22 – Investigating with Inert Electrodes (LC 2027) Revision Notes for Leaving Cert Chemistry

22 – Investigating with Inert Electrodes

What is electrolysis with inert electrodes?

Electrolysis involves passing an electric current through a solution or molten compound to cause chemical decomposition. Inert electrodes are electrodes that don't react during the process - they simply provide a surface for reactions to occur. Common inert electrodes include platinum, carbon, and graphite.

infoNote

The key characteristic of inert electrodes is their chemical stability - they remain unchanged throughout the electrolysis process. This allows us to study the behaviour of the electrolyte solution without interference from electrode reactions.

When we use inert electrodes, the products formed depend entirely on the ions present in the solution, not on the electrode material itself. This makes them perfect for investigating what happens during electrolysis of different compounds.

Experiment 1: Electrolysis of acidified water

Apparatus and method

The electrolysis of water requires adding a small amount of dilute sulfuric acid to help the water conduct electricity. The experiment uses either a voltameter (a specialised apparatus for studying electrical conduction) or simple inverted test tubes to collect the gases produced.

Key equipment:

Water acidified with dilute sulfuric acid
Platinum or carbon electrodes (inert)
Gas collection tubes
Battery or DC power supply

Observations and gas tests

When the electric current flows through the acidified water, you'll notice:

Gas bubbles form at both electrodes
The volume of gas at the negative electrode is twice that at the positive electrode
Gas tests reveal:
- Gas at positive electrode relights a glowing splint → this is oxygen
- Gas at negative electrode ignites with a 'pop' → this is hydrogen

chatImportant

The 2:1 volume ratio of hydrogen to oxygen matches perfectly with the chemical formula H₂O! This provides direct evidence for the composition of water molecules.

Electrode reactions

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Worked Example: Water Electrolysis Reactions

At the negative electrode (cathode - reduction):

H⁺ ions from the sulfuric acid are attracted to the negative electrode
Each H⁺ ion gains an electron to become a hydrogen atom
Two hydrogen atoms combine to form a hydrogen molecule

Chemical equation: $2\text{H}^+ + 2e^- \rightarrow \text{H}_2$

At the positive electrode (anode - oxidation):

Water molecules lose electrons
This breaks them down into hydrogen ions and oxygen gas

Chemical equation: $\text{H}_2\text{O} \rightarrow 2\text{H}^+ + \frac{1}{2}\text{O}_2 + 2e^-$

Experiment 2: Electrolysis of copper(II) chloride solution

Apparatus and method

This experiment uses a simple beaker containing copper(II) chloride solution with two carbon electrodes connected to a DC power supply.

Observations

When you switch on the power supply, you'll observe:

A brown solid forms at the electrode connected to the negative terminal - this is copper metal
Bubbles of gas are produced at the positive electrode
Testing this gas with moist blue litmus paper causes the paper to bleach - this indicates chlorine gas

infoNote

The bleaching of litmus paper is a definitive test for chlorine gas. Unlike other gas tests that produce sounds or flames, chlorine's oxidising properties cause the permanent colour change in the indicator.

Electrode reactions

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Worked Example: Copper(II) Chloride Electrolysis

At the negative electrode (cathode - reduction):

Copper ions gain electrons to form copper metal $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$

At the positive electrode (anode - oxidation):

Chloride ions lose electrons to form chlorine gas $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-$

Overall reaction: $\text{Cu}^{2+} + 2\text{Cl}^- \rightarrow \text{Cu} + \text{Cl}_2$

Experiment 3: Electrolysis of potassium iodide solution

Apparatus and method

This experiment is particularly interesting because it uses phenolphthalein indicator to help identify one of the products. The potassium iodide solution is colourless initially.

Observations and indicators

During electrolysis, you'll notice:

A brown colour develops at the positive electrode - this is iodine forming
The solution turns pink at the negative electrode - phenolphthalein indicates alkaline conditions (OH⁻ ions present)
Gas bubbles form at the negative electrode
Testing shows this gas ignites with a 'pop' - confirming it's hydrogen

infoNote

Phenolphthalein is colourless in neutral and acidic solutions but turns bright pink in alkaline conditions. This colour change directly indicates the formation of hydroxide ions (OH⁻) at the cathode.

Electrode reactions

lightbulbExample

Worked Example: Potassium Iodide Electrolysis

At the positive electrode (anode - oxidation):

Iodide ions lose electrons to form iodine atoms
Two iodine atoms combine to form an iodine molecule $2\text{I}^- \rightarrow \text{I}_2 + 2e^-$

At the negative electrode (cathode - reduction):

Water molecules gain electrons and break down
This produces hydrogen gas and hydroxide ions $2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2\text{OH}^-$

Overall reaction: $2\text{I}^- + 2\text{H}_2\text{O} \rightarrow \text{I}_2 + \text{H}_2 + 2\text{OH}^-$

Understanding electrode terminology

It's crucial to remember the CROA acronym for electrode processes:

Cathode - Reduction (electrons gained)
Oxidation - Anode (electrons lost)

chatImportant

Critical Distinction: In electrolytic cells (like our experiments), the cathode is the negative electrode and the anode is the positive electrode. This is opposite to galvanic cells (batteries), where the cathode is positive and the anode is negative.

Many students confuse these relationships, so always remember: the type of cell determines which electrode is positive or negative!

Remember!

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Key Points to Remember:

Inert electrodes don't participate in reactions - they only provide surfaces for reactions to occur
Gas test results: Hydrogen pops, oxygen relights glowing splints, chlorine bleaches litmus paper
CROA helps you remember: Cathode = Reduction (gain electrons), Anode = Oxidation (lose electrons)
Water electrolysis produces hydrogen and oxygen in a 2:1 ratio, matching the H₂O formula
Phenolphthalein indicator turns pink when OH⁻ ions are present, showing alkaline conditions

22 – Investigating with Inert Electrodes (LC 2027) (Leaving Cert Chemistry): Revision Notes