Lesson Note on Electrolysis: Meaning, Terms and Electrolysis of Water and Brine for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Third Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Energy and Chemical Reactions
Previous Lesson: Ionic Theory: Meaning, Electrolytes and Electrochemical Series.
Topic: ELECTROLYSIS
Subject Matter: Meaning of electrolysis (chemical decomposition by electric current), terminologies (electrodes, electrolyte, electrolytic cell, electrochemical cells), electrolysis of acidified water, copper(II) sulphate solution, brine.

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Define electrolysis.
• Distinguish between strong and weak electrolytes.
• Differentiate between electrolytic and electrochemical cells.
• Identify the products formed at the electrodes during the electrolysis of acidified water, copper(II) sulphate solution, and brine.
• Write simple electrode equations for the reactions occurring during electrolysis.

Affective Domain:

• Appreciate the industrial importance of electrolysis.
• Show interest in understanding electrochemical processes.

Psychomotor Domain:

• Construct a simple electrolytic cell.
• Observe and record the products formed at the electrodes during electrolysis experiments.

Social Domain:

• Collaborate with peers in discussing observations from electrolysis experiments.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• New School Chemistry for Senior Secondary Schools by Osei Yaw Ababio.

Instructional Materials

The teacher will teach this lesson with the aid of:

• Electrolytic cell apparatus
• Hoffman’s voltammeter
• Electrochemical cell apparatus
• Copper sulphate solution
• Dilute H2SO4
• Brine (concentrated NaCl solution)
• Connecting wires
• Power source (battery)

Rationale for the Lesson

This lesson helps pupils understand how electricity can cause chemical changes, a fundamental concept in chemistry. It also provides the basis for understanding various industrial processes like electroplating and metal extraction.

Prerequisite/Previous Knowledge

Pupils have a basic understanding of electricity, chemical reactions, ions, and the concept of conductors and non-conductors.

Lesson Content/Board Summary

ELECTROLYSIS

Meaning of Electrolysis

Electrolysis is the chemical decomposition of a compound (electrolyte) by the passage of electric current through it, either in molten state or in aqueous solution.

Key Terminologies in Electrolysis

• Electrolyte: A substance that conducts electricity in its molten state or aqueous solution and is simultaneously decomposed. Electrolytes contain mobile ions.
• Non-electrolyte: A substance that does not conduct electricity in any state (e.g., sugar, ethanol).
• Electrodes: Conductors (usually inert metals or graphite) through which electricity enters or leaves the electrolyte.
• Anode: The positive electrode where oxidation occurs (loss of electrons). Anions migrate to the anode.
• Cathode: The negative electrode where reduction occurs (gain of electrons). Cations migrate to the cathode.
• Electrolytic Cell: An apparatus used for carrying out electrolysis. It converts electrical energy into chemical energy.
• Anion: Negatively charged ion that moves towards the anode.
• Cation: Positively charged ion that moves towards the cathode.

Strong and Weak Electrolytes

• Strong Electrolytes: Substances that completely dissociate into ions in solution and conduct electricity very well (e.g., strong acids like HCl, strong bases like NaOH, most salts like NaCl).
• Weak Electrolytes: Substances that partially dissociate into ions in solution and conduct electricity poorly (e.g., weak acids like CH₃COOH, weak bases like NH₃).

Distinction between Electrolytic and Electrochemical Cells

• Electrolytic Cell:
  • Converts electrical energy into chemical energy.
  • Requires an external power source.
  • Non-spontaneous reaction.
  • Anode is positive, cathode is negative.
• Electrochemical (Voltaic/Galvanic) Cell:
  • Converts chemical energy into electrical energy.
  • Generates its own electricity.
  • Spontaneous reaction.
  • Anode is negative, cathode is positive.

Electrolysis of Acidified Water (Dilute H₂SO₄)

Ions present: H⁺, OH⁻ (from water), H⁺, SO₄²⁻ (from H₂SO₄).

• At the Cathode (negative electrode): H⁺ ions are discharged in preference to SO₄²⁻ ions.

  2H⁺(aq) + 2e⁻ → H₂(g)

  Observation: Colourless gas (hydrogen) is evolved.

• At the Anode (positive electrode): OH⁻ ions are discharged in preference to SO₄²⁻ ions.

  4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻

  Observation: Colourless gas (oxygen) is evolved.

• Overall Reaction:

  2H₂O(l) → 2H₂(g) + O₂(g)

  Note: The volume of hydrogen produced is twice the volume of oxygen.

Electrolysis of Aqueous Copper(II) Sulphate Solution (using Carbon Electrodes)

Ions present: Cu²⁺, SO₄²⁻ (from CuSO₄), H⁺, OH⁻ (from water).

• At the Cathode (negative electrode): Cu²⁺ ions are discharged in preference to H⁺ ions (due to lower position in electrochemical series).

  Cu²⁺(aq) + 2e⁻ → Cu(s)

  Observation: A reddish-brown deposit (copper metal) is formed.

• At the Anode (positive electrode): OH⁻ ions are discharged in preference to SO₄²⁻ ions.

  4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻

  Observation: Colourless gas (oxygen) is evolved.

• Overall Reaction:

  2CuSO₄(aq) + 2H₂O(l) → 2Cu(s) + O₂(g) + 2H₂SO₄(aq)

  Note: The solution gradually turns from blue to colourless as copper ions are removed and sulfuric acid is formed.

Electrolysis of Brine (Concentrated Sodium Chloride Solution)

Ions present: Na⁺, Cl⁻ (from NaCl), H⁺, OH⁻ (from water).

• At the Cathode (negative electrode): H⁺ ions are discharged in preference to Na⁺ ions.

  2H⁺(aq) + 2e⁻ → H₂(g)

  Observation: Colourless gas (hydrogen) is evolved.

• At the Anode (positive electrode): Cl⁻ ions are discharged in preference to OH⁻ ions due to their higher concentration (overvoltage effect for oxygen).

  2Cl⁻(aq) → Cl₂(g) + 2e⁻

  Observation: Pale yellowish-green gas (chlorine) is evolved.

• Overall Reaction:

  2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g)

  Note: Sodium hydroxide (caustic soda) solution is formed in the electrolyte.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher introduces the lesson by asking pupils about their understanding of electricity and how it can cause changes in substances.
Pupils’ Activity: Pupils respond by sharing their prior knowledge about electricity and its effects.
Learning Point: Pupils are engaged and connect previous knowledge to the new topic.

Step 2: Meaning of Electrolysis and Terminologies

Time: 10 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher guides pupils to define electrolysis and explains key terminologies such as electrolyte, non-electrolyte, electrodes (anode, cathode), and ions (anions, cations), differentiating between strong and weak electrolytes.
Pupils’ Activity: Pupils listen, take notes, and contribute to the definitions.
Learning Point: Pupils understand the fundamental concepts and vocabulary of electrolysis.

Step 3: Electrolytic vs. Electrochemical Cells

Time: 5 minutes
Teaching Skill: Comparison/Differentiation
Teacher’s Activity: The teacher explains the differences between electrolytic cells and electrochemical cells, highlighting their energy conversions and electrode polarities.
Pupils’ Activity: Pupils observe the setup for each cell (if available) and note the distinctions.
Learning Point: Pupils can differentiate between the two types of electrochemical cells.

Step 4: Electrolysis of Acidified Water

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher demonstrates the electrolysis of acidified water using a Hoffman’s voltammeter, explaining the ions present and predicting the products at the anode and cathode, then writing the electrode equations.
Pupils’ Activity: Pupils observe the demonstration, record observations, and identify the gases evolved.
Learning Point: Pupils understand the process and products of water electrolysis.

Step 5: Electrolysis of Aqueous Copper(II) Sulphate

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher sets up and explains the electrolysis of aqueous copper(II) sulphate solution using carbon electrodes, focusing on the competitive discharge of ions and writing electrode equations.
Pupils’ Activity: Pupils observe the changes at the electrodes and the solution, then record their observations.
Learning Point: Pupils learn about the selective discharge of ions based on the electrochemical series and concentration.

Step 6: Electrolysis of Brine

Time: 6 minutes
Teaching Skill: Explanation/Discussion
Teacher’s Activity: The teacher explains the electrolysis of concentrated sodium chloride solution (brine), discussing the ions present and the factors determining the products at each electrode (concentration effect).
Pupils’ Activity: Pupils engage in discussion and write down the electrode equations and observations for brine electrolysis.
Learning Point: Pupils understand the industrial importance of brine electrolysis and the overvoltage effect.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define electrolysis.
2. Differentiate between a strong electrolyte and a weak electrolyte, giving one example for each.
3. State two differences between an electrolytic cell and an electrochemical cell.
4. During the electrolysis of acidified water, what are the products at the anode and cathode? Write the electrode equations.
5. What are the products when aqueous copper(II) sulphate is electrolysed using carbon electrodes?

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, emphasizing the importance of electrolysis in various applications. The teacher assigns homework for pupils to research industrial applications of electrolysis.
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: Pupils consolidate their learning and are encouraged to explore further.

Lesson Keywords

• Electrolysis – Chemical decomposition by electric current
• Electrolyte – Substance that conducts electricity and is decomposed
• Electrode – Conductor that makes contact with an electrolyte
• Anode – Positive electrode where oxidation occurs
• Cathode – Negative electrode where reduction occurs
• Anion – Negatively charged ion
• Cation – Positively charged ion
• Electrolytic cell – Apparatus for electrolysis
• Electrochemical cell – Cell that produces electricity from chemical reactions
• Brine – Concentrated sodium chloride solution

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional challenges such as predicting products of electrolysis for other molten salts. For those who need more support, the teacher will use simplified diagrams and repeat explanations, focusing on defining key terms and identifying products at electrodes.

Note for teachers using this lesson plan

Ensure all safety precautions are strictly observed during practical demonstrations, especially when handling electrical apparatus and chemicals. Emphasize the industrial relevance of electrolysis to motivate pupils. Encourage pupils to draw and label the different electrolytic setups in their notebooks.