Meaning of Redox Reaction
A redox reaction is a chemical reaction in which oxidation and reduction occur simultaneously.
The word redox comes from:
RED → Reduction
OX → Oxidation
In every redox reaction:
One substance is oxidized
Another substance is reduced
They always happen together — you cannot have oxidation without reduction.
Definitions of Oxidation
Oxidation can be defined in several ways depending on the concept used:
1. Oxidation in terms of Oxygen
Oxidation is the addition of oxygen to a substance.
Example:
2Mg + O2 →2MgO
Magnesium is oxidized because it gains oxygen.
2. Oxidation in terms of Hydrogen
Oxidation is the removal of hydrogen from a substance.
Example:
H2S → S + H2
Hydrogen sulphide is oxidized because hydrogen is removed.
3. Oxidation in terms of Electrons
Oxidation is the loss of electrons by a substance.
Example:
Zn →Zn2+ + 2e-
Zinc is oxidized because it loses electrons.
4. Oxidation in terms of Oxidation Number
Oxidation is an increase in oxidation number.
Example:
Fe²⁺ → Fe³⁺
Iron is oxidized because its oxidation number increases from +2 to +3.
5. Oxidation in terms of Electrochemistry
Oxidation occurs at the anode in an electrochemical cell.
Definitions of Reduction
Reduction is the opposite of oxidation and can also be defined in different ways:
1. Reduction in terms of Oxygen
Reduction is the removal of oxygen from a substance.
Example:
CuO + H2 →Cu + H2O
Copper (II) oxide is reduced because oxygen is removed.
2. Reduction in terms of Hydrogen
Reduction is the addition of hydrogen to a substance.
Example:
N2 + 3H2 → 2NH3
Nitrogen is reduced because it gains hydrogen.
3. Reduction in term of Electrons
Reduction is the gain of electrons by a substance.
Example:
Cu2+ + 2e- →Cu
Copper ions are reduced because they gain electrons.
4. Reduction in terms of Oxidation Number
Reduction is a decrease in oxidation number.
Example:
Cl₂0 → Cl⁻-1
0 -1
Chlorine is reduced because its oxidation number decreases.
5. Reduction in terms of Electrochemistry
Reduction occurs at the cathode in an electrochemical cell.
Oxidizing and Reducing Agents
Oxidizing Agent
An oxidizing agent is a substance that:
Causes oxidation of another substance
Itself gets reduced
Example:
KMnO₄, O₂, Cl₂
Reducing Agent
A reducing agent is a substance that:
Causes reduction of another substance
Itself gets oxidized
Example:
Zn, H₂, CO
Example of a Redox Reaction
Zn + CuSO4 → ZnSO4 + Cu
Zinc is oxidized (loses electrons)
Copper (II) ions are reduced (gain electrons)
Zinc is the reducing agent
Copper (II) sulphate is the oxidizing agent
Importance and Examples of Redox Reactions
Redox reactions are very important in daily life and industry, such as:
Respiration in living cells
Rusting of iron
Burning of fuels
Electrolysis
Production of metals
Batteries and cells
Photosynthesis
Summary Table
| Oxidation | Reduction |
|---|---|
| Gain of oxygen | Loss of oxygen |
| Loss of hydrogen | Gain of hydrogen |
| Loss of electrons | Gain of electrons |
| Increase in oxidation number | Decrease in oxidation number |
| Occurs at anode | Occurs at cathode |
Test for oxidizing agents
To common test or reactions that are used to test for an oxidizing agent involves the action on iron (II) chloride and hydrogen sulphide.
a) Reaction with FeCl2
When an oxidizing agent is added to green iron (II chloride; the iron (II) ions become oxidized to yellow or brown Fe3+.
Fe2+ → Fe3+ + e–
green yellow/brown
b) Reaction with hydrogen sulphide
When hydrogen sulphide is bubbled through a solution of an oxidizing agent, the sulphide ions S2– becomes oxidized to elemental sulphur; and this is seen or observed as yellow deposits sulphur, i.e. S2– → S(s) + 2e–.
Test for reducing agents
Two commonest reagents that are used to test for a reducing agent are
1 Acidified potassium tetraoxomanganate(VI) (KMnO4) and acidified potassium heptaoxodichromate(I) (K2Cr2O7).
a) Action of potassium hyptaoxodichromate (VI) (K2Cr2O7)
When acidified potassium heptaoxodichromate (VI) (K2Cr2O7) is added to a sample of a reducing agent, its colour changes from orange to green, due to the reduction of the dichromate (VI) ion (Cr6+) (orange) to chromium (III) (Cr3+) ion green
Cr6+ + 3e– → Cr3+Orange green
When acidified potassium tetraoxomanganate (VII) to a sample of reducing agent, the purple colour changes to colourless: due to the reduction of the manganate ion from (+7) which is purple to (+2) which is colourless and a more stable oxidation state.
MnO4- + 8H+ + 5e– →Mn2+ + 4H2O
purple colourless
Mn7+ + 5e– → Mn2
purple colourless
This reaction is reversible as the purple colour is restored when an oxidizing agent is reintroduced into the mixture.
Mn2+ + 5e →Mn7+colourless purple
OBJECTIVE QUESTIONS
1. A redox reaction is one in which
A. acids react with bases
B. electrons are transferred
C. salts are formed
D. heat is evolved
2. Oxidation is the process involving
A. gain of electrons
B. loss of electrons
C. gain of neutrons
D. loss of protons
3. Reduction is best defined as
A. loss of hydrogen
B. gain of oxygen
C. gain of electrons
D. loss of oxygen and hydrogen
4. In the reaction
Zn + Cu²⁺ → Zn²⁺ + Cu
the species oxidized is
A. Cu²⁺
B. Zn²⁺
C. Zn
D. Cu
5. Which of the following is a reducing agent?
A. A substance that gains electrons
B. A substance that is reduced
C. A substance that loses electrons
D. A substance that gains oxygen
6. Which of the following statements is correct?
A. Oxidation and reduction occur separately
B. Reduction involves loss of electrons
C. Oxidation involves gain of electrons
D. Oxidation and reduction occur simultaneously
7. In terms of oxygen, oxidation is defined as
A. removal of oxygen
B. addition of oxygen
C. removal of hydrogen
D. addition of hydrogen
8. In the reaction
2Mg + O₂ → 2MgO
Magnesium is
A. reduced
B. oxidized
C. neutralized
D. displaced
9. Which of the following is an oxidizing agent?
A. A substance that is oxidized
B. A substance that loses electrons
C. A substance that gains electrons
D. A substance that donates protons
10. The oxidation number of chlorine in Cl₂ is
A. +1
B. −1
C. 0
D. +2
11. Reduction involves
A. increase in oxidation number
B. decrease in oxidation number
C. no change in oxidation number
D. increase in atomic mass
12. Which reaction is a redox reaction?
A. NaOH + HCl → NaCl + H₂O
B. AgNO₃ + NaCl → AgCl + NaNO₃
C. Zn + H₂SO₄ → ZnSO₄ + H₂
D. CaCO₃ → CaO + CO₂
13. The electrode at which oxidation occurs is called the
A. cathode
B. anode
C. electrolyte
D. salt bridge
14. Which of the following processes is an example of oxidation?
A. Rusting of iron
B. Freezing of water
C. Melting of ice
D. Dissolving sugar
15. A substance that causes reduction and is itself oxidized is called
A. oxidizing agent
B. electrolyte
C. reducing agent
D. catalyst
16. In the reaction
Fe²⁺ → Fe³⁺ + e⁻
iron is
A. reduced
B. oxidized
C. neutralized
D. displaced
17. Which of the following is NOT a redox reaction?
A. Combustion
B. Respiration
C. Electrolysis
D. Neutralization
18. In a redox reaction, electrons are transferred from
A. oxidizing agent to reducing agent
B. reducing agent to oxidizing agent
C. acid to base
D. salt to water
19. Which of the following changes represents reduction?
A. Cu → Cu²⁺
B. Fe²⁺ → Fe³⁺
C. Cl₂ → 2Cl⁻
D. Na → Na⁺
20. The main feature of a redox reaction is
A. formation of precipitate
B. evolution of gas
C. transfer of electrons
D. formation of salt
THEORY QUESTIONS
Short Answer Questions
1. Define oxidation and reduction using the electron concept.
2. What is a redox reaction?
3. State two ways by which oxidation can be defined.
4. State two ways by which reduction can be defined.
5. What is meant by an oxidizing agent?
6. What is meant by a reducing agent?
7. Why must oxidation and reduction always occur together?
8. Identify the oxidizing and reducing agents in the reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
Structured / Calculation-Based Questions
9. For the reaction:
2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺
(a) State the species oxidized
(b) State the species reduced
(c) Identify the oxidizing agent
(d) Identify the reducing agent
10. In the reaction:
Mg + 2HCl → MgCl₂ + H₂
(a) Which substance is oxidized?
(b) Which substance is reduced?
(c) State the role of magnesium.
Essay Questions
11. Define oxidation and reduction. Explain each using:
(a) oxygen
(b) hydrogen
(c) electrons
(d) oxidation numbers.
12. With examples, explain what is meant by oxidizing and reducing agents.
13. Describe an experiment to show that oxidation and reduction occur simultaneously.
14. Explain the importance of redox reactions in everyday life and industry.
15. Using suitable examples, distinguish between oxidation and reduction.
Practical-Oriented Questions
16. Explain how rusting of iron is a redox reaction.
17. Describe the redox processes that occur during electrolysis of molten sodium chloride.
18. Explain how respiration in living cells is a redox process.
19. Balance the following redox equation using the ion-electron method:
MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺ (in acidic medium)
20. State three industrial applications of redox reactions.