Electrolytic Cell

Electrolyte and Non-electrolyte

Definition of electrolytes

Substances that can conduct electricity in either the molten state or aqueous solution and undergo chemical changes

Definition of non-electrolytes

Substances that cannot conduct electricity in all states

Classification of substances into electrolytes and non-electrolytes

Apparatus setup for electrolysis

Ion discharged at cation and anion in an electrolysis

Conductor	Electrolyte
Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.	Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.
Substances that conduct electricity without undergoing decomposition.	Substances that conduct electricity and undergo decomposition into their constituent elements.
Can conduct electricity due to the presence of free moving electrons.	Can conduct electricity due to the presence of free moving ions.
Electrical conductivity decreases as temperature increases.	Electrical conductivity increases as temperature increases.
Examples of conductors are metals and graphite.	Examples of electrolytes are ionic compounds, acids and alkalis.

Definition of electrolysis

A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.

Apparatus setup for electrolysis of molten lead(II) bromide

The Eº value mentioned in the table is the Eº value in the standard electrode potential series.

Factors affecting the electrolysis of an aqueous solution

E° value

Concentration of solution

Type of electrode

Electrode	Ion chosen to be discharged
Anode	Anion with a more negative or less positive E° value will be easier to be discharged and oxidised.
Cathode	Cation with a more positive or less negative E° value will be easier to be discharged and reduced.

Electrode	Ion chosen to be discharged
Anode	This factor is only considered for the selection of ions at the anode if the aqueous solution contains halide ions. Halide ions with a higher concentration in the electrolytes will be discharged at the anode, even though the E° value of the halide ions are more positive.
Cathode	Cation with a more positive or less negative E° value will be easier to be discharged.

Electrode	Ion chosen to be discharged
Anode	For active electrodes (e.g. copper and silver) No anions are discharged Metal atoms at the anode releases electrons to form metal ions
Cathode	Cation with a more positive or less negative E° value will be easier to be discharged.

Category	Electrolytic cell	Chemical cell
Electric sources	Electrodes are connected to a battery or any electrical source	Electrodes are not connected to any electrical sources
Electrolyte	Both electrodes are dipped in the electrolyte
Type of metal for electrodes	Usually carbon electrodes	Different metals if being dipped in the same electrolyte
Type of metal for electrodes	Usually carbon electrodes	The same metal if being dipped in a different electrolyte
Negatively charged electrode	Cathode	Positive terminal (less electropositive metal)
Positively charged electrode	Anode	Negative terminal (more electropositive metal)
Energy conversion	Electric energy -> chemical energy	Chemical energy → electric energy
Electron transfer	Anion releases electrons at the anode	The atom at the negative terminal releases electrons
Electron transfer	Cation receives electrons at the cathode	Ions in the electrolyte receive electrons
Oxidation	At anode	At the negative terminal
Reduction	At cathode	At the positive terminal

Electroplating of metals

Electroplating of metals through electrolysis is done by making the object being electroplated as the cathode, the electroplating metal as the anode, and an aqueous solution containing the ions of the electroplating metal as the electrolyte.
For example, to electroplate an iron ring with copper, Cu, the copper anode ionises to become copper(II) ions, \(Cu^{2+}\).
- Anode: \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
Copper(II) ions, \(Cu^{2+}\) move to the cathode, are discharged and deposited as a thin layer of copper, Cu on the iron ring.
- Cathode: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
The blue colour of copper(II) sulphate, \(CuSO_4\) solution does not change because the concentration of copper(II) ions, \(Cu^{2+}\) remains the same.
The rate of ionisation of copper, Cu at the anode is the same as the rate of discharged copper(II) ions, \(Cu^{2+}\) at the cathode.

Purification of metal

Copper is an important mineral and element in our daily life.
It is an important industrial metal due to its ductility, malleability, electrical conductivity and resistance towards corrosion.
Copper used in electrical wiring must have a 99.99% purity.
The purity of copper extracted by the process of melting is about 99.5%.
Even a slight difference in copper purity will negatively impact its conductivity.
To determine whether a copper metal is pure, one must conduct the purification of metals through electrolysis.
The purification of copper by electrolysis is carried out with a piece of pure, thin copper as the cathode; impure copper as the anode; and an aqueous salt solution of copper, such as copper(II) nitrate, \(Cu(NO_3)_2\) as electrolyte.
Impure copper anode ionises to form copper(II) ions, \(Cu^{2+}\).
- \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
Copper dissolves to become copper(II) ions, \(Cu^{2+}\) and impurities accumulate below the impure copper anode.
The anode becomes thinner.
At the pure copper cathode, copper(II) ions, \(Cu^{2+}\) are discharged to form copper atoms, Cu.
- \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
Solid copper is deposited and the copper cathode becomes thicker.