25.04.201 7 LECTURE 12 ELECTROLYSIS

compounds and aqueous solutions
Describe the electrolysis of an aqueous solution
Describe

the electrolysis of a molten ionic compounds
Write half equations for the discharge of ions at the anode and the cathode,
Laws of electrolysis – Faradays laws

in aqueous solution, conducts an electric current and is decomposed

in the process.
An electrode is a rod or plate where electricity enters or leaves an electrolyte during electrolysis. Reactions occur at the electrodes (and not inside the electrolyte). (inert/reactive)
The anode is the positive electrode connected to the positive terminal of the d.c. power source. Oxidation occurs at the anode.
The cathode is the negative electrode connected to the negative terminal of the d.c. power source. Reduction occurs at the cathode.
An anion is an ion with a negative charge. During electrolysis, it is attracted to the anode.
A cation is an ion with a positive charge. During electrolysis, it is attracted to the cathode.
In electrolysis, a compound in the molten state or in aqueous solution, conducts electricity and is decomposed by it.

or liquid state and undergo chemical change.

Electrolysis is a

process where the electrolytes are broken down into its constituent elements by passing electricity through it.

The term electrolysis was introduced by Michael Faraday: “Lysis” means loosening in Greek, thus electrolysis means “loosening by electricity”.

by passing electricity through molten compounds or aqueous solutions of

compounds.
Electricity is used to produce chemical changes.
The apparatus used for electrolysis is called an electrolytic cell. An electrolytic cell is an electrochemical cell in which an electric current drives an otherwise non-spontaneous reaction.

charged ions (cations)
The anode is positively charged. It attracts negatively

charged ions (anions)

The power source provides electrical current that gives the electrodes their respective charges

These are known as electrodes. They are usually a metallic (or in the case of graphite – a non-metallic) conductor

The electrolyte is the ionic compound in its molten or aqueous state. It provides mobile electrons that allow electrical conduction

so, they lose electrons to become a neutral element:
nXn- -

ne- -> Xn

The cations move towards this negatively charged electrode (cathode).
In doing so, they gain electrons to become an electrically neutral element:
Yn+ + ne- -> Y

The electrons from the anions then move along the circuit through the power source to the negative electrode

These electrons then provide the negative charge for the negative electrode (cathode)

+

-

cathode

anode

of ions to be discharged during electrolysis is based on:
Factors

affecting products of electrolysis:
Type of electrolyte (molten or solution)
The electrochemical series
Molarity / Concentration of Solution
Type of Electrodes (inert or active)

can be either

Molten

or Solution

Crucible
(+)
(–)
Heat

… silvery liquid… Pb

that represent the electrolysis of molten lead (ll) bromide:

( – ) Cathode: (+) Anode:

Cathode:

(+) Anode:

electrolysis of solution is based on:

in order of their ability to be reduced.
For cations, the

higher the element in the series, the less likely it is that this will gain electrons (that is be reduced).
For anions, the higher it is on the series the less likely will it lose electrons (that is be oxidized)

Oxidative ability of anion increases

Reducing ability of cation increases

value of standard potential) is liberated first at the anode
if

anions are halogens i.e.
chloride Сl- , bromide Вr- and iodide I- the halogen is produced:

if – ions are not halogens eg sulphate SO42-, nitrate NO3-, carbonate CO32- and other, oxygen is produced, because OH- ion of water is electrolysed:

- CATHODE: the ion which is stronger oxidizing agent (high value of standard potential) is discharged first at the cathode

if cations (metals) are more reactive than hydrogen (before H atom in ecs):
K, Na, Ca, Mg, Zn, Fe ...... H2
then hydrogen is produced:

if cations (metals) are less reactive than hydrogen (after H atom in ecs): Cu, Ag, Au, Pt
then the metal is produced:

RULES FOR IONIC SOLUTIONS

Cathode:

(+) Anode:

Cathode:

(+) Anode:

reduced from the solution
W = 0%
These cations are reduced from

the solution with hydrogen ions
W < 50%

These cations are total reduced from the solution
W = 100%

but just provide electrical current (graphite, platinum, mercury)
Active electrodes actually

participate in electrolysis while providing electrical current. Usually made of the metal that corresponds to the metallic ion in the electrolyte: Zn, Cu, Al, Cr, Ni

of a substance produced at an electrode during electrolysis is

proportional to the number of moles of electrons (the quantity of electricity) transferred at that electrode.

of a substance deposited or liberated at any electrode on

passing a certain amount of charge is directly proportional to its chemical equivalent weight:

mass of Ag, Cu and Al, deposited on their respective

electrodes would be directly proportional to their chemical equivalent masses

better
Most commonly used metals for electroplating: Copper, Chromium, Silver, Tin
The

anode usually is made of the plating metal. The object to be plated is the cathode.

ELECTROPLATING
An electrolytic process of Formation of a thin protective coating of a non-reactive or superior metal on an article made of a more reactive or inferior metal.

by electrolysis.
Manufacture of metals: The metals like sodium, potassium,

magnesium, calcium aluminum, etc., are obtained by electrolytes of fused electrolytes.
Manufacture of non-metals: Non-metals like hydrogen, fluorine, chlorine are obtained by electrolysis.
Electro-refining of metals: This is the process of refining the metal. i.e. removing impurity from metal by the use of electrolysis method. The metals like copper, silver, gold, aluminum, tin, etc., are refined by electrolysis.
Manufacture of compounds: Compounds like NaOH, KOH, Na2CO3 KCIO3, white lead, KMnO4, etc., are manufactured by electrolysis.
Electroplating: The process of coating an inferior metal with a superior metal by electrolysis is known as electroplating. The aims of electroplating are:
To prevent the inferior metal from corrosion.
To make it more attractive in appearance.