4. ο When an electric current is passed through a molten ionic compound the
compound decomposes or breaks down
ο The process also occurs for aqueous solutions of ionic compounds
ο Covalent compounds cannot conduct electricity hence they do not undergo
electrolysis
ο Ionic compounds in the solid state cannot conduct electricity either since they
have no free ions that can move and carry the charge
ο Particles in ionic compounds are in fixed position in the solid state but
can move around when molten or in solution
Electrolysis: Basics
5. Electrolysis: Basics
Particles in ionic compounds are in fixed position in the solid state but can move
around when molten or in solution
6. ο Electrode is a rod of metal or graphite through which an electric current
flows into or out of an electrolyte
ο Electrolyte is the ionic compound in molten or dissolved solution that
conducts the electricity
ο Anode is the positive electrode of an electrolysis cell
ο Anion is a negatively charged ion which is attracted to the anode
ο Cathode is the negative electrode of an electrolysis cell
ο Cation is a positively charged ion which is attracted to the cathode
Key terms
SIR.MUHAMMAD ABDUL MAGEID
7. The basic set-up of an electrolysis cell
SIR.MUHAMMAD ABDUL MAGEID
10. Exam Tip
ο Positive is Anode Negative is Cathode
ο Cations are attracted to the cathode and anions are attracted to
the anode.
ο Electron flow in electrochemistry occurs in alphabetical order as electrons
flow from the Anode to the Cathode.
SIR.MUHAMMAD ABDUL MAGEID
11. Electrolysis: General Principles
SIR.MUHAMMAD ABDUL MAGEID
Rules
ο Aqueous solutions will always have water (H2O)
ο H+ and OHβ ions from the water are involved as well
Positive electrode (anode)
ο OHβ ions and non-metal ions (anions) are attracted to the positive electrode
ο Either OHβ or non-metal ions will lose electrons and oxygen gas or gas of
non-metal in question is released, eg. chlorine, bromine, nitrogen
ο The product formed depends on which ion loses electrons more readily, with
the more reactive ion remaining in solution
A reactivity series of anions is shown below:
More reactive SO4
2- β NO3
- β OH- β Cl- β Br- β I- Less reactive
12. Electrolysis: General Principles
SIR.MUHAMMAD ABDUL MAGEID
Negative electrode (cathode)
ο H+ and metal ions attracted to the
negative electrode but only one will
gain electrons
ο Either hydrogen or metal will be produced
ο If the metal is above hydrogen in reactivity
series, then hydrogen will be produced and
bubbling will be seen at the cathode
The reactivity series of metals including
hydrogen and carbon
13. Electrolysis: General Principles
SIR.MUHAMMAD ABDUL MAGEID
Concentrated and dilute solutions
ο Concentrated and dilute solutions of the same compound
give different products
ο For anions, the more concentrated ion will tend to get discharged over
a more dilute ion
Electrolysis of binary molten compound
ο For a binary molten compound of a metal and a nonmetal, the cathode
product will always be the metal
ο The product formed at the anode will always be the non-metal
14. Electrolysis of Aqueous Solutions
SIR.MUHAMMAD ABDUL MAGEID
Diagram showing
the electrolysis of
aqueous solutions
15. Electrolysis of Aqueous Solutions
SIR.MUHAMMAD ABDUL MAGEID
Method:
ο Add aqueous solution into a beaker
ο Add two Graphite rods as the electrodes
and connect this to a power pack or
battery
ο Turn on power pack or battery and allow
electrolysis to take place
17. Electrolysis: Reactions at the Electrodes
SIR.MUHAMMAD ABDUL MAGEID
Determining what gas is produced
ο If the gas produced at the cathode burns with a βpopβ when a sample is lit
with a lighted splint then the gas is hydrogen
ο If the gas produced at the anode relights a glowing splint dipped into a
sample of the gas then the gas is oxygen
ο The halogen gases all produce their own colours (bromine is red-brown,
chlorine is yellow-green and fluorine is pale yellow)