GCSE · Chemistry · AQA · Spec 8462

Oxidation and reduction in terms of electrons (HT)

When magnesium meets copper sulfate solution, the sulfate does absolutely nothing. The real action is electrons jumping between particles. Follow the electrons and these reactions start making sense.

Higher

Chemistry · Redox

Who lost electrons? Who gained them?

Reaction 1: Mg + CuSO₄ → MgSO₄ + Cu. Reaction 2: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. Sort each particle by what happens to its electrons.

Still to sort

Oxidised: loses electrons (0)

Its charge goes up, for example a neutral atom becoming a positive ion.

Where the line is: Only electrons count. If it gave electrons away, it was oxidised, whatever it ends up as.

Reduced: gains electrons (0)

Its charge goes down, for example a positive ion becoming a neutral atom.

Where the line is: Look for electrons taken in. In a half equation they sit on the left, with the reactants.

Neither: a spectator ion (0)

Same particle, same charge, before and after.

Where the line is: It is in the beaker and in the full equation, but it neither loses nor gains electrons. Being there is not the same as taking part.

8 of 8 still to sort.

Two displacement reactions and two half equations. Pick a particle (chemists call each one a species), decide what happened to its electrons, then read why.

Exam line: Always say it with electrons: 'magnesium is oxidised because each atom loses two electrons', not just 'magnesium is oxidised'.
Watch out: Copper turns up twice. In reaction 1, Cu²⁺ ions gain electrons (reduced). In reaction 2, Cu atoms lose electrons (oxidised). Oxidised or reduced describes what one particle does in one reaction, not what an element always does.
Higher

Watch it done: from full equation to half equations

Problem

Zinc is added to iron(II) sulfate solution: Zn + FeSO₄ → ZnSO₄ + Fe. Write the ionic equation and both half equations, and say which species is oxidised and which is reduced.

Higher

Your turn

Finish it: when the charges don't match

Magnesium is added to silver nitrate solution: Mg + 2AgNO₃ → Mg(NO₃)₂ + 2Ag. Write the ionic equation and the half equations.

  1. Ions: Mg + 2Ag⁺ + 2NO₃⁻ → Mg²⁺ + 2NO₃⁻ + 2AgSilver nitrate and magnesium nitrate are dissolved, so they are split into ions.
  2. missing step
Which line is step 2?

Exam line: Balance the electrons, not just the atoms: electrons lost must equal electrons gained.

Higher

Spot the slip

Where does this answer go wrong?

Zinc is added to copper chloride solution: Zn + CuCl₂ → ZnCl₂ + Cu. Write the ionic equation and the half equations, and say which species is oxidised and which is reduced.

A student's answer — which line goes wrong?

Exam line: Before you label a half equation, check the total charge is the same on both sides.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Whoever loses electrons is oxidised. Whoever gains them is reduced. Whoever does neither is just watching.

What you need to know

  • Define oxidation and reduction in terms of electrons.
  • Decide which species is oxidised and which is reduced in a reaction, a symbol equation or a half equation, and say why.
  • Write the ionic equation for a metal displacement reaction, leaving out the spectator ions.

The big picture

Oxidation is the loss of electrons and reduction is the gain of electrons, and they always happen together. In a displacement reaction the more reactive metal's atoms lose electrons to the less reactive metal's ions. Cross out the spectator ions and you have the ionic equation; split that in two and you have the half equations.

Key points

1Oxidation is loss of electrons. Reduction is gain of electrons.
2The two happen together: every electron one species loses, another species gains.
3In a metal displacement reaction, atoms of the more reactive metal are oxidised and ions of the less reactive metal are reduced.
4A spectator ion has the same charge before and after. It neither loses nor gains electrons, so it is left out of the ionic equation.
5In a half equation, electrons on the right are lost (oxidation) and electrons on the left are gained (reduction). The total charge must be the same on both sides.

Worked example

Problem

Aluminium displaces copper from copper sulfate solution. The half equations are Al → Al³⁺ + 3e⁻ and Cu²⁺ + 2e⁻ → Cu. Combine them into the ionic equation and say which species is oxidised and which is reduced.

⚠ Watch out

Calling every ion in the equation oxidised or reduced. Check each one: a spectator ion such as SO₄²⁻ has exactly the same charge before and after, so it gained and lost nothing. It is neither, and it does not belong in the ionic equation.

🧠

Memory hook

OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). And picture the spectator ion up in the stands: it's at the match, but it never touches the ball.

✓

Check yourself

Try one on your own: Fe + CuSO₄ → FeSO₄ + Cu. Which species is oxidised, which is reduced, which is the spectator, and what is the ionic equation?

Flashcards

(6)
What are oxidation and reduction, in terms of electrons?
Oxidation is loss of electrons. Reduction is gain of electrons. (OIL RIG)
What is a spectator ion?
An ion that is present but neither loses nor gains electrons. It is the same on both sides, so it is left out of the ionic equation.
In a metal displacement reaction, what is oxidised and what is reduced?
Atoms of the more reactive metal are oxidised (they lose electrons). Ions of the less reactive metal are reduced (they gain those electrons).
In a half equation, how can you tell oxidation from reduction?
Electrons on the right-hand side are given out: oxidation. Electrons on the left-hand side are taken in: reduction.
How do you check a half equation is written correctly?
Add up the charges. The total charge must be the same on both sides.
Why can't oxidation happen without reduction?
Electrons are transferred, not created or destroyed. Every electron one species loses must be gained by another, so electrons lost always equal electrons gained.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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