5.4.1.4Oxidation and reduction in terms of electrons (HT only)
AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemical changes › Reactivity of metals
Practise Oxidation and reduction in terms of electrons (HT only). 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Higher tier only. Oxidation and reduction described in terms of electrons, used to write ionic equations for displacement reactions and to say which species are oxidised and which are reduced. Usually 2 to 3 mark questions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
Define oxidation and reduction using electronsOxidation is the loss of electrons and reduction is the gain of electrons (OIL RIG).
6
Identify the species oxidised and reducedAn atom that becomes a positive ion is oxidised; a positive ion that becomes an atom is reduced.
7
Write half equations for displacement reactionsFor example, Mg → Mg2+ + 2e− and Cu2+ + 2e− → Cu.
8
Write ionic equations for displacement reactionsLeave out the spectator ions, e.g. Mg + Cu2+ → Mg2+ + Cu.
9
Balance ionic equations with different ion chargesMatch the electrons lost and gained, e.g. 2Al + 3Cu2+ → 2Al3+ + 3Cu.
Notes
OIL RIG
Oxidation Is Loss of electrons. Reduction Is Gain of electrons.
A metal atom that loses electrons to form a positive ion is oxidised: Zn → Zn2+ + 2e−.
A metal ion that gains electrons to form an atom is reduced: Cu2+ + 2e− → Cu.
Non-metals work the same way: chlorine gaining electrons is reduced (Cl2 + 2e− → 2Cl−); bromide ions losing electrons are oxidised.
The electrons lost by one species are gained by another, so oxidation and reduction always happen together: a redox reaction.
Ionic equations for displacement reactions
In a displacement reaction in solution, the negative ion (e.g. sulfate) does not change. It is a spectator ion and is left out of the ionic equation.
Full equation: Zn + CuSO4 → ZnSO4 + Cu
Ionic equation: Zn + Cu2+ → Zn2+ + Cu
Zinc atoms lose electrons, so zinc is oxidised. Copper(II) ions gain electrons, so they are reduced.
Halogen displacement works the same way: Cl2 + 2Br− → 2Cl− + Br2. Chlorine is reduced and bromide ions are oxidised.
Building and balancing an ionic equation
Write the two half equations, one for the species losing electrons and one for the species gaining them.
Multiply them so the number of electrons is the same in both, then add them and cancel the electrons.
Example: Al → Al3+ + 3e− (× 2) and Cu2+ + 2e− → Cu (× 3). Both now involve 6 electrons, giving 2Al + 3Cu2+ → 2Al3+ + 3Cu. grade 9+
Check: the number of each atom and the total charge must be the same on both sides.
Cheatsheet
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
Metal atom → metal ion + electrons: oxidation
Metal ion + electrons → metal atom: reduction
Mg + Cu2+ → Mg2+ + Cu
Cl2 + 2Br− → 2Cl− + Br2
Spectator ion: an ion that does not change, left out of the ionic equation
Electrons on the left of a half equation = reduction; on the right = oxidation
How to answer each type of question
Which species is oxidised? Explain in terms of electrons
2 marks6
Look at each species: has it lost electrons (charge becomes more positive) or gained electrons?
Loss of electrons = oxidised; gain of electrons = reduced.
Name the species exactly, e.g. 'iron atoms' or 'copper(II) ions'.
Example. Fe + Cu2+ → Fe2+ + Cu Which species is oxidised? Explain your answer in terms of electrons.
Show the model answer
Iron / iron atoms (1) because they lose electrons (to form Fe2+ ions) (1)
Complete a half equation and explain a reduction
3 marks7
Balance the atoms, then add electrons to make the charges equal on both sides.
Say which species gains electrons and link it to reduction.
Example. Chlorine reacts with potassium iodide solution: Cl2 + 2I− → 2Cl− + I2 (a) Complete the half equation: 2I− → I2 + ........ (b) Explain why chlorine is reduced in this reaction.
Show the model answer
(a) 2e− (1) (b) Each chlorine molecule gains (two) electrons to form chloride ions (1) Gain of electrons is reduction (1)
Write an ionic equation for a displacement reaction
2 marks8
Split the soluble ionic compounds into their ions.
Cross out the ions that are the same on both sides (spectator ions).
Balance the atoms and check the total charge is the same on both sides.
Example. Magnesium reacts with silver nitrate solution: Mg + 2AgNO3 → Mg(NO3)2 + 2Ag Write the ionic equation for this reaction.
Charge goes up (e.g. 0 to 2+) = electrons lost = oxidised. Charge goes down (e.g. 2+ to 0, or 0 to 1−) = electrons gained = reduced.
In a metal displacement reaction, the more reactive metal is always the one that is oxidised.
Electrons go on whichever side makes the total charges equal.
Where marks are lost
Mixing up the two definitions: gain of oxygen and loss of electrons are both oxidation.
Leaving spectator ions such as SO42− or NO3− in an ionic equation.
Not balancing charge: Mg + Ag+ → Mg2+ + Ag has 1+ on the left but 2+ on the right.
Using the wrong number of electrons, e.g. Cu2+ + e− → Cu.
Saying 'copper is reduced' in a displacement reaction: it is the copper ions that gain electrons.
Exam technique
If the question says 'in terms of electrons', use 'loses electrons' or 'gains electrons', not oxygen.
Name the species with its charge where you can, e.g. 'Cu2+ ions are reduced'.
Write the half equations first: they make the ionic equation easier to build and to check.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentences. Oxidation is the .................... of electrons. Reduction is the .................... of electrons.
loss; gain
Zinc displaces copper from copper(II) sulfate solution: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) Write the ionic equation for this reaction.
Zn + Cu2+ → Zn2+ + Cu
When chlorine water is added to potassium bromide solution, the solution turns orange: Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq) Write the ionic equation for this reaction.
Cl2 + 2Br− → 2Cl− + Br2
Complete the ionic equation. Mg + Fe2+ → ....................
Mg2+ + Fe
Sodium atoms form sodium ions: Na → Na+ + e− Is this oxidation or reduction?
oxidation
Complete the half equation for the oxidation of zinc. Zn → Zn2+ + ....
2e−
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Oxidation and reduction can be described in terms of electrons.
(a) Which statement describes reduction? Tick (✓) one box.[1]
gain of electrons
loss of electrons
gain of protons
loss of neutrons
(b) Sodium atoms form sodium ions: Na → Na+ + e− Is this oxidation or reduction?[1]
(c) Complete the half equation. Cl2 + 2e− → ....................[1]
(d) Magnesium reacts with copper(II) ions: Mg + Cu2+ → Mg2+ + Cu Which species loses electrons in this reaction?[1]
Show the answer and mark scheme
(a)Answer: gain of electrons
(b)Answer: oxidation
oxidation
(c)Answer: 2Cl−
2Cl−
(d)Answer: magnesium atoms, Mg
magnesium / Mg
Question 2Medium5 marks
Oxidation and reduction can be described in terms of electrons.
(a) Complete the sentences. Oxidation is the .................... of electrons. Reduction is the .................... of electrons.[2]
(b) State whether each half equation shows oxidation or reduction. Fe → Fe2+ + 2e− Cl2 + 2e− → 2Cl− Cu2+ + 2e− → Cu[3]