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.
An investigation starts with a scientific question, for example:
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:
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.'
The plan needs to address these points:
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.
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.
Any gases produced can be collected in the test tubes. They need to be stoppered and tested later. Gas tests include:
The electrodes need to be examined carefully each time, to see if a metal has been deposited on them.
In an electrolysis cell, a gas that bleached litmus paper was produced at the anode. What is the identity of the gas?
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.
|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|
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.
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|
It is vital to make observations carefully, and to record them in detail at the time they are made.
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?
The evaluation should address these points: