Required practical

Investigating the electrolysis of aqueous solutions using inert electrodes

This required practical involves developing a hypothesis. This is also an opportunity to use appropriate apparatus and techniques to draw, set up and use electrochemical cells to monitor chemical reactions, produce elements from compounds and to identify gases.

Developing the hypothesis

An investigation starts with a scientific question, for example:

  • What are the products of electrolysis of aqueous solutions?
  • Is there a pattern in the products of electrolysis of aqueous solutions?

The first step in answering a scientific question is to develop a hypothesis. A hypothesis is an idea to be tested, which is backed up by scientific knowledge. Suitable hypotheses are:

  • a non-metal will be produced at the positive electrode because non-metal ions are negative
  • solutions that include ions of metals that are low in the reactivity series produce the metal at the negative electrode (not hydrogen) because ions of unreactive metals have a greater tendency to gain electrons

The hypothesis can then be used to make predictions, such as 'In the hydrolysis of copper chloride, the product at the positive electrode will be chlorine.'

Planning the investigation

The plan needs to address these points:

  • which apparatus to use, and a diagram showing how it will be set up
  • which solutions to test
  • how to test for, and identify, the products of electrolysis
  • a consideration of hazards, risks and precautions

Apparatus

The set up below is suitable. The positive electrode is connected to the positive terminal of a dc power pack. The negative electrode is connected to the negative terminal of the power pack.

A negative and positive electrode in upside-down test tubes which have been placed in a beaker of electrolyte liquid. The electrodes are attached to a 12 V DC charge.

Test solutions

It is best to test at least five solutions. Suitable solutions include copper sulfate, copper chloride, sodium chloride, sodium nitrate, sodium bromide. There are many more.

Identifying the products

Any gases produced can be collected in the test tubes. They need to be stoppered and tested later. Gas tests include:

  1. hydrogen - lighted splint goes out with a squeaky pop
  2. oxygen - a glowing splint relights
  3. chlorine - damp blue litmus paper turns red and is then bleached white

The electrodes need to be examined carefully each time, to see if a metal has been deposited on them.

Question

In an electrolysis cell, a gas that bleached litmus paper was produced at the anode. What is the identity of the gas?

Chlorine.

Hazards, risks and precautions

It is vital to identify hazards, the possible harm they can cause, and suitable precautions. The table below shows how to do this. It does not include all possible hazards.

HazardHarmPrecaution
Copper sulfate solutionCauses skin irritationWear gloves
Copper sulfate solutionCauses serious eye irritationWear eye protection
dc electricity supplyElectric shockMake sure electrodes do not touch; make sure that electricity supply is switched off before handling apparatus
Question

Chlorine gas is produced in the electrolysis of chlorides. The gas is toxic. What precautions could be taken to prevent harm from chlorine?

Ensure small quantities only are produced. Work in a well-ventilated laboratory.

Making observations

Suitable column headings for a results table are given below.

ElectrolyteObservations at positive electrode, including results of gas testsObservations at negative electrode, including results of gas tests

It is vital to make observations carefully, and to record them in detail at the time they are made.

Analysing the results

The observations in the table, including gas test results, need to be carefully considered. Are there any patterns in the results? Do the results, and any patterns in them, support the original hypothesis?

Evaluating the practical

The evaluation should address these points:

  • Do the results agree with the predictions and what are the reasons for any differences?
  • What actions were taken to achieve accurate results?
  • What improvements could be made to the method?

Fran Scott demonstrates how to perform practical experiments with electrolysis