Advanced Chemical Principles and Molecular Dynamics · 6th Year · Oxidation and Reduction · Summer Term

Electrolysis

Investigate the process of using electrical energy to drive non-spontaneous chemical reactions, predicting the products of electrolysis.

TL;DR:Challenge your students to become chemical architects, using electricity as a tool to force reactions that wouldn't happen on their own.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Physical Chemistry - Electrochemistry

About This Topic

This topic, Electrolysis, is a cornerstone of the Electrochemistry section within the Irish Senior Cycle Chemistry syllabus. It builds directly upon students' prior understanding of redox reactions and electrochemical cells, which they will have covered previously. Where electrochemical (voltaic) cells harness spontaneous reactions to produce electrical energy, electrolytic cells do the opposite: they use an external electrical energy source to drive non-spontaneous chemical reactions. This concept is fundamental for Leaving Certificate Chemistry, frequently appearing in examination questions that test both theoretical understanding and practical application.

The core of this topic involves predicting the products of electrolysis under different conditions, specifically for molten ionic compounds and aqueous solutions. For molten salts, the process is straightforward: the cation is reduced at the cathode and the anion is oxidised at the anode. However, the electrolysis of aqueous solutions introduces a layer of complexity due to the presence of water, which can also be oxidised or reduced. Students must learn to use the electrochemical series to determine which species will be preferentially discharged at each electrode, while also considering the effect of ion concentration. A thorough grasp of writing half-equations for the reactions at the anode and cathode is essential for success.

Key Questions

1. Explain the difference between an electrochemical cell and an electrolytic cell.
2. Analyse the factors that determine which ion is discharged at an electrode during the electrolysis of an aqueous solution.
3. Compare the products of electrolysis of molten sodium chloride and aqueous sodium chloride.

Learning Objectives

• Distinguish between an electrochemical cell and an electrolytic cell in terms of energy conversion and spontaneity.
• Define electrolysis, electrolyte, anode, and cathode in the context of an electrolytic cell.
• Predict the products formed at the anode and cathode during the electrolysis of molten salts.
• Use the electrochemical series to predict the products of the electrolysis of specified aqueous solutions.
• Construct balanced half-equations for the reactions occurring at each electrode during electrolysis.

Key Vocabulary

Electrolysis	The process of using a direct electric current (DC) to drive a non-spontaneous chemical reaction, causing the decomposition of a compound.
Electrolyte	A substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. It contains mobile ions.
Anode	The electrode at which oxidation occurs. In an electrolytic cell, it is the positive electrode.
Cathode	The electrode at which reduction occurs. In an electrolytic cell, it is the negative electrode.
Electrochemical Series	A series of chemical elements arranged in order of their standard electrode potentials, used to predict the outcome of redox reactions.
Half-equation	An equation that shows either the oxidation or reduction reaction in a redox process, including the electrons gained or lost.

Watch Out for These Misconceptions

Common Misconception

Electrons flow through the electrolyte solution to complete the circuit.

What to Teach Instead

Electrons do not travel through the electrolyte. Instead, mobile ions (cations and anions) move towards the oppositely charged electrodes, carrying the charge through the solution. Electrons only flow through the external circuit (the wires and power supply).

Common Misconception

The anode is always the negative electrode.

What to Teach Instead

This is only true for an electrochemical (voltaic) cell. In an electrolytic cell, the anode is the positive electrode because it is connected to the positive terminal of the external power supply. Oxidation always occurs at the anode in both types of cells.

Common Misconception

In the electrolysis of an aqueous salt solution, only the ions from the salt are involved in the reaction.

What to Teach Instead

Water molecules can also be oxidised (at the anode) or reduced (at the cathode). The actual product at each electrode depends on the relative positions of the ions and water in the electrochemical series and, in some cases, their concentration.

Active Learning Ideas

Simulation Game

Electrolysis of Water (Hoffman Voltameter Demo)

Set up a Hoffman voltameter with dilute sulfuric acid to demonstrate the decomposition of water into hydrogen and oxygen gas. Students can observe the 2:1 volume ratio of gases produced and test for each gas using a lit splint (for H₂) and a glowing splint (for O₂).

20 min·Whole Class

Simulation Game

Electroplating a 2c Coin

Students use a simple circuit with a power pack, a copper strip (anode), and a 2 cent coin (cathode) immersed in copper(II) sulfate solution. They will observe the coin becoming coated in a fresh layer of copper.

30 min·Pairs

Simulation Game

Predict and Test: Electrolysis of Aqueous Solutions

In small groups, students predict the products of electrolysis for various aqueous solutions (e.g., CuSO₄, KI, NaCl). They then perform the electrolysis on a small scale using carbon electrodes and test for the products, for example using universal indicator or starch solution.

45 min·Small Groups

Real-World Connections

• The extraction of reactive metals like aluminium from its ore (bauxite) using the Hall-Héroult process.
• Electroplating objects with a thin layer of another metal, such as silver-plating cutlery or chrome-plating car parts for decoration and corrosion resistance.
• The production of essential industrial chemicals like chlorine gas, hydrogen gas, and sodium hydroxide via the chlor-alkali process.
• Anodising aluminium to create a thick, protective layer of aluminium oxide, which can be dyed to produce colourful finishes on products.
• Recharging secondary batteries (like car batteries or lithium-ion batteries), which involves using electrical energy to reverse the spontaneous discharge reaction.

Assessment Ideas

Quick Check

Use mini-whiteboards for students to quickly write and display the predicted products and half-equations for the electrolysis of a given compound, allowing for instant feedback.

Quick Check

Assign a past Leaving Certificate examination question on electrolysis. This will assess their ability to apply their knowledge to predict products, write equations, and explain the principles in an exam context.

Quick Check

Provide students with a checklist of skills for the topic (e.g., 'I can define electrolysis', 'I can predict the products for molten salts', 'I can use the electrochemical series for aqueous solutions'). Students rate their confidence level for each skill.

Frequently Asked Questions

Why do we use a direct current (DC) supply for electrolysis and not an alternating current (AC) supply?

A DC supply ensures that one electrode is always positive (the anode) and the other is always negative (the cathode). This allows for a continuous, one-way process of oxidation and reduction. An AC supply would cause the polarity of the electrodes to constantly reverse, meaning any products formed would immediately react back, resulting in no net chemical change.

What is the purpose of the electrochemical series in electrolysis?

The electrochemical series is a list of elements and ions ranked by their standard electrode potentials. In the electrolysis of aqueous solutions, it helps us predict which species will be discharged. At the cathode, the cation that is easier to reduce (has a more positive E⁰ value) will be discharged. At the anode, the anion that is easier to oxidise (has a more negative E⁰ value) will be discharged.

Why does the electrolysis of molten NaCl produce different products from aqueous NaCl?

In molten NaCl, the only ions present are Na⁺ and Cl⁻, so they are discharged to form sodium metal and chlorine gas. In aqueous NaCl, water (H₂O) is also present. At the cathode, H₂O is more easily reduced than Na⁺, so hydrogen gas is produced. At the anode, Cl⁻ is preferentially oxidised over H₂O (especially in concentrated solutions), producing chlorine gas.