Redox

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.20 — Oxidation, reduction, redox, oxidising agent and reducing agent in terms of oxygen and electrons
• 2.21 — Practical: investigate reactions between dilute acids and metals (required practical)

Specification section 2(d) Reactivity series — also taught in The Gases in the Air, Combustion and Carbonates, The Reactivity Series, Rusting and its Prevention.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Chemistry has two definitions of oxidation, an older one and a broader one, and both are examined. The older is the obvious one: OXIDATION IS GAINING OXYGEN and REDUCTION IS LOSING IT. Magnesium burning gains oxygen and is oxidised; copper oxide heated with hydrogen loses its oxygen and is reduced.

The trouble with that definition is that a great many reactions with the same character involve no oxygen at all. Magnesium burning in chlorine behaves exactly like magnesium burning in oxygen — vigorous, exothermic, giving an ionic compound — and yet by the oxygen definition it is not oxidation. Something is clearly being missed.

The broader definition fixes this by asking what actually happens to the ELECTRONS. Magnesium burning in oxygen loses two electrons per atom, and magnesium burning in chlorine loses exactly the same two. So: OXIDATION IS LOSS of electrons, and REDUCTION IS GAIN of electrons. The mnemonic is OIL RIG — Oxidation Is Loss, Reduction Is Gain.

Think of it like describing a transaction by the goods or by the money

A shop sale can be described two ways. You could say the customer received a loaf, which is accurate and works only for sales involving bread. Or you could say money moved from the customer to the shop, which describes every sale there has ever been. The oxygen definition is the loaf: correct, vivid, and limited to reactions where oxygen happens to be involved. The electron definition is the money — the thing that is actually changing hands in every case. Both describe the same event, and one of them generalises.

Why the two definitions never disagree

1. Oxygen is a non-metal that GAINS electrons when it reacts, becoming O²⁻.
2. So when something gains oxygen, that something must have GIVEN UP the electrons the oxygen took.
3. Gaining oxygen therefore always means losing electrons — the two definitions agree wherever oxygen is present.
4. The electron definition simply also covers the cases where no oxygen is involved, such as reaction with chlorine.
5. So it is not a replacement for the old definition but an extension of it.

A REDOX reaction is one in which both happen together, and the crucial point is that they always do. Electrons do not vanish: if one substance loses electrons, another must gain exactly those electrons. There is no such thing as oxidation on its own, which is why the word redox exists at all.

The AGENT words follow from this and are the ones most often reversed. An OXIDISING AGENT is the substance that CAUSES oxidation in something else — so it takes the electrons, which means it is itself REDUCED. A REDUCING AGENT causes reduction in something else by giving electrons away, so it is itself OXIDISED. The agent always undergoes the opposite of what it causes.

One reaction analysed both ways

Magnesium reacts with copper(II) sulfate: Mg + CuSO₄ → MgSO₄ + Cu. Identify what is oxidised, what is reduced, and the two agents.

1. Write the half-equations. Magnesium: Mg → Mg²⁺ + 2e⁻ — it LOSES two electrons.
2. Copper: Cu²⁺ + 2e⁻ → Cu — it GAINS two electrons.
3. OIL RIG: magnesium loses electrons so magnesium is OXIDISED; copper ions gain electrons so they are REDUCED.
4. The copper ions took the electrons and so caused the oxidation of the magnesium: copper(II) sulfate is the OXIDISING AGENT, and it is itself reduced.
5. The magnesium gave the electrons and so caused the reduction of the copper ions: magnesium is the REDUCING AGENT, and it is itself oxidised.
6. Note that no oxygen takes part at all, and the sulfate is a spectator ion that changes in no way.

Answer: Magnesium oxidised and is the reducing agent; copper ions reduced and are the oxidising agent.

That worked example is also the whole reactivity series restated. A metal high in the series loses electrons readily, which is exactly what being a good reducing agent means — so the reactivity series is an order of how readily each metal is oxidised. Every displacement reaction met on the previous pages was a redox reaction all along.

The required practical, statement 2.21, is the reaction of dilute acids with metals, and it is redox too. A metal above hydrogen gives its electrons to the hydrogen ions in the acid, so the metal is oxidised to its ion and the hydrogen ions are reduced to hydrogen gas — which is the fizzing. The rate of fizzing ranks the metals, and the ranking is the reactivity series once again.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.20 — the five words, defined both ways

Oxidation

Gain of oxygen, OR loss of electrons. Remember OIL — Oxidation Is Loss.

Reduction

Loss of oxygen, OR gain of electrons. Remember RIG — Reduction Is Gain.

Redox

A reaction in which oxidation and reduction both occur. They always do, since electrons lost by one substance are gained by another.

Oxidising agent

The substance that causes oxidation by TAKING electrons — so it is itself REDUCED.

Reducing agent

The substance that causes reduction by GIVING electrons — so it is itself OXIDISED.

Analysing a redox reaction

1. Write the half-equation for each species that changes.
2. Identify which loses electrons: that one is OXIDISED.
3. Identify which gains electrons: that one is REDUCED.
4. The substance that was reduced took the electrons, so it is the OXIDISING AGENT.
5. The substance that was oxidised gave the electrons, so it is the REDUCING AGENT.
6. Check that the electrons lost equal the electrons gained.

The two definitions of oxidation

A. IN TERMS OF OXYGEN — oxidation is GAINING oxygen. Correct, but only applies where oxygen is involved.

B. IN TERMS OF ELECTRONS — oxidation is LOSING electrons. Covers every case, including reactions with no oxygen at all.

Required practical 2.21 — reactions of dilute acids with metals

1. Measure equal volumes of the same dilute acid, such as 25 cm³ of hydrochloric acid, into several test tubes.
2. Take equal masses of each metal — magnesium, zinc, iron and copper — in the same physical form, such as similar-sized pieces of ribbon or turnings.
3. Add one metal to each tube and start a stopwatch.
4. Observe and record the rate of fizzing in each tube, and note any temperature change by feeling the tube or using a thermometer.
5. Test the gas produced with a lit splint; a squeaky pop confirms hydrogen.
6. Rank the metals by how vigorously they react, most vigorous first.
7. Note that any metal producing no fizzing at all lies below hydrogen in the reactivity series.

Variables

Independent (changed) — The metal added to the acid
Dependent (measured) — The rate of reaction, judged by how vigorously hydrogen is given off

Sources of error

Mg + CuSO₄ → MgSO₄ + Cu

Conditions: Redox with no oxygen involved. Mg → Mg²⁺ + 2e⁻ (oxidised); Cu²⁺ + 2e⁻ → Cu (reduced).

Model answer [4 marks]

In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, state what is oxidised and what is reduced, and identify the reducing agent. Explain your answers in terms of electrons. [4]

Zinc is oxidised, because each zinc atom loses two electrons to become a zinc ion: Zn → Zn²⁺ + 2e⁻. Copper ions are reduced, because each gains two electrons to become a copper atom: Cu²⁺ + 2e⁻ → Cu. The reducing agent is the zinc, because it is the substance that gives away the electrons and so causes the reduction of the copper ions. Being the reducing agent, the zinc is itself oxidised.

Model answer [3 marks]

Explain why the definition of oxidation in terms of electrons is more useful than the definition in terms of oxygen. [3]

The definition in terms of oxygen only applies to reactions in which oxygen takes part, so it cannot describe reactions such as magnesium burning in chlorine, which behave in exactly the same way. The definition in terms of electrons applies to all such reactions, because electron transfer is what is happening in every case. The two definitions never conflict, because oxygen gains electrons when it reacts, so anything gaining oxygen must also be losing electrons.

Not this: An oxidising agent is a substance that is oxidised.

This: It is the substance that CAUSES oxidation in something else, which it does by TAKING the electrons — so an oxidising agent is itself REDUCED. The agent always undergoes the opposite of what it causes.

Mark-losing trap. OIL RIG — Oxidation Is Loss, Reduction Is Gain. Of ELECTRONS.

Mark-losing trap. An oxidising agent is REDUCED and a reducing agent is OXIDISED. The agent does the opposite.

Mark-losing trap. Oxidation never happens alone. Electrons lost by one substance are gained by another — hence redox.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — In terms of electrons, what is oxidation?
2. Grade 7 · Identify [2 marks] — In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, which species is oxidised?
3. Grade 8 · Explain [3 marks] — In the reaction Mg + 2HCl → MgCl₂ + H₂, explain which substance is the reducing agent.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of the redox changes in the reaction Mg + CuSO₄ → MgSO₄ + Cu.
5. 9+ · Analyse [6 marks] — A student says: 'The reaction between magnesium and chlorine cannot be a redox reaction, because there is no oxygen involved, so nothing can be oxidised.' Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who gave oxidation its name and its first definition, and the one whose measurements pointed at electrons before anyone knew of them. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antoine Lavoisier — the one who gave oxidation its name and its first definition

The older definition on this page is Lavoisier's, and the word itself is his — oxidation means combination with oxygen because he named the gas and the process together. He is also the right person to ask about a definition being outgrown: he was so convinced oxygen was the essential ingredient of reactions that he named it from the Greek for acid-former, believing every acid must contain it. Davy later showed that hydrochloric acid contains none. A definition can be a great advance and still turn out to have named the wrong thing as essential.

• “Why did you name the process after oxygen?”
• “What did you think oxygen was essential to?”
• “Why did you believe every acid must contain it?”
• “How did you feel about being wrong on that point?”
• “What does a substance gain when it combines with oxygen?”

Michael Faraday — the one whose measurements pointed at electrons before anyone knew of them

The electron definition on this page could not be stated until the electron was known in 1897, and Faraday's electrolysis work sixty years earlier is the strongest hint that something like it existed. He established that the mass of a substance released at an electrode is proportional to the charge passed, and that the quantities for different substances stand in simple whole-number ratios tied to their charges. That is very hard to explain unless charge comes in fixed indivisible units — an inference he declined to make outright, and which turned out to be exactly right.

• “What made you think charge came in definite amounts?”
• “Why did you not simply say there was a particle of electricity?”
• “How did your measurements relate to the charge on an ion?”
• “What is actually transferred when a metal displaces another?”
• “How much did you think was still missing from the picture?”

Carry on through the course

• Previous lesson: Rusting and its Prevention
• Next lesson: Extracting Metals from their Ores
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification