MINISTRY OF EDUCATION
KASEMPA DAY SECONDARY SCHOOL
CHEMISTRY TEACHING MODULE FORM 2 TERM 3
DEPARTMENT: NATURAL SCIENCE NAME OF TEACHER: MR MASUMBA .D.
TOPIC 1: REDOX REACTIONS
INTRODUCTION
Redox reactions are chemical reactions in which oxidation and reduction occur simultaneously. In everyday life in Zambia learners meet redox processes in iron corrosion (rusting of nails), in the use of batteries, and during the reaction of metals with acids. Understanding oxidation and reduction helps learners interpret why some materials lose electrons (are oxidised) and others gain electrons (are reduced). This topic focuses on the sub-topic Oxidation and Reduction and shows how to analyse reactions by oxygen/hydrogen exchange, by electron transfer and by changes of oxidation number. Practical identification of oxidising and reducing agents using common reagents such as potassium iodide, potassium permanganate and acidified potassium dichromate will be modelled with simple apparatus and local examples (e.g., galvanised nails, copper coins).
GENERAL COMPETENCES:
• Analytical Thinking
• Collaboration
• Communication
• Critical Thinking
• Problem Solving
SUB-TOPIC 1: OXIDATION AND REDUCTION
INTRODUCTION
This sub-topic explains what is meant by oxidation and reduction in three complementary ways: (a) loss/gain of oxygen or hydrogen, (b) transfer of electrons between species, and (c) change in oxidation numbers. Learners will examine example reactions (such as a zinc strip in copper(II) sulphate solution) to identify oxidised and reduced species, identify oxidising and reducing agents, practise assigning oxidation numbers to elements with variable states, and distinguish redox from non-redox reactions. Practical tests using potassium iodide, acidified potassium permanganate and acidified potassium dichromate are included to identify oxidising and reducing agents by observed colour changes.
SPECIFIC COMPETENCE(S):
Interpret redox reactions
KEY TERMS:
• Oxidation - a process in which a species loses electrons, or gains oxygen, or loses hydrogen
• Reduction - a process in which a species gains electrons, or gains hydrogen, or loses oxygen
• Oxidising agent - a substance that causes another species to be oxidised and is itself reduced
• Reducing agent - a substance that causes another species to be reduced and is itself oxidised
• Oxidation number - a signed number that indicates the degree of oxidation of an atom in a compound or ion
• Half-reaction - the separate oxidation or reduction equation showing electrons transferred
• Non-redox reaction - a chemical reaction where there is no change in oxidation numbers (no oxidation or reduction)
• Electron transfer - the movement of electrons from one species to another in a redox process
KEY CONCEPTS:
• Oxidation and reduction can be defined by oxygen/hydrogen exchange, by electron transfer, or by changes in oxidation numbers.
• In a redox reaction one species is oxidised (loses electrons) while another is reduced (gains electrons); electrons must balance when half-reactions are combined.
• Oxidising agents accept electrons and are reduced; reducing agents donate electrons and are oxidised.
• Oxidation numbers are assigned by a set of rules and are used to detect redox changes for elements with variable oxidation states.
• Some reactions (acid–base neutralisation, precipitation without change in oxidation states) are non-redox.
• Laboratory tests (KI + starch; acidified potassium permanganate; acidified potassium dichromate) indicate oxidising or reducing behaviour by characteristic colour changes.
LEARNING ACTIVITIES
LEARNING ACTIVITY 1: ANALYSING OXIDATION AND REDUCTION IN TERMS OF OXYGEN/HYDROGEN EXCHANGE, ELECTRON TRANSFER, AND OXIDATION STATE CHANGES
This activity helps learners recognise the three equivalent descriptions of oxidation and reduction and practise converting between them.
ACTIVITY 1.1: CLASS DEMONSTRATION AND GROUP ANALYSIS OF METAL DISPLACEMENT
Purpose: Show a zinc strip placed in copper(II) sulphate solution and analyse the process by oxygen/hydrogen exchange, electron transfer, and oxidation numbers.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: zinc strip (or galvanised nail), copper(II) sulphate solution (CuSO4), beaker, forceps, safety goggles, white sheet of paper
• Natural Materials: copper coin (for comparison with deposited copper)
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Chemistry laboratory bench with running water and waste container
• Natural Environment: School compound for observation of rusting (related example)
HOOK: If a galvanised nail is placed in copper sulphate solution, why does the nail remain metallic while reddish copper appears on its surface?
TEACHER'S ROLES:
• Demonstrate placing the zinc strip into CuSO4 solution while asking learners to predict observations.
• Guide learners to observe the copper deposition and change in solution colour.
• Lead questions to elicit oxygen/hydrogen exchange view, ask for electron transfer interpretation, and finally show oxidation number changes.
• Write the half-reactions and overall equation on the board and balance electrons.
• Summarise on the board: which species is oxidised, which reduced, and identify oxidising/reducing agents.
LEARNERS' TASKS:
• Observe the demonstration and record visual changes (copper deposit, fading blue colour).
• In groups, write half-reactions, assign oxidation numbers, and identify oxidised and reduced species.
• Present group findings to the class.
ASSESSMENT:
• Collect group worksheets showing half-reactions, oxidation numbers and identification of agents.
• Oral questioning: learners must state which species donates electrons and which accepts them; teacher checks balanced electron transfer.
EXPLANATION (CONCEPTUALISATION):
Example reaction: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Separate ionic species: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Oxidation half-reaction: Zn(s) → Zn2+ + 2e-
Reduction half-reaction: Cu2+ + 2e- → Cu(s)
Electron transfer: 2e- transferred from Zn to Cu2+.
Oxidation numbers: Zn goes from 0 → +2 (oxidised); Cu goes from +2 → 0 (reduced).
Oxidising agent: Cu2+ (it oxidises Zn); Reducing agent: Zn(s).
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
• Problem Solving
LEARNING ACTIVITY 2: EXAMINING A REDOX REACTION (REACTION INVOLVING BOTH OXIDATION AND REDUCTION)
This activity deepens learners’ ability to deduce half-reactions and combine them to produce a balanced redox equation.
ACTIVITY 2.1: PAIR WORK — DEDUCE HALF-REACTIONS FOR GIVEN REACTIONS
Purpose: Learners practise splitting reactions into oxidation and reduction halves and balancing electrons.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: worksheets with example equations (e.g., 2Fe + 3Cl2 → 2FeCl3, H2 + Cl2 → 2HCl), pens, calculators
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Classroom with chalkboard and group desks
HOOK: Which atoms change their oxidation numbers when iron reacts with chlorine gas to form iron(III) chloride?
TEACHER'S ROLES:
• Provide step-by-step modelling for one example on the board.
• Circulate and support pairs as they assign oxidation numbers and balance half-reactions.
• Lead plenary to compare methods and common errors.
LEARNERS' TASKS:
• Assign oxidation numbers, write oxidation and reduction half-reactions, and show balanced overall equations.
• Present one worked example to the class.
ASSESSMENT:
• Review written group work for correct half-reactions and electron balance.
• Spot-check common misconceptions (wrong oxidation states for chlorine in compounds).
EXPLANATION (CONCEPTUALISATION):
Example: 2Fe(s) + 3Cl2(g) → 2FeCl3(s)
Oxidation: Fe(s) → Fe3+ + 3e-
Reduction: Cl2(g) + 2e- → 2Cl-
Balance electrons: multiply the Fe half by 2 and the Cl half by 3 then combine.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
LEARNING ACTIVITY 3: IDENTIFYING THE CHARACTERISTICS OF OXIDISING AND REDUCING AGENTS
This activity helps learners recognise the defining features and common examples of oxidising and reducing agents.
ACTIVITY 3.1: CLASSIFICATION EXERCISE WITH REAGENT CARDS
Purpose: Learners sort reagents into oxidising agents and reducing agents and justify choices.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: reagent cards (KI, KMnO4, K2Cr2O7, H2, Zn), marker cards
• Natural Materials: galvanised nail, charcoal (as reducing example)
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Classroom with group tables
HOOK: Which is more likely to appear as brown/blue with starch: iodine formed by oxidation of iodide, or potassium permanganate itself?
TEACHER'S ROLES:
• Provide reagent cards and lead discussion on behaviour and typical colour changes.
• Ask probing questions: what does it mean to accept electrons? Give local examples.
• Guide learners to link reagent behaviour to oxidising/reducing definitions.
LEARNERS' TASKS:
• Sort reagent cards, give reasons, and predict outcomes when paired with a test species.
ASSESSMENT:
• Use quick oral questions and group justifications; collect written cards with reasons.
EXPLANATION (CONCEPTUALISATION):
Oxidising agents accept electrons (e.g., KMnO4, K2Cr2O7, acidified nitrate). Reducing agents donate electrons (e.g., Zn, H2, I- under certain conditions). Identify by observing which species undergoes decrease or increase in oxidation number.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
LEARNING ACTIVITY 4: OXIDISING AGENTS (IDENTIFIED USING POTASSIUM IODIDE SOLUTION AS A REDUCING AGENT IN THE PRESENCE OF STARCH OR ACIDIFIED POTASSIUM IODIDE PAPER)
This activity demonstrates how oxidising agents convert iodide to iodine, detected with starch.
ACTIVITY 4.1: PRACTICAL TEST — KI + STARCH PAPER TO DETECT OXIDISERS
Purpose: Learners test unknown solutions with acidified KI and starch to identify oxidising agents.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: 0.1 M potassium iodide (KI) solution, dilute acid (H2SO4 or HCl) for acidification, starch solution or starch-impregnated paper, test tubes, pipettes, protective gloves, safety goggles
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Chemistry laboratory with sink and waste disposal
HOOK: A pale brown solution turns blue-black when a drop of starch is added. What happened to the iodide?
TEACHER'S ROLES:
• Demonstrate safe preparation of acidified KI and addition to a sample.
• Show how starch indicator is used and what a positive result looks like.
• Emphasise safety: use small volumes and proper disposal.
LEARNERS' TASKS:
• Perform tests on labelled unknown solutions in small groups, record colour changes and interpret results.
ASSESSMENT:
• Evaluate learners’ recorded observations and conclusions; ask students to write balanced half-reaction showing I- → I2.
EXPLANATION (CONCEPTUALISATION):
Oxidising agent reacts with iodide ions:
2I- → I2 + 2e- (oxidation)
I2 + starch → blue-black complex (visual indicator)
Thus appearance of blue-black indicates an oxidising agent was present.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
• Practical Manipulation
LEARNING ACTIVITY 5: REDUCING AGENTS (IDENTIFIED USING ACIDIFIED POTASSIUM DICHROMATE OR POTASSIUM PERMANGANATE AS OXIDIZING AGENTS AND OBSERVE COLOUR CHANGES ONLY)
This activity shows how strong oxidising reagents change colour when reduced by reducing agents.
ACTIVITY 5.1: PRACTICAL DEMONSTRATION — COLOUR CHANGE OF KMNO4 AND K2CR2O7
Purpose: Learners observe purple KMnO4 becoming colourless or brown, and orange dichromate becoming green when reduced by a reducing agent.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: dilute KMnO4 solution, acidified K2Cr2O7, reducing samples (e.g., iron filings, zinc dust), test tubes, pipettes, safety goggles, gloves
• Natural Materials: rusted iron sample (for discussion)
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Laboratory with fume extraction if available
HOOK: Why does purple permanganate disappear when added to a sample of iron filings and acid?
TEACHER'S ROLES:
• Demonstrate addition of reducing sample to KMnO4 and acidified dichromate, pointing out colour changes and linking to redox half-reactions.
• Explain the meaning of colour change in terms of change of oxidation state (Mn7+ → Mn2+, Cr6+ → Cr3+).
LEARNERS' TASKS:
• Observe and record colour changes and suggest half-reactions for oxidant reduction.
• Explain which reagent is oxidised and which is reduced.
ASSESSMENT:
• Check written half-reactions and identification of reducing agent; oral questioning for justification.
EXPLANATION (CONCEPTUALISATION):
Example: Permanganate in acidic solution is reduced:
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
Colour change: purple → colourless (or pale pink) indicates reduction of Mn7+.
Acidified dichromate: Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O (orange → green).
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
• Practical Manipulation
LEARNING ACTIVITY 6: DETERMINING OXIDATION NUMBERS OF ELEMENTS WITH VARIABLE OXIDATION STATES
This activity teaches rules and practice for assigning oxidation numbers to atoms in compounds and ions.
ACTIVITY 6.1: WORKSHEET PRACTICE — ASSIGN OXIDATION NUMBERS
Purpose: Learners assign oxidation numbers in examples such as H2O, H2O2, ClO3-, SO42-, MnO4- and identify changes in redox reactions.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: worksheet, periodic table, pens
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Classroom with board
HOOK: What is the oxidation number of sulphur in SO42- and how do you find it?
TEACHER'S ROLES:
• Explain the rules for oxidation numbers and model several examples on the board.
• Provide guided practice and correct common errors (e.g., mis-assigning hydrogen in metal hydrides).
LEARNERS' TASKS:
• Complete worksheet problems in pairs; show steps used to assign numbers.
ASSESSMENT:
• Teacher marks worksheets focusing on method and correct assignments; oral mini-quiz at the end.
EXPLANATION (CONCEPTUALISATION):
Rules (summary):
• Element in free form has oxidation number 0 (e.g., O2, Zn).
• Monatomic ion oxidation number equals its charge (e.g., Na+ = +1).
• Oxygen usually −2 (exceptions: peroxides O22- where O = −1).
• Hydrogen usually +1 when bonded to non-metals, −1 in metal hydrides.
• Sum of oxidation numbers in a neutral compound = 0; in an ion = ion charge.
Example: In SO42-, let sulphur be x: x + 4(−2) = −2 → x = +6.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
LEARNING ACTIVITY 7: DEDUCING A REDOX REACTION USING OXIDATION NUMBERS
This activity consolidates assigning oxidation numbers to deduce which elements are oxidised or reduced and to balance electrons.
ACTIVITY 7.1: GROUP TASK — IDENTIFY REDOX PAIRS AND BALANCE BY INSPECTION
Purpose: Use oxidation numbers to determine changing elements and to write balanced redox equations.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: problem set with reactions (e.g., Cl2 + H2O → HCl + HOCl; Fe2+ + MnO4- + H+ → Fe3+ + Mn2+), chalkboard
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Classroom
HOOK: Which species is oxidised when hypochlorous acid forms from chlorine in water?
TEACHER'S ROLES:
• Model one complex example using oxidation numbers to find electrons transferred, then ask groups to attempt others.
• Provide hints for balancing electrons and for acidic/alkaline medium adjustments.
LEARNERS' TASKS:
• Assign oxidation numbers, identify oxidised and reduced elements, write half-reactions and combine them.
ASSESSMENT:
• Mark group answers for correct identification, balanced equations and explanation of electron transfers.
EXPLANATION (CONCEPTUALISATION):
Use oxidation numbers to detect change: if oxidation number increases the element is oxidised; if it decreases it is reduced. Use half-reactions to balance electrons before adding.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
LEARNING ACTIVITY 8: DESCRIBING A NON-REDOX REACTION (REACTION IN WHICH THERE IS NEITHER OXIDATION NOR REDUCTION INVOLVED)
This activity distinguishes reactions with no change in oxidation numbers from redox reactions.
ACTIVITY 8.1: COMPARE AND CONTRAST — NEUTRALISATION VS REDOX
Purpose: Learners compare neutralisation reaction (e.g., HCl + NaOH → NaCl + H2O) with redox reactions and explain why one is non-redox.
SUGGESTED TEACHING AND LEARNING MATERIALS:
• Artificial Materials: examples on worksheet, pH indicators for demonstration
LEARNING ENVIRONMENT SET-UP:
• Artificial Environment: Classroom or laboratory bench
HOOK: When hydrochloric acid neutralises sodium hydroxide to form salt and water, do any atoms change their oxidation number?
TEACHER'S ROLES:
• Demonstrate a neutralisation reaction and lead learners to assign oxidation numbers to show no change.
• Highlight differences with a redox example.
LEARNERS' TASKS:
• Assign oxidation numbers for all species in the neutralisation example and confirm there is no change.
• Explain in writing why neutralisation is non-redox.
ASSESSMENT:
• Collect written explanations showing oxidation numbers and final conclusion that there is no redox change.
EXPLANATION (CONCEPTUALISATION):
Example: HCl + NaOH → NaCl + H2O
Oxidation numbers remain: H(+1), Cl(−1), Na(+1), O(−2). No element changes oxidation number; therefore the reaction is non-redox.
COMPETENCIES DEVELOPED:
• Analytical Thinking
• Communication
• Collaboration
• Critical Thinking
EXPECTED STANDARD(S):
Ability to interpret and analyse redox reactions
ASSESSMENT QUESTIONS
PART A: MULTIPLE CHOICE QUESTIONS
1. In the reaction Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s), which species is the reducing agent?
A. Zn(s)
B. CuSO4(aq)
C. Cu(s)
D. SO42-
2. Which statement is correct about an oxidising agent?
A. It donates electrons and is oxidised.
B. It accepts electrons and is reduced.
C. It always contains oxygen.
D. It is always a metal.
3. What is the oxidation number of sulphur in SO42-?
A. +4
B. +6
C. −2
D. +2
4. A solution turns blue-black when acidified KI and starch are added. The blue-black colour indicates:
A. Presence of a reducing agent that produces iodine
B. Presence of an oxidising agent that oxidises iodide to iodine
C. That the solution is neutral
D. That permanganate is present
(Answers: 1 A; 2 B; 3 B; 4 B)
PART B: SHORT ANSWER QUESTIONS
1. Define oxidation in terms of electron transfer and give one example equation.
2. Assign oxidation numbers to each atom in KMnO4 and identify the oxidation number of Mn.
3. Write the two half-reactions and the balanced overall redox equation for the reaction between iron and chlorine: 2Fe + 3Cl2 → 2FeCl3.
4. Explain briefly why the neutralisation reaction HCl + NaOH → NaCl + H2O is not a redox reaction.
PART C: ESSAY/APPLICATION QUESTIONS
1. A student places a strip of zinc metal into a solution of copper(II) sulphate. After some time copper is deposited on the zinc strip and the solution becomes colourless. Explain, using electron transfer, oxidation numbers and half-reactions, what has occurred. Identify the oxidising and reducing agents.
2. Describe how you would carry out a simple laboratory test to show that an unknown solution contains an oxidising agent using acidified potassium iodide and starch. Include expected observations and relevant half-reactions.
KEY LEARNING POINTS
• Oxidation is loss of electrons (or gain of oxygen / loss of hydrogen); reduction is gain of electrons (or gain of hydrogen / loss of oxygen).
• In any redox reaction, electrons lost in oxidation equal electrons gained in reduction; write half-reactions to balance.
• Oxidising agents accept electrons and are themselves reduced; reducing agents donate electrons and are themselves oxidised.
• Assign oxidation numbers using the rules (free elements 0, monatomic ion = charge, O usually −2, H usually +1, sum equals charge of species).
• Some reactions (e.g., neutralisation) show no change in oxidation numbers and are non-redox.
• Laboratory tests: acidified KI + starch indicates oxidisers (I- → I2 blue-black); KMnO4 and K2Cr2O7 show colour changes when reduced by reducing agents.
TOPIC 1 SUMMARY: REDOX REACTIONS
KEY LEARNING POINTS FOR THE ENTIRE TOPIC:
• Sub-Topic: Oxidation and Reduction
• Definitions: Oxidation (loss of electrons/gain of oxygen/loss of hydrogen), Reduction (gain of electrons/gain of hydrogen/loss of oxygen).
• Electron transfer model: write oxidation and reduction half-reactions and combine ensuring electrons balance (example: Zn → Zn2+ + 2e-; Cu2+ + 2e- → Cu).
• Oxidation numbers: assign using rules (free elements 0; monatomic ions = charge; O usually −2, H usually +1; sum equals charge of compound/ion). Use these to detect redox changes.
• Identifying agents: oxidising agents accept electrons (are reduced); reducing agents donate electrons (are oxidised).
• Practical identification: acidified KI + starch indicates oxidisers by blue-black iodine; KMnO4 and K2Cr2O7 show distinctive colour changes when reduced by reducing agents.
• Non-redox reactions: reactions with no change in oxidation numbers (e.g., acid–base neutralisation) are not redox processes.
• Skills: interpret redox reactions by oxygen/hydrogen exchange, electron transfer and oxidation number changes; deduce half-reactions and balance electrons; perform simple qualitative tests to identify oxidising and reducing agents.
END OF TOPIC 1: REDOX REACTIONS