Electrolysis Principles

Section 1 of 7What electrolysis is

Definition

• Definition Electrolysis is the breaking down of an ionic compound, when it is molten or dissolved in water, by passing an electric current through it.

Why the compound must be molten or dissolved

• Why the compound must be molten or dissolved In a solid ionic compound the ions are locked in a lattice and cannot move, so no current flows.
• When the compound is melted or dissolved the ions are free to move and carry charge.

Covalent compounds

• Covalent compounds Simple covalent compounds contain no ions and no free charged particles, so they cannot be electrolysed.

Section 2 of 7Key terms

Term	Meaning
Electrode	A rod, usually graphite or metal, that carries current into or out of the electrolyte.
Electrolyte	The molten or dissolved ionic compound that conducts electricity and is decomposed.
Anode	The positive electrode; negative ions (anions) move to it.
Cathode	The negative electrode; positive ions (cations) move to it.

Section 3 of 7Predicting the products

1

Positive ions (metals and hydrogen) move to the cathode and non-metal negative ions move to the anode.

2

At the cathode, a metal is formed if it is less reactive than hydrogen, for example copper or silver (HT only).

3

If the metal is more reactive than hydrogen, such as sodium or magnesium, hydrogen gas is made instead from a solution (HT only).

4

At the anode, non-metals other than hydrogen are formed, for example chlorine or bromine.

5

In a molten binary compound there is only one possible product at each electrode: a metal at the cathode and a non-metal at the anode.

Section 4 of 7Charge transfer in the circuit (HT only)

1Power supply

• Power supply Electrons are pushed out of the negative terminal of the power supply towards the cathode.

2At the cathode

• At the cathode The electrons do not travel through the solution.
  • Cations arrive and gain electrons, so reduction takes place.

3In the electrolyte

• In the electrolyte Ions carry the charge: cations move towards the cathode and anions towards the anode.

4At the anode

• At the anode Anions lose electrons, so oxidation takes place.
  • The electrons travel back through the wire to the positive terminal.

Section 5 of 7Helpful memory aids

PANIC

• PANIC Positive is Anode, Negative Is Cathode.

Charge carriers

• Charge carriers Electrons carry charge in the external wires; ions carry charge in the electrolyte (HT only).

Direction

• Direction Electrons flow from negative to positive, while conventional current is drawn from positive to negative.

Section 6 of 7Key terms

Term	Meaning
Electrolysis	The decomposition of a molten or dissolved ionic compound by an electric current.
Electrolyte	An ionic compound, molten or in solution, that conducts electricity and is decomposed.
Anode	The positive electrode, where oxidation occurs.
Cathode	The negative electrode, where reduction occurs.
Anion	A negatively charged ion that is attracted to the anode.
Cation	A positively charged ion that is attracted to the cathode.

Section 7 of 7Exam technique

1

Saying that electrons travel through the electrolyte; only ions move in the solution or melt.

2

Saying solid ionic compounds conduct because they contain ions; the ions cannot move in a solid.

3

Mixing up the electrodes; remember PANIC, and that positive ions go to the negative cathode.

4

Forgetting that a reactive metal such as sodium is not produced from an aqueous solution, where hydrogen forms instead (HT only).