Required practical - electrolysis of aqueous solutions
Investigating the electrolysis of aqueous solutions using inert electrodes
This required practical involves developing a A starting point for investigation. An idea or explanation to be tested.. This is also an opportunity to use appropriate apparatus and techniques to draw, set up and use Chemical reaction resulting in the production of electric current. cells to monitor chemical reactions, produce A substance made of one type of atom only. from A substance formed by the chemical union of two or more elements. 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.
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:
- hydrogen - lighted splint goes out with a squeaky pop
- oxygen - a glowing splint relights
- 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.
| Hazard | Harm | Precaution |
| Copper sulfate solution | Causes skin irritation | Wear gloves |
| Copper sulfate solution | Causes serious eye irritation | Wear eye protection |
| dc electricity supply | Electric shock | Make sure electrodes do not touch; make sure that electricity supply is switched off before handling apparatus |
| Hazard | Copper sulfate solution |
|---|---|
| Harm | Causes skin irritation |
| Precaution | Wear gloves |
| Hazard | Copper sulfate solution |
|---|---|
| Harm | Causes serious eye irritation |
| Precaution | Wear eye protection |
| Hazard | dc electricity supply |
|---|---|
| Harm | Electric shock |
| Precaution | Make 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.
| Electrolyte | Observations at positive electrode, including results of gas tests | Observations at negative electrode, including results of gas tests |
| Electrolyte | |
|---|---|
| Observations at positive electrode, including results of gas tests | |
| Observations 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