Chapter 12: Redox Reactions

Chapter 12: Redox Reactions

What is Redox?

Redox is a combination of two words: Reduction and Oxidation.

There are 4 definitions that students must know.

Definition	Reduction	Oxidation
Oxygen	Loss	Gain
Hydrogen	Gain	Loss
Electron	Gain	Loss
Oxidation State	Decrease (more negative)	Increase (more positive)

Definition: Oxygen
Reduction	Loss
Oxidation	Gain

Definition: Hydrogen
Reduction	Gain
Oxidation	Loss

Definition: Electron
Reduction	Gain
Oxidation	Loss

Definition: Oxidation state
Reduction	Decrease (more negative)
Oxidation	Increase (more positive)

In this post, we will be focusing on electron transfer and oxidation states, which are more commonly tested in examinations.

Let’s look at the reaction between magnesium metal and hydrochloric acid.

Mg(s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

The ionic equation is

Mg(s) + 2H⁺(aq) → Mg²⁺ (aq) + H₂(g)

• 1. Electron transfer

• Let’s break down the ionic equation further to half equations:

Mg (s) → Mg²⁺ (aq) + 2e- Mg is oxidized as 1 mol of Mg atoms lose 2 mol of electrons to form 1 mol of Mg²⁺ ions. OIL: Oxidation is loss (of electrons). 2H⁺ (aq) + 2e- → H₂ (g) HCl is reduced as 2 mol of H⁺ ions gain 2 mol of electrons to form 1 mol of H₂ molecules. RIG: Reduction is gain (of electrons).

As both reduction and oxidation are taking place simultaneously, this is a redox reaction.

• 2. Oxidation states

• Another way to determine if a reaction is a redox reaction will be by looking at the oxidation states of the element.

• Firstly, what are oxidation states? They are the charge the atom will have if it is an ion.

• The oxidation state of an element in its free form is zero.
• The oxidation state of a monatomic ion is the charge of the ion.

Question 1
In which reaction does the oxidation state of iron remain unchanged?

A     3Cl₂ + 2Fe → 2FeCl₃

B     2FeCl₂ + Cl₂ → 2FeCl₃

C      Fe + 2FeCl₃ → 3FeCl₂

D      Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O

Answer

Option A:
Before reaction, oxidation state of Fe metal = 0
After reaction, oxidation state of Fe in FeCl₃ = +3 Option B:
Before reaction, oxidation state of Fe in FeCl_₂= +2
After reaction, oxidation state of Fe in FeCl_₃ = +3 Option C:
Before reaction, oxidation state of Fe metal= 0, Fe in FeCl_₃ = +3
After reaction, oxidation state of Fe in FeCl₂ = +2 Option D:
Before reaction, oxidation state of Fe in Fe_₂O_₃ = +3
After reaction, oxidation state of Fe in FeCl₃ = +3

Therefore, answer is D.



Oxidising agents vs Reducing agents

Oxidising agent: Substance that oxidises another substance, itself will be reduced.
Reducing agent: Substance that reduces another substance, itself will be oxidized.

Using the same example (when Mg reacts with HCl), Mg is the reducing agent while HCl is the oxidizing agent.



One common oxidizing agent is acidified potassium manganate(VII), KMnO₄. To test for the presence of a reducing agent, we add acidified KMnO₄ to the solution and observe if there is a colour change from purple to colourless.

• If solution remains purple in colour, there is no reducing agent present.
• If solution changes colour from purple to colourless, there is a reducing agent present.

A common reducing agent is acidified potassium iodide, KI. To test for the presence of an oxidizing agent, we add acidified KI and observe if there is a colour change from colourless to brown.

• If solution remains colourless, there is no oxidizing agent present in the solution.
• If the solution changes colour to brown, there is an oxidizing agent present in the solution.

Solution	Acidified KMnO₄	Acidified KI	Conclusion
A	Purple to colourless	Remains colourless.	There is a reducing agent present (as acidified KMnO₄ changes colour from purple to colourless) but no oxidizing agent (as acidified KI remains brown) in Solution A.
B	Remains purple	Colourless to brown	There is an oxidizing agent present (as acidified KI changes from colourless to brown) but no reducing agent (as acidified KMnO4 remains purple) in Solution B.
C	Remains purple	Remains colourless	There are no reducing agent and oxidizing agent in Solution C.
D	Purple to colourless	Colourless to brown	There are both oxidizing agent and reducing agent in Solution D.

Solution: A
Acidified KMNO4	Purple to colourless
Acidified KI	Remains colourless
Conclusion	There is a reducing agent present (as acidified KMnO4 changes colour from purple to colourless) but no oxidizing agent (as acidified KI remains brown) in Solution A.

Solution: B
Acidified KMNO4	Remains purple
Acidified KI	Colourless to brown
Conclusion	There is an oxidizing agent present (as acidified KI changes from colourless to brown) but no reducing agent (as acidified KMnO4 remains purple) in Solution B.

Solution: C
Acidified KMNO4	Remains purple
Acidified KI	Remains colourless
Conclusion	There are no reducing agent and oxidizing agent in Solution C.

Solution: D
Acidified KMNO4	Purple to colourless
Acidified KI	Colourless to brown
Conclusion	There are both oxidizing agent and reducing agent in Solution D.

Question 2
A portion of acidified potassium manganate(VII) solution was added to a sample of solution X. Solution X decolourised the potassium manganate(VII) solution. The resulting solution Y was brown. When starch solution was added to a sample of solution Y, the solution turned dark blue.

What conclusion can be drawn about solution X?

A     It is a reducing agent and it contains bromide ions.

B     It is a reducing agent and it contains iodide ions

C      It is an oxidizing agent and it contains bromide ions.

D      It is an oxidizing agent and it contains iodide ions.

Answer

Acidified potassium manganate(VII), which is an oxidizing agent, is used to test for the presence of reducing agent. Since there is a colour change (purple to colourless), X is a reducing agent.

Furthermore, as the resulting solution Y turned dark blue when starch was added, this indicates that iodine is present in solution Y. How did iodine come about? It is due to the iodide ion initially present in solution X, getting oxidized by oxidizing agent KMnO_₄, to form iodine in Solution Y. Therefore, X contains iodide ions.

Answer : B

Non-redox reactions

Students are recommended to remember these 4 types of non-redox reactions:

A – Ammonium compound + alkali

N – Neutralisation

D – Decomposition of carbonate

P – Precipitation

These reactions may appear in Paper 1 where the question requires student to identify which reactions are redox and which are not. An example is shown below.

Question 3
Which equation does not represent a redox reaction? 

A     3Cl₂ (g) + 2Fe(s) → 2FeCl₃ (S)

B    Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄(S)

C    Fe^²⁺ (aq) + Mg(s) → Fe(s) + Mg²⁺(aq)

D   Zn(s) + 2HCl (aq) → ZnCl_₂ (aq) + H₂(g)

Answer

For option A:
Cl₂ is reduced as the oxidation state of Cl decreases from 0 in Cl₂ to -1 in FeCl_₃.
Fe is oxidized as the oxidation state of Fe increases from 0 in Fe to +3 in FeCl_₃.
Since oxidation and reduction is occurring simultaneously in this reaction, this is a redox reaction. For option B:
The oxidation state of Ba in BaSO₄ is +2. There, is no change in the oxidation state of Ba before and after the reaction.
The oxidation state of O in SO₄^²⁻  ion and BaSO_₄ is +6. There is no change in the oxidation state of O before and after the reaction.
Students should also realise that this reaction is a precipitation reaction, which is a non-redox reaction. Thus, the answer is B. For option C:
Fe²⁺ is reduced as the oxidation state of Fe decreases from +2 in Fe^²⁺ to 0 in Fe.
Mg is oxidized as the oxidation state of Mg increases from 0 in Mg to +2 in Mg^²⁺.
Since oxidation and reduction is occurring simultaneously in this reaction, this is a redox reaction. For option D:
HCl  is reduced as the oxidation state of H decreases from +1 in HCl to 0 in H_₂.
Zn is oxidized as the oxidation state of Zn increases from 0 in Zn to +2 in ZnCl_₂.
Since oxidation and reduction is occurring simultaneously in this reaction, this is a redox reaction.

Therefore, answer is B.



Ammonium compound + Alkali



As there is no change in oxidation states of the elements before and after the reaction, the reaction between ammonia compound and alkali is not a redox reaction.

The following reactions below are also not redox reactions.

Neutralisation



Decomposition of carbonate



Precipitation

Question 4
Which reaction does not involve either oxidation or reduction? 

A     CuO + H_₂  → Cu + H₂O

B    2SO₂ + O₂  → 2SO₃

C    2H⁺ + CO₃²⁻ → H₂O + CO₂

D   Cl₂(g) + 2NaOH  → NaCl + NaOCl + H₂O

Answer : C