Achievers Dream - O & A Level Chemistry Tuition In Singapore
Categories
01 Free Studying Resources 05 Secondary 3 Chemistry

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.
    O.S of Cu²⁺ = +2
  • O.S of N³⁻ = -3
  • The sum of the oxidation states of all elements in a compound is zero.

FeO

2(x) + 3 (-2) = 0

2x -6 = 0

2x = +6

x = +3

The oxidation state of oxide ion is -2.
Let the oxidation state of Fe be x.

The sum of the oxidation states
=charge of the compound which is 0.

  • The sum of the oxidation states of all elements in a polyatomic ion is equals to the charge of the ion.

SO²⁻

x + 4 (-2) = -2

x - 8 = -2

x = +6

The oxidation state of oxide ion is -2.
Let the oxidation state of S be x.

The sum of the oxidation states
=charge of the ion which is -2 for this ion.

  • Some elements have fixed oxidation state in thier compounds.
  • Oxidation State of H in compounds = +1
  • Oxidation State of O in compounds = -2
  • Oxidation State of Group I elements in compounds = +1
  • Oxidation State of Group II elements in compounds = +2

Now that we know what are oxidation states, how do we differentiate between reduction and oxidation in terms of oxidation states?

  • - If oxidation state increases (more positive), the substance is oxidized.
  • - If oxidation state decreases (more negative), the substance is reduced.

Again, let’s look at the reaction between Mg and HCl and explain why it’s a redox reaction in terms of oxidation states.

what is redox

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

   3Cl₂ + 2Fe → 2FeCl

   2FeCl₂ + Cl2FeCl

C      Fe + 2FeCl3FeCl

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
After reaction, oxidation state of Fe in FeCl = +3

Option C:
Before reaction, oxidation state of Fe metal= 0, Fe in FeCl= +3
After reaction, oxidation state of Fe in FeCl₂ = +2

Option D:
Before reaction, oxidation state of Fe in FeO = +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.

what is redox

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?

   It is a reducing agent and it contains bromide ions.

   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, 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? 

   3Cl₂ (g) + 2Fe(s) → 2FeCl (S)

  Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄(S)

C    Fe²⁺ (aq) + Mg(s) → Fe(s) + Mg²⁺(aq)

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

Answer

For option A:
Cl is reduced as the oxidation state of Cl decreases from 0 in Cl₂ to -1 in FeCl.
Fe is oxidized as the oxidation state of Fe increases from 0 in Fe to +3 in FeCl.
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 SO₄²⁻  ion and BaSO 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 Fe²⁺ to 0 in Fe.

Mg is oxidized as the oxidation state of Mg increases from 0 in Mg to +2 in Mg²⁺.
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.

Zn is oxidized as the oxidation state of Zn increases from 0 in Zn to +2 in ZnCl.
Since oxidation and reduction is occurring simultaneously in this reaction, this is a redox reaction.

Therefore, answer is B.

Ammonium compound + Alkali

what is redox

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

what is redox

Decomposition of carbonate

what is redox

Precipitation

what-is-redox

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

   CuO + H   Cu + H₂O

  2SO₂ + O₂   2SO₃

C    2H⁺ + CO₃²⁻  H₂O + CO₂

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

Answer : C