3.2 Electrolysis Flashcards
Electrolysis
When an electric current is passed through a molten or aqueous ionic compound the compound decomposes or breaks down.
Liquids and solutions that are able to conduct electricity are called electrolytes.
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis.
During electrolysis the electrons move from the power supply towards the cathode.
Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode.
Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode.
Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode.
Electrolytic cell
An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following:
Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte.
Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity.
Anode: the positive electrode of an electrolysis cell.
Anion: negatively charged ion which is attracted to the anode.
Cathode: the negative electrode of an electrolysis cell.
Cation: positively charged ion which is attracted to the cathode.
Positive electrode
Negatively charged OH– ions and non-metal ions are attracted to the positive electrode.
If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine).
If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen gas.
In both cases, the other negative ion remains in solution.
Negative electrode
H+ ions and metal ions are attracted to the negative electrode but only one will gain electrons.
Either hydrogen or a metal will be produced.
If the metal is above hydrogen in reactivity series, hydrogen will be produced – bubbling will be seen at the cathode.
Electrolysing copper chloride
Copper is below hydrogen so copper(II) ions are preferentially discharged at the cathode.
Chlorine is a halogen, so is preferentially discharged at the anode.
Electrolysing sodium chloride
Sodium is above hydrogen so hydrogen ions are preferentially discharged at the cathode.
Chlorine is a halogen, so is preferentially discharged at the anode.
Electrolysing sodium sulphate
Sodium is above hydrogen so hydrogen ions are preferentially discharged at the cathode.
Hydroxide ions are preferentially discharged over sulfate ions, so oxygen is produced at the anode.
Electrolysing Acidified water (Dilute sulphuric acid)
Hydrogen ions are discharged at the cathode.
Oxygen from water molecules is preferentially discharged at the anode.
Electrolysing lead bromide (molten)
Binary ionic compounds consists of just two elements joined together by ionic bonding.
When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge.
These compounds undergo electrolysis and decompose into their constituent elements.
Lead(II) bromide is an ionic solid with a relatively low melting point and can be used to illustrate the electrolysis of a molten compound.
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off.
Positive lead ions move to the negative electrode (cathode) and gain electrons to form grey lead metal which deposits on the bottom of the electrode.
Predicting the products of electrolysis
To predict the products of any binary molten compound first identify the ions present.
The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode.
Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal.
Electrolysis and redox
Oxidation is when a substance loses electrons and reduction is when a substance gains electrons.
As the ions come into contact with the electrode, electrons are either lost or gained and they form neutral substances.
These are then discharged as products at the electrodes.
At the anode, negatively charged ions lose electrons and are thus oxidised.
At the cathode, the positively charged ions gain electrons and are thus reduced.
Electrolysis of copper sulphate using inert (graphite) electrodes
A solution of copper sulphate (CuSO4) contains four different ions: Cu 2+, SO4 2-, H+ and OH-. When you electrolyse a solution of CuSO4 with inert electrodes:
Copper metal is less reactive than hydrogen. So, at the cathode, copper metal is produced and coats the electrode: Cu2+ + 2e- → Cu
There aren’t any halide ions present so at the anode oxygen and water are produced. The oxygen can be seen as bubbles: 4OH- → О₂ + 2H₂O + 4е-
Electrolysis of copper sulphate using non-inert (copper) electrodes
As the reaction continues, the mass of the anode will decrease and the mass of the cathode will increase. This is because copper is transferred from the anode to the cathode. The reaction is quite slow, so the cell should be left for around 30 minutes in order to achieve a measurable change in mass.
You can measure how the mass of the electrodes has changed during the experiment by finding the difference between the masses of the electrodes before and after the experiment.
Anode – Cu → Cu 2+ + 2e-
Cathode – Cu 2+ + 2e- → Cu
Purification of copper using electrolysis of copper sulphate using non-inert (copper) electrodes
Copper can be extracted from its ore by reduction with carbon, but copper made in this way is impure. Electrolysis can be used to purify the copper using an electrochemical cell with copper electrodes.
When copper is purified using electrolysis, the anode starts off as a big lump of impure copper and the cathode starts off as a thin piece of pure copper. The electrolyte is copper sulfate solution (which contains Cu²+ ions). Here’s what happens during the process:
Copper in the impure copper anode forms copper ions which dissolve into the electrolyte – Cu → Cu 2+ + 2e-
The copper ions move to the pure copper cathode, and react to form a layer of pure copper – Cu 2+ + 2e- → Cu
Any impurities from the impure copper anode sink to the bottom of the cell, forming a sludge.
Practical 4 (electrolysis of copper sulphate)
Aim: To electrolyse copper(II) sulphate solution using inert(graphite) electrodes.
Procedure - Pour copper sulphate solution into a beaker.
Place two graphite rods into the copper sulfate solution. Attach one electrode to the negative terminal of a DC supply, and the other electrode to the positive terminal.
Completely fill two small test tubes with copper sulphate solution and position a test tube over each electrode as shown in the diagram.
Turn on the power supply and observe what happens at each electrode.
Test any gas produced with a glowing splint and a burning splint.
Record your observations and the results of your tests.
Practical 4 (Electrolysis with Active Electrodes)
Aim: To electrolyse copper(II) sulphate solution using active( copper) electrodesDiagram.
Procedure - Pour copper sulphate solution into a beaker.
Measure and record the mass of a piece of copper foil. Attach it to the negative terminal of a DC supply, and dip the copper foil into the copper sulphate solution.
Repeat with another piece of copper foil, but this time attach it to the positive terminal.
Make sure the electrodes do not touch each other, then turn on the power supply.
Adjust the power supply to achieve a constant current and leave for 20 minutes.
Remove one of the electrodes and wash it with distilled water, then dip it into propanone.
Lift the electrode out and allow all the liquid to evaporate. Do not wipe the electrodes clean. Measure and record the mass of the electrode.
Repeat with the other electrode making sure you can identify which electrode is which.
Repeat the experiment with fresh electrodes and different currents..
