CDS Chemistry · Chemical Reactions

Oxidation and Reduction

What oxidation and reduction mean, which everyday changes are oxidation, the thermite reaction, and why the standard hydrogen electrode's zero is a convention.

All Chemical Reactions notes

Why this matters

Five CDS questions. Two ask which everyday change is NOT an oxidation; two are on the thermite reaction; one HARD 2016 question is on the standard hydrogen electrode.

Concept 1 of 3: Oxidation and reduction in everyday changes

Oxidation is gaining oxygen or losing electrons; reduction is the reverse. They happen together: whatever is oxidised hands its electrons to something that is reduced. Rusting, rancid butter, burning and wine turning sour are all oxidation. A precipitate forming, or gas fizzing out of soda, is not.

Definition

Definitions:

Oxidation: gain of oxygen, loss of hydrogen, or loss of electrons.
Reduction: loss of oxygen, gain of hydrogen, or gain of electrons.
The oxidising agent is itself reduced; the reducing agent is itself oxidised.
Everyday oxidation: rusting, rancidity (butter or oil going stale), combustion, wine turning sour (ethanol → acetic acid).
Not oxidation: a precipitation such as BaCl₂ + Na₂SO₄ → BaSO₄ (no change of oxidation state), and opening a soda bottle (CO₂ simply leaves the liquid).

Change	Oxidation?
Rusting of iron	Yes
Butter turning rancid	Yes
Wine turning sour	Yes
Combustion	Yes
White BaSO₄ forming from BaCl₂ and Na₂SO₄	No — double displacementQ
Opening a soda bottle	No — physical release of CO₂Q

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (I) 2023 — General Knowledge · Q14Easy

Example 1 · Chemical Reactions · Oxidation and Reduction

Which one of the following is not an oxidation reaction?

A precipitate is not an oxidation

Forming BaSO₄ from barium chloride and sodium sulphate changes no oxidation state, so it is not redox. Rusting, rancidity and souring wine are.

The reducing agent is oxidised

The substance that reduces something else gives up electrons, so it is itself oxidised. Don't confuse the agent with what happens to it.

Drill 1 more on oxidation and reduction in everyday changes

Concept 2 of 3: The thermite reaction

Aluminium is more reactive than iron, so it can pull oxygen away from iron oxide. The reaction gives out so much heat that the iron comes out molten, hot enough to weld railway tracks where they meet.

Definition

The reaction:

Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat.
Aluminium is the reducing agent (it is oxidised); iron oxide is reduced.
The heat melts the iron, which is run into the gap to weld rails and machine parts.
The products are molten iron and aluminium oxide, not molten aluminium.

Statement	True or false
Iron oxide reacts with aluminium to join railway tracks	TrueQ
The heat given out is used for welding	True
Aluminium acts as an oxidising agent	False — it is the reducing agent
Molten iron and molten aluminium are formed	False — molten iron and aluminium oxide

Practice this concept3 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (I) 2023 — General Knowledge · Q54Moderate

Example 2 · Chemical Reactions · Oxidation and Reduction

Which of the following statements are true for the reaction of

\mathrm{Fe_2O_3}

with aluminium? 1. It is known as the 'thermite reaction'. 2. The heat evolved is used for welding purpose. 3. Aluminium metal acts as an oxidizing agent. 4. Molten Fe and Al are formed at the end of the reaction. Select the correct answer using the code given below.

Aluminium reduces, it does not oxidise

In thermite, aluminium takes oxygen from iron oxide, so aluminium is the reducing agent. 'Aluminium acts as an oxidising agent' is false.

Drill 1 more on the thermite reaction

Concept 3 of 3: The standard hydrogen electrode

You cannot measure the potential of one electrode alone, only the difference between two. So chemists agreed to call the hydrogen electrode zero and measure every other electrode against it. The zero is a convention, not a fact about the electrode.

Definition

The facts:

The standard hydrogen electrode (SHE): hydrogen gas at 1 bar over platinum, in 1 mol/L H⁺, at 298 K.
Its potential is assigned 0 V by convention, at every temperature.
Its absolute potential is not zero; no single electrode's absolute potential can be measured.

Statement about the SHE	True or false
Its standard potential is taken as 0 V by convention	True
Its absolute electrode potential is not zero	TrueQ
Its potential is zero only at 25 °C	False

Practice this concept2 quick reps

The same idea in a real exam question:

CDS · 2016 · CDS (II) 2016 — General Knowledge · Q28Hard

Example 3 · Chemical Reactions · Oxidation and Reduction

In case of a standard hydrogen electrode

The hydrogen electrode's zero is agreed, not measured

The SHE is assigned 0 V so that other electrodes can be compared with it. Its absolute potential is not zero, and the zero is not limited to 25 °C.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Oxidation and reduction in everyday changes6 rows

Change	Oxidation?
Rusting of iron	Yes
Butter turning rancid	Yes
Wine turning sour	Yes
Combustion	Yes
White BaSO₄ forming from BaCl₂ and Na₂SO₄	No — double displacementQ
Opening a soda bottle	No — physical release of CO₂Q

The thermite reaction4 rows

Statement	True or false
Iron oxide reacts with aluminium to join railway tracks	TrueQ
The heat given out is used for welding	True
Aluminium acts as an oxidising agent	False — it is the reducing agent
Molten iron and molten aluminium are formed	False — molten iron and aluminium oxide

The standard hydrogen electrode3 rows

Statement about the SHE	True or false
Its standard potential is taken as 0 V by convention	True
Its absolute electrode potential is not zero	TrueQ
Its potential is zero only at 25 °C	False

Watch out for (4)

A precipitate is not an oxidation
→ Oxidation and reduction in everyday changes
The reducing agent is oxidised
→ Oxidation and reduction in everyday changes
Aluminium reduces, it does not oxidise
→ The thermite reaction
The hydrogen electrode's zero is agreed, not measured
→ The standard hydrogen electrode

Test yourself on a real paper

Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit a CDS past paper Drill all 5 questions