Investigating electrolysis

In a nutshell

The aim of this practical is to investigate the change in mass of the anode and cathode when an aqueous solution is electrolysed using copper electrodes. Electrolysis is the breakdown of ionic compounds when an electric current is passed through them.

Equations

The first equation is the oxidation of copper, which occurs at the positive electrode. The second equation is the reduction of copper ions to solid copper, which occurs at the negative electrode.

Note: The anode is the positive electrode and the cathode is the negative electrode.

Word equation	Formula equation
$copper\ \rightarrow copper\ ion\ +\ 2\ electrons$	$Cu(s)\ \rightarrow\ Cu^{2+}(aq)\ +\ 2e^-$
$copper\ ion\ +\ 2\ electrons \ \rightarrow\ copper$	$Cu^{2+}(aq)\ + \ 2e^- \rightarrow\ Cu(s)$

Variable definitions

Quantity name	Symbol	Unit name	Unit
$mass$	$m$	$gram$	$g$
$current$	$I$	$ampere$	$A$
$time$	$t$	$second$	$s$

Equipment list

The following equipment list can be used to carry out electrolysis on an aqueous solution.

Equipment	Use
Copper sulfate solution	Aqueous solution which will undergo electrolysis
Low voltage power supply	To provide current for electrolysis to occur
$2\times$ carbon electrodes	Placed in solution and used as electrodes for electrolysis in experiment $1$
$2\times$ copper electrodes	Placed in solution and used as electrodes for electrolysis in experiment $2$
$2\times$ crocodile clips	To connect the power supply to the electrodes
Beaker	Where the aqueous solution is held
Safety goggles	To protect eyes from hazards
Safety gloves	To protect hands from hazards
$\pm\ 0.01\, g$ balance	To measure the weight of electrodes before and after electrolysis

Experiment 1: Electrolysis of aqueous solution using carbon electrodes

Experimental variables

The independent variable is the one you change. The dependent variable is the one which depends on what has been changed, therefore it is the one you measure. The control is the one which is kept constant.

Independent variable	$Time$
Dependent variable	$Rate\ of\ gas\ release$
Control variable	$Current$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

Hazard	Risk	Safety Measure
Copper sulfate solution	Leads to skin irritation	Wear gloves
Copper sulfate solution	Causes eye irritation	Wear safety goggles
Low voltage power supply	Electric shock	Make sure that the electricity supply is switched off before handling and make sure the electrodes are not touching

Method

This is your instructions for how to complete the experiment.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating electrolysis

1.	Pour some copper sulfate solution into a beaker.
2.	Set up your electrolysis apparatus. Firstly, whilst the power supply is unplugged, connect the cables to the crocodile clips .
3.	Place the two graphite rods into the copper sulfate solution.
4.	Connect the cables to the power supply and to the electrodes.
5.	Turn on the power supply and observe what happens at each electrode.
6.	Record your observations.

Analysis

What reaction is occuring at each electrode?

Electrode	Observation
Negative electrode	A brown / pink solid forms
Positive electrode	Bubbles of colourless gas

Conclusion

At the negative electrode you should see that a solid forms (copper) and at the positive electrode a gas will be released (oxygen).

Evaluation

Once you have completed the experiment it is important to understand why the reactions occur at the positive and negative electrode. Were your predictions correct about what would be formed at the respective electrodes? What improvements could be made to the method? Did you keep the current the same? The current affects the rate of electrolysis. Therefore it has to stay constant throughout the experiments.

Experiment 2: Electrolysis of copper sulfate solution using copper electrodes

Experimental variables

The independent variable is the one you change. The dependent variable is the one which depends on what has been changed, therefore it is the one you measure. The control is the one which is kept constant.

Independent variable	$Current$
Dependent variable	$Mass$
Control variable	$Time$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

Hazard	Risk	Safety Measure
Copper sulfate solution	Leads to skin irritation	Wear gloves
Copper sulfate solution	Leads to eye irritation	Wear safety goggles
Low voltage power supply	Electric shock	Make sure that the electricity is switched off before handling and make sure that the electrodes are not touching

Method

This is your instructions for how to complete the experiment.

1.	Measure and record the mass of each copper electrode.
2.	Pour the copper sulfate solution into a beaker.
3.	Set up your electrolysis apparatus by connecting the crocodile clips to the copper electrodes.
4.	Connect the cables to the power supply and to the electrodes.
5.	Make sure that the electrodes do not touch each other and then turn on the power supply.
6.	After a specified amount of time (ask your teacher), turn off the power supply.
7.	Remove the electrodes, wash in distilled water and then dip in propanone. Let the propanone evaporate.
8.	Measure and record the mass of each electrode.
9.	Repeat the experiment with fresh electrodes and different currents.

Analysis

Record the mass of the electrodes in tables below.

Current

$(A)$

Starting mass

(g)

Final mass

(g)

Change

(g)

Have a table for both the positive and negative electrodes.

Plot a graph of the change in mass on the $y$ axis with the current on the $x$ axis.

Conclusion

Your graph should show that as current increases so does the amount of mass gained by the electrodes.

Evaluation

Once you have completed the experiment you will be expected to understand why a higher current causes an increase in the change in mass of the electrodes.