction is gain. In terms of oxygen, oxidation is gain and reduction is loss.
Section 3
Oxidising and reducing agents
- An oxidising agent oxidises another substance. It takes electrons (or gives oxygen) and is itself reduced.
- A reducing agent reduces another substance. It gives electrons (or takes oxygen) and is itself oxidised.
Example: CuO + H₂ → Cu + H₂O. Hydrogen is the reducing agent because it removes oxygen from copper(II) oxide. Copper(II) oxide is the oxidising agent.
The reducing agent is the substance that gets oxidised, and the oxidising agent is the one that gets reduced.
Section 4
Half-equations
A half-equation shows just the oxidation or just the reduction, with electrons (e⁻) written in. The charges must balance on both sides.
- Oxidation: Mg → Mg²⁺ + 2e⁻ (electrons on the right)
- Reduction: Cu²⁺ + 2e⁻ → Cu (electrons on the left)
For the displacement Zn + CuSO₄ → ZnSO₄ + Cu, the half-equations are Zn → Zn²⁺ + 2e⁻ (zinc oxidised, the reducing agent) and Cu²⁺ + 2e⁻ → Cu (copper ions reduced, the oxidising agent). You do not need oxidation numbers.
In an oxidation half-equation the electrons are products (right). In a reduction half-equation they are reactants (left).
Section 5
Redox in everyday reactions
- Combustion: fuels gain oxygen, so they are oxidised. For example, methane burning.
- Displacement: a more reactive metal displaces a less reactive metal from its solution. The more reactive metal loses electrons (oxidised) and the metal ions gain them (reduced). Example: Fe + CuSO₄ → FeSO₄ + Cu.
- Rusting: iron reacts with oxygen and water to form hydrated iron(III) oxide (rust). Iron gains oxygen and loses electrons, so it is oxidised: Fe → Fe³⁺ + 3e⁻.
Rusting needs both oxygen and water.
For rusting, say that iron is oxidised and that it needs both oxygen and water.
Must know
- Oxidation: gain of oxygen or loss of electrons. Reduction: loss of oxygen or gain of electrons (OIL RIG)
- Redox: oxidation and reduction happen together
- Oxidising agent takes electrons and is reduced; reducing agent gives electrons and is oxidised
- Half-equations: Mg → Mg²⁺ + 2e⁻ (oxidation), Cu²⁺ + 2e⁻ → Cu (reduction)
- Combustion, displacement and rusting are all redox reactions
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Oxidation and reduction
- A student in Athens burns a strip of magnesium ribbon in air. It burns with a bright white flame and leaves a white powder of magnesium oxide, MgO, as shown by the equation 2Mg + O₂ → 2MgO. Magnesium oxide is an ionic compound made of Mg²⁺ ions and O²⁻ ions.Write the half-equation for magnesium atoms forming Mg²⁺ ions and explain why this is oxidation.2 marks
- A teacher heats black copper(II) oxide, CuO, in a stream of hydrogen gas. The black solid turns into a pink-brown solid, copper, and steam is formed: CuO + H₂ → Cu + H₂O.Explain why this is a redox reaction, in terms of oxygen.2 marks
- Students at a school in Manila investigate the conditions needed for iron to rust. They set up three test tubes, each containing a clean iron nail. Tube A has boiled water with a layer of oil on top. Tube B has dry air and a drying agent, calcium chloride. Tube C has tap water and is open to the air. They leave the tubes for one week.State a hypothesis for this investigation, with a scientific reason.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).