Science with Tara

Redox (Reduction-Oxidation) reactions are an important topic in O-Level and IGCSE Chemistry. In this post, we cover the basic concepts of oxidation, reduction, oxidation states, and oxidising/reducing agents.

What is Oxidation and Reduction?

Oxidation and reduction can be defined in several ways:

Oxidation – gain of oxygen, loss of hydrogen, loss of electrons, increase in oxidation state

Reduction – loss of oxygen, gain of hydrogen, gain of electrons, decrease in oxidation state

	Oxidation	Reduction
Hydrogen	Lose	Gain
Oxygen	Gain	Lose
Electrons	Lose (LEO)	Gain (GER)
Oxidation number	Increase	Decrease

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Oxidation Number / Oxidation State

Oxidation state (or oxidation number) is the charge an atom of an element would have if it existed as an ion in a compound.

Rules to work out oxidation state/number of an atom

Rule	Example	Oxidation State
1. Oxidation state of a free element is zero.	Cu H₂	0 0
2. Oxidation state of a simple ion is the same as the charge on the ion.	Na⁺ Cl⁻	+1 -1
3. Oxidation states of the atoms present in the formula of a compound add up to zero.	Ca C O CaCO₃	+2 +4 3(-2) +2 + (+4) + 3(-2) = 0
4. Sum of the oxidation states of the atoms in a polyatomic ion is equal to the charge on the ion.	S O SO₄²⁻	+6 -2 +6 + 4(-2) = -2

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Oxidising Agent and Reducing Agent

Oxidising agent : A substance that causes another substance to be oxidised while itself being reduced.

Reducing agent: A substance that causes another substance to be reduced while itself being oxidised.

Common Oxidising Agents

• Oxygen (O₂)
• Halogens (Fluorine is the strongest, Astatine is the weakest)
• Concentrated sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Acidified potassium manganate (VII) (purple to colourless) (MnO₄⁻ → Mn²⁺)
• Iron (III) solution (yellow to green) (Fe³⁺→Fe²⁺)
• Iodine solution (brown to colourless) (I₂ →I^–)

Common Reducing Agents

• Hydrogen (H₂)
• Halides (Astatide is the strongest, Fluoride is the weakest)
• Carbon (C)
• Carbon monoxide (CO)
• Hydrogen sulfide (H₂S)
• Metals (e.g. Mg, Zn)
• Sulfur dioxide (SO₂)
• Potassium iodide (colourless to brown) (KI →I₂)
• Iron (II) solution (green to yellow) (Fe²⁺→Fe³⁺)
• Starch iodide paper (white to blue-black)

Chemical reactions that are not redox reactions

• Neutralization reactions
• Precipitation reaction
• Acids reacting with carbonates
• Decomposition of carbonates by heat

Test for the presence of a reducing agent

Method: Acidified potassium manganate (VII)

For gases:
Place a piece of filter paper soaked with acidified potassium manganate(VII) at the mouth of test tube.

For solutions:
Add acidified potassium manganate(VII) to the unknown solution.

Positive test observation:
Acidified potassium manganate(VII) turned from purple to colourless

Equation:
MnO₄^– (aq) + 8H⁺ (aq) + 5e^{– →} Mn²⁺ (aq) + 4H₂O (l)
(purple)   (acidified)           (colourless)

Note: Potassium manganate (VII) has to be acidified. Acid helps to liberate Mn⁷⁺ in MnO₄^–to Mn²⁺ by providing electrons for liberation of bonded oxygen to form water.

What is the significance of (VII) in potassium mangante (VII)?

Potassium manganate: KMnO₄
Oxidation state of K = +1, O = -2
Oxidiation state of , a compound, is 0

Since Mn is a transition element, it can have more than 1 oxidiation state.
Let’s work it out.

Let x be the oxidation number of Mn
+1 + x + 4(-2) = 0
x = +7

Hence, oxidation state of Mn in KMnO₄ is +7.
(VII) is the oxidiation number of the transition element (Mn).

Test for the presence of an oxidising agent

Method 1: Aqueous potassium iodide

Add aqueous potassium iodide, KI, to the unknown solution.

Positive test observation: 
A brown solution is formed.

Reason:
Iodide ions are oxidised to iodine by the oxidising agent.

Equation:
2I⁻(aq) → I₂(aq) + 2e⁻

Method 2: Starch-iodide paper

Dip a piece of starch−iodide paper in the unknown solution.

Positive test observation: 
White starch−iodide paper turns blue to blue-black.

Reason:
Iodide ions (I⁻) are oxidised by the oxidising agent to form iodine (I₂).

Equation:
2I⁻(aq) → I₂(aq) + 2e⁻

The iodine then reacts with starch to produce the characteristic blue-black colour. This confirms the presence of an oxidising agent.