HighMarks

Redox reactions — KCSE Chemistry

KCSE Chemistry · 44 practice questions · 3 syllabus objectives · 3 revision lessons

16 easy15 medium13 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Assign oxidation states to atoms in compounds and ions

Define oxidation and reduction in terms of electron transfer and change in oxidation number

Identify and explain redox reactions, oxidising agents and reducing agents

Revision Notes

Concise lesson notes for Redox reactions, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Assigning Oxidation States in Compounds

Oxidation states are important for understanding redox reactions. To assign oxidation states, follow these rules:

  • Elemental state: The oxidation state of an element in its pure form is 0.
  • Monatomic ions: The oxidation state is equal to the charge of the ion. For example, Na^+ has an oxidation state of +1.
  • Oxygen: Usually has an oxidation state of -2, except in peroxides where it is -1.
  • Hydrogen: Generally has an oxidation state of +1, except when bonded to metals, where it is -1.
  • Sum of oxidation states: In a neutral compound, the sum of oxidation states equals 0. In a polyatomic ion, it equals the ion's charge.

For example, in H₂O:

  • Hydrogen (H) = +1 (2 H atoms contribute +2)
  • Oxygen (O) = -2
  • Total = +2 + (-2) = 0, confirming the compound is neutral.

In SO₄²⁻:

  • Let sulfur (S) be x. Oxygen = -2 (4 O atoms contribute -8)
  • x + (-8) = -2 → x = +6, so S has an oxidation state of +6.

Key points to remember

  • Oxidation state of an element in pure form is 0.
  • Monatomic ion's oxidation state equals its charge.
  • Oxygen usually has an oxidation state of -2.
  • In compounds, the total oxidation states equal 0.
  • In ions, the total equals the ion's charge.

Worked example

Assign oxidation states in NaCl: Na = +1, Cl = -1; Total = +1 + (-1) = 0.

Read all 3 Redox reactions lessons free

Sign up free to unlock the full set of revision notes, all 44 practice questions with marking schemes, plus a personalised study plan that adapts to the topics you keep getting wrong.

More lessons in this topic

Lesson 2: Understanding Oxidation and Reduction

Objective: Define oxidation and reduction in terms of electron transfer and change in oxidation number

In redox reactions, oxidation and reduction are defined based on electron transfer and changes in oxidation numbers:

  • Oxidation: This is the process where a substance loses electrons and its oxidation number increases.
  • Reduction: This is the process where a substance gains electrons and its oxidation number decreases.

To determine oxidation states, remember the following rules:

  • The oxidation state of an element in its standard state is 0.
  • For monoatomic ions, the oxidation state is equal to the charge of the ion.
  • In compounds, the sum of oxidation states must equal the overall charge of the compound.

For example, in the reaction:

Zn + Cu^2+ → Zn^2+ + Cu

  • Oxidation: Zinc (Zn) loses 2 electrons, changing from 0 to +2.
  • Reduction: Copper (Cu^2+) gains 2 electrons, changing from +2 to 0.

Thus, zinc is oxidized, and copper is reduced.

  • Oxidation involves loss of electrons and increase in oxidation number.
  • Reduction involves gain of electrons and decrease in oxidation number.
  • Identify oxidation states using standard rules for elements and compounds.
  • In redox reactions, one substance is oxidized while another is reduced.

Define oxidation and reduction in terms of electron transfer. Oxidation is the loss of electrons, increasing oxidation number. Reduction is the gain of electrons, decreasing oxidation number.

Lesson 3: Understanding Redox Reactions

Objective: Identify and explain redox reactions, oxidising agents and reducing agents

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between substances. In these reactions, one substance is oxidized (loses electrons) while another is reduced (gains electrons). This electron transfer is crucial in various chemical processes.

Key Components:

  • Oxidizing Agent: The substance that gains electrons and is reduced.
  • Reducing Agent: The substance that loses electrons and is oxidized.

Example: In the reaction between zinc and copper sulfate:

  • Zinc (Zn) is oxidized to zinc ions (Zn²⁺), losing electrons.
  • Copper ions (Cu²⁺) are reduced to copper metal (Cu), gaining electrons.

Thus, zinc acts as the reducing agent, while copper sulfate acts as the oxidizing agent. Understanding these roles helps in identifying redox reactions in various contexts, such as in batteries and metabolic processes in living organisms.

  • Redox reactions involve electron transfer between substances.
  • Oxidizing agents gain electrons and are reduced.
  • Reducing agents lose electrons and are oxidized.
  • Identifying agents helps in understanding chemical reactions.
  • Examples include metals reacting with metal salts.

Identify the oxidizing and reducing agents in the reaction:
2H₂ + O₂ → 2H₂O.

  • Hydrogen (H₂) is the reducing agent (oxidized).
  • Oxygen (O₂) is the oxidizing agent (reduced).

Sample Questions

Read 3 questions and answers free. Sign up to access all 44 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

Describe the process of a redox reaction using the example of iron(III) oxide and carbon. Identify the oxidising agent and reducing agent. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
Iron(III) oxide (Fe₂O₃) is the oxidising agent (1 mk)
It gains electrons and is reduced to iron (1 mk)
Part (b) — 2 marks
Carbon (C) is the reducing agent (1 mk)
It donates electrons and is oxidised to carbon dioxide (CO₂) (1 mk)
2
easySHORT ANSWER4 marks

State the oxidation states of chlorine in the following compounds: (a) NaCl (b) Cl₂ (c) ClO₃ (d) HCl. (4 marks)

Answer & marking scheme

Part (a) — 1 mark
Chlorine in NaCl has an oxidation state of -1 (1 mk)
Part (b) — 1 mark
Chlorine in Cl₂ has an oxidation state of 0 (1 mk)
Part (c) — 1 mark
Chlorine in ClO₃ has an oxidation state of +5 (1 mk)
Part (d) — 1 mark
Chlorine in HCl has an oxidation state of -1 (1 mk)
3
easySHORT ANSWER4 marks

Explain how the reaction between magnesium and oxygen demonstrates a redox reaction. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
Magnesium is oxidised because it loses electrons (1 mk)
Its oxidation state increases from 0 to +2 (1 mk)
Part (b) — 2 marks
Oxygen is reduced because it gains electrons (1 mk)
Its oxidation state decreases from 0 to -2 (1 mk)
4

Define the terms 'oxidising agent' and 'reducing agent' in the context of redox reactions. (4 marks)

+41 More Questions

Sign up free to access all 44 questions with marking schemes, track your progress, and get personalised recommendations.

Frequently asked questions

What does the KCSE Chemistry topic "Redox reactions" cover?

Electron transfer, oxidation numbers, displacement reactions as redox, identifying oxidising and reducing agents

How many practice questions are available for Redox reactions?

HighMarks has 44 Redox reactions practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Redox reactions for the KCSE exam?

Start with the revision notes on this page to refresh the core concepts, then work through the practice questions in increasing difficulty. Sign up for HighMarks to get a personalised study plan that adapts to the topics you keep getting wrong, plus mock exams, subject-wide practice, and detailed performance tracking. See pricing.

Why Practise Redox reactions?

KNEC Aligned

Questions match the KCSE syllabus objectives and exam format exactly.

Detailed Marking Schemes

Every answer shows exactly what examiners award marks for.

Track Your Mastery

See your score improve as you practise and identify remaining gaps.

Related topics in KCSE Chemistry

Continue revising adjacent topics from the Chemistry syllabus.

Chemistry
Detergents and polymers
Chemistry
Alkynes
Chemistry
Chemical families and group properties
Chemistry
Radioactivity
Chemistry
Corrosion and prevention
Chemistry
Extraction of metals

More KCSE resources

Round out your Chemistry preparation with full mock exams, past papers, and every Chemistry topic in one place.

All Chemistry topics

Browse every Chemistry revision topic on HighMarks.

KCSE past papers

Years of past KCSE papers with worked answers.

KCSE mock exams

Full-length, timed mocks scored against KNEC-style rubrics.

KCSE practice questions

All KCSE practice questions across every subject.

Master Redox reactions for KCSE

Sign up free to unlock all 44 questions, track your progress, and get a personalised study plan for Chemistry.