Ionic theory and Electrolysis

Electrolysis is the decomposition of an electrolyte caused by passing an electric current through it.

Conductors are substances that conduct electricity eg. metals and carbon (graphite).

Non-conductors are substances that do not conduct electricity eg. plastics, paper and rubber.

Metals and carbon (graphite) have free electrons which are responsible for conduction of electricity.

Electrolyte is a compound which conducts electric current in molten or in aqueous solution and is decomposed by it.

Non-electrolytes are compounds in molten or solution state that do not conduct electricity. Examples are ethanol, sugar solution and molten wax.

Ion is charged atom or group of atoms(radicals).

Ions are of two types:

Molten/aqueous ionic compounds conduct electricity due to the presence of charged particles called ions that are free to move and carry electric current.

Solid ionic compounds do not conduct electricity because the ions in the solid state are not able to move freely and carry an electric current. The ions in a solid ionic compounds are packed tightly together in a crystalline structure and held in their fixed position by strong electrostatic forces.

Again, covalent compounds do not conduct electricity because they do not have ions. They possess molecules which exist as molecules when dissolved in water. No free particles to carry the electric charge.

Electrolytes

There are two types of electrolytes:

Strong electrolyte is the compound which ionizes completely into ions in its aqueous solution. Example HCl, H2SO4, HNO3, KOH

Weak electrolyte is the compound in aqueous which ionizes to a small extent into ions. Eg CH3COOH, NH4OH.

Strong electrolytes Weak electrolytes
Allow a large amount of electricity to flow through them. Allow small amounts of electricity to flow through.
Dissociate completely into ions in aqueous solution Dissociate partially into ions in aqueous solution.
Contain almost only free mobile ions. Contain ions as well as molecules.

Differences between electrolytes and (solid)conductors

Electrolytes Solid conductors
Conduct electricity only in solution or molten state. Conduct electricity in solid state.
Have mobile ions. Have mobile electrons.
They are decomposed by electricity. They remain chemically unchanged when electric current flows.
They are compounds of metals or hydrogen and non-metals as well as radicals. They are elements that is metals or graphite.

Ionic theory
According to the ionic theory, electrolytes are present as ions in solution or in molten state and the function of electricity is to direct those ions to their respective electrodes.

Electrolytic cell

Electrolysis takes place in electrolytic cell. Electrolytic cell is made up of a container carrying two electrodes and an electrolyte.

The electrodes are connected to an external power supply such as direct current(DC) source or battery by using connecting wires.

Electrodes are two metals or carbon rods or plates through which electricity enters or leaves an electrolyte.

Electrodes are of two types:
i. Anode is the positively charged electrode, where oxidation (loss of electrons) occurs. (Meaning that negative ions lose electrons and form neutral atoms or molecules). Anode is connected to the positive terminal of the power source.
ii. Cathode is the negatively charged electrode, where reduction (gain of electrons) occurs.(Meaning that positive ions gain electrons and form neutral atoms or molecules). Cathode is connected to the negative terminal of the power source.

Mechanism of Electrolysis

When no current is passing, the ions move freely in the electrolyte. When the circuit is switched on, the current passes through the electrolyte and immediately the electrodes become charged, the cathode negatively and the anode positively.

Cations migrate to the cathode. When they reach the cathode they gain electrons from the cathode.
Cu2+ + 2e-→ Cu → Gain of electrons is reduction.
2H+ + 2e- → H2 → Reduction always occurs at the cathode.

Anions migrate to the anode. When they reach the anode the lose electrons to the anode.
2Cl- → Cl2 + 2e- → Loss of electrons is oxidation.
2O2- → O2 + 4e- → Oxidation always occurs at the anode.

Note

When ion loses or gains electrons it becomes an atom/molecule.

If metal atoms are formed, they may be deposited as a layer of metal on the cathode. If hydrogen gas is formed, it bubbles off.

Discharge – the removal of electrons from negative ions to form atoms or the gain of electrons of positive ions to become atoms.

(A) Electrolysis of Molten (Fused) Compounds

Molten ionic compounds conduct electricity because ions are free to move.

When molten binary compound is electrolysed, metal atom or hydrogen gas is formed at the cathode while non-metal except hydrogen is formed anode.

To make any compound molten we strongly heat the solid until it melts.

Worked Examples

1. Electrolysis of molten lead (II) bromide (PbBr2).


Ions present: Pb2+, Br-

Electrode reactions:

Reaction at cathode:

Lead ions gain electrons at cathode to become lead atoms.
Pb2+ + 2e- → Pb

Reaction at anode:

Bromine ions (Br-) lose electrons at anode to become bromine atoms. Br atoms created form bond together to make Br2 gas.
2Br- → Br2 + 2e-

2. Electrolysis of molten sodium chloride by using carbon rods (graphite).

Electrolyte: molten sodium chloride.

Ions present in an electrolyte: Na+ and Cl-

Reaction at the cathode: Na+ ions are attracted to the cathode. Each Na+ ion gains one electron from the cathode to form one solution.
Na+ + e- → Na ( Reduction)

Reaction at the anode:
Cl- ions are attracted to the anode. Each Cl- ions loses one electrons to the anode to form one chlorine atom. Chlorine gas is liberated at anode. 2Cl- → Cl2 + 2e- (Oxidation)

3. Electrolysis of molten oxide (MgO) using carbon rods (graphite).

Electrolyte: molten magnesium oxide
Ions present: Mg2+ and O2-
Electrode reactions:
Reaction at the cathode: Mg2+ + 2e-→ Mg (reduction)
Reaction at the anode: 2O2- → O2 + 4e- (oxidation)

(B) Electrolysis of Aqueous solutions

When the electrolyte is in aqueous solution state, there might be two ions of similar charge.

For examples, sodium chloride solution.
Ions from water: H+ and OH-
Ions from from: Na+ and Cl-

Factors affecting the selective discharge

There are three main factors affect the selective discharge of ions during electrolysis.
  1. The position of the ion in the electrochemical series.
  2. The concentration of ion in the electrolyte.
  3. The nature of the electrode.

(a) Position of ion in the electrochemical series

The electrochemical series is the arrangement of ions according to their ease in gaining or losing electrons. The lower the position of the ions in the electrochemical series, the more likely their ions to be discharged, hence the more preference for them been discharged.
Cations Ease of discharge Anions
K+ ↓ SO42-
Na+ NO3-
Ca2+Cl-
Mg2+Br-
Al3+I-
Zn2+OH-
Fe2+
Pb2+
H+
Cu2+
Hg2+
Ag+
Au2+

Worked Examples

1. Electrolysis of aqueous sodium chloride solution

Solution

Electrolyte: sodium chloride solution
Ions present: Na+, Cl- , OH- , H+

Electrode reactions:

At anode:
Both Cl- and OH- will migrate to anode but OH- will be preferentially discharged.
4OH- → 2H2O + O2 + 4e-
At cathode:
Both Na+ and H+ will migrate to cathode, but H+ will be preferentially discharged.
2H+ (aq) + 2e- → H2(g)

2. Electrolysis of dilute sulphuric acid by Hoffman’s voltammeter

Solution

Electrolyte: dilute sulphuric acid
Ions present: H+, OH- , SO42-

Electrode reactions:

At cathode:
Hydrogen ions will be discharged at cathode.
2H+ + 2e- → H2
At anode:
Both OH- and SO42- migrate to anode but hydroxide being lower in the electrochemical series, it will be preferentially discharged. 4OH- → 2H2O + O2 + 4e-

(b)The concentration of ion in the electrolyte

The more concentrated a type of ion in an aqueous solution, the more readily it is discharged during electrolysis. This factor is commonest to anions, thus it mainly affect the products formed at the anode.

Example

Electrolysis of concentrated calcium chloride. Both Cl- and OH- migrate to the anode; the chloride (Cl-) ion will be preferentially discharged over the OH- because of its higher concentration. However, at anode H+ will be discharged in the preference to Ca2+ despite the higher concentration of the calcium ion in the solution. (distance factor in the ES).

(c)Nature of electrodes

There are two types of electrodes

Examples

1.In the electrolysis of sodium chloride solution using mercury and platinum electrodes.
  1. When platinum cathode is used H+ ions are discharged in preference to Na+ ions to produce hydrogen gas.
    2H+(aq) + 2e- → H2(g)
  2. When mercury cathode is used Na+ is discharged in preference to H+ ions. The sodium metal mixes with mercury to form sodium amalgam
    Na+(aq) + e- → Na(s)

2.Electrolysis of copper (II) sulphate solution with carbon and copper electrodes
Carbon/graphite electrodes Copper electrodes
→Ions present: Cu2+, SO42- , H+, OH-
Electrode reactions:
a. At anode:
4OH- → H2O + O2 + 4e-
b. At cathode:
Cu2+ + 2e- → Cu 		→Ions present: Cu2+, SO42- , H+, OH-
→Electrode reactions:
a. Anode reactions:
Cu2+→ Cu2+ + 2e-
b. Cathode reactions:
Cu2+ + 2e- → Cu
Both hydroxide and sulphate ions are not selectively discharged.
The remaining hydrogen and sulphate ions combine to form sulphuric acid. The resulting solution (electrolyte) becomes increasingly more acidic.

Laws of electrolysis

Michael Faraday investigated the relationship between the amount of a substance liberated at electrodes and the quantity of electricity passed through the electrolyte. He came up with two important laws:

Faraday’s First law of electrolysis

“The mass of substance formed (or dissolved from) at electrode during electrolysis is directly proportional to the quantity of electricity passing through the electrolyte.”

Mathematically this can be expressed as:
M ∝ Q ……………………………….(i)
Where m= mass of substance formed
Q= quantity of electricity in coulombs (C)
Coulomb is the quantity of electricity passed when a current of one ampere flows in one second.
But
Q=I.t ……………………………… ii
Where I = current in amperes (A) t=time in second
Removing the sign of proportionality (∝) by introducing the equal sign and a constant of proportionality Z equation (i) becomes:
M = Z.Q ………………………...iii
Where Z is electrochemical equivalent.

Electrochemical equivalent of a substance is the mass in grams of that substance that can be formed by the passage of one coulombs of electricity.
To calculate the value of Z, the formula used is
Z = Ar /V.F ……………………….iv

Where Z = electrochemical equivalent (g/C)
Ar= relative atomic mass of the element.
F = Faraday’s constant = 96500 coulombs.
V= valency of the element.

Now substituting the value of Z in the equation for mass
M=Ar. I.t/V.F …………………….v

WORKED EXAMPLES

1. Calculate the mass of silver that will be deposited at an electrode during electrolysis of a silver nitrate solution if the current of 0.5A is passed through the solution for 2 hours. Ar for silver is 108.

2. Calculate the mass of hydrogen that will evolve at electrode during electrolysis of water given that the current of of 2A is passed through the solution for 1 hour.

Faraday’s second law of electrolysis

“When the same quantity of electricity is passed through solutions of different electrolytes, the amounts of the elements deposited is proportional to the chemical equivalents of the elements.”
Chemical equivalent is denoted by E, 
Chemical equivalent = Relative atomic mass
                      Charge of an ion.

Or the law can be stated this way:
“ If the same quantity of electricity is passed through electrolytes, the amount of substances deposited at respective electrodes is in ratio of their equivalent masses.”
m1 = E1
m2   E2

Applications of electrolysis

Electroplating:

Electroplating is a process in which a thin layer of metal is deposited onto a surface of another material using an electric current.

The object to be plated is made the cathode in the electrolytic cell. The anode is made of plating metal. Electrolyte is a solution containing its ions. When a current passes, the plating metal metal is transferred from the anode to the cathode.

The objects are electroplated for the following reasons:

Purification of metals

Electrolysis is used to refine metals such as copper by dissolving impurities and depositing pure metal at cathode.

Production of chemicals.

Electrolysis is used to produce variety of chemicals, such as hydrogen gas and chlorine gas.

Extraction of metals:

Highly reactive metals such as potassium, sodium and aluminium are extracted from their ores by electrolytic reduction.

Electrolytic polishing:

Electrolysis is used to polish metal surfaces by removing a thin layer of metal through electrolysis, leaving a smooth and shiny surface.

Anodizing:

This involves thickening of an oxide layer formed on the surface of aluminium by electrolysis. It is mainly done to prevent aluminium from corrosion.
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