IONIC THEORY
Ionic theory as proposed by Arrhenius states that when an ionic compound is dissolved in water or melted, some or all its particles dissociate (break up) into free moving charged particles called ions. This dissociation into ions is called ionization.
These free ions move randomly in all directions inside the solution. as seen in fig. A
But the ions lose their freedom as soon as an electric current is passed through the solution and they become orderly, surrounding themselves around the opposite pole or electrode as they begin to pull electrons from the electrodes or lose electrons to the electrodes and come out of the solution.
Electrolysis is defined as the chemical decomposition of a compound by the passage of electricity through the solution of the compound or its molten form.
Terms commonly used in Electrolysis
i. ELECTROLYTE: An electrolyte is a compound which allows the passage of electricity through its solution or its molten state and is decomposed in the process.
Examples of electrolytes include dilute Acids and Alkalis and all electrovalent compounds like NaCl.
Electrolytes are grouped in two
1. Strong Electrolytes: These are compounds which ionize completely in solutions.
They usually have large amounts of ions in solution and hence are good conductors. Examples are all sodium and potassium salts, minerals acids, and caustic alkalis.
NaCl(aq) →Na+(aq) + Cl-(aq)
Weak Electrolytes: These are compounds that ionize partially in solution
There is slight dissociation of the ions in dilute solutions, and so they contain less ions in solution. Examples include organic acids, aqueous ammonia, etc.
CH3COOH(aq) →CH3COO-(aq) + H+((aq)
Non-Electrolytes: These are compounds which do not conduct electricity at all, whether molten or in solution
Non-electrolytes are mostly covalent compounds and only exist as molecules. Examples include vegetable oils, organic solvents like alcohols, benzene sugar solution
Conductor and Non-conductor
Conductors: These are metals which allow the passage of electricity through them.
Examples include all metals in general Silver is the best conductor followed by copper and ionic solutions
Non-conductor (Insulators): These are substances that do not conduct or allow electricity to pass through them. Examples include wood, paper, air, rubber, plastic.
Electrodes: these are wires rods or plates through which an electric current enters or leaves the electrolytes
2. Anodes: This is the positive electrode through which electrons leave electrolytes. It is the electrode where oxidation occurs
3. Cathode: This is the negative electrode through which electrons enter the electrolyte. It is also the electrode where reduction takes place
4. Cations: - these are positively charged ions. They migrate to the cathode (negative electrode) during electrolysis.
5. Anions: these are negatively charged ions. They migrate to the anode (positive electrode) during electrolysis.
5. Electrolytic Cell: This is a vessel or container containing two electrons connected to a battery and an electrolyte. It is used for Electrolysis.
When electrolysis is carried out on the solution of an ionic compound. There are usually two cations and two anions which migrate to the cathode and anode respectively but only one of each ion is preferentially discharged at the electrodes.
The following factors determine which ion gets discharged at each electrode.
1. Position of ions in the electrochemical series.
The ions at the bottom of the series for the positive ions are preferentially discharged to the ions at the top of the series, for example for a solution of sodium chloride containing Na+ and Cl-, H+ and OH- hydrogen is below sodium in the series and so will be preferentially discharged. For non-metals the less electronegative element is preferentially discharged to the more electronegative. So OH- is preferentially discharged to Cl-
2. Concentration of ion in the electrolyte. For a concentrated solution of a salt, the more concentrated ion is preferentially discharged to the less concentrated ion. but concentration does not matter where there is a large gap between the two ions in the series.
3. Nature of the electrodes: - Electrodes that have affinity for certain ions will cause those ions to be preferentially discharged during electrolysis. But both Platinum and Carbon are two electrodes that are considered as neutral or passive electrodes since they have no affinity for any element.
Examples of Electrolysis
1. Electrolysis of Acidified Water (water containing drops of H2SO4)
The Hoffman Voltameter is used; both the anode and cathode are platinum foil.
The ions present in the electrolyte are:
Cations Anions
H2SO4 → 2H+(g) + SO42-
H2O → H+(aq) + OH-(aq)
At the Cathode: H+ ion migrate to the cathode and take up electrons to form neutral hydrogen atoms.
H+(aq) + e-→ H(g)
The hydrogen atoms then combine to form hydrogen gas molecule
H(g) + H(g)→ H2(g)
Overall equation
2H+(aq) + 2e- →H2(g)
At the anode: Both SO42- and OH- ions migrate to the anode where OH- ions being lower in the electrochemical series is preferentially discharged and lose its electrons to the anode to become a neutral - OH group.
OH-(aq) → OH + e
The neutral –OH group combines in pairs to form one molecule of water and one atom of oxygen.
OH + OH → H2O(l) + O(g)
The oxygen atoms then combine with another free oxygen atom to form an oxygen gas molecule.
O(g) + O(g)→O2(g)
Overall equation
4OH-(aq)→2H2O(l) + O2(g) + 4e-
Note: At the end of the electrolysis the solution becomes more concentrated or more acidic as the components of water (H2 and O)
H+(aq) + e–→ H(g)
The hydrogen atoms then combine in pairs to form diatomic hydrogen gas molecule
H(g) + H(g)→H2(g).
Overall equation
2H+(aq) + 2e-→ H2(g)
Thus, hydrogen gas is obtained at the cathode.
At the anode: both Cl- and OH- ions migrate to the anode where Cl- ions are preferentially discharged. This is because it is higher in concentration than OH- ion and the two ions are close to each other in the series.
Cl-(aq)→Cl(g) + e-
The chlorine atoms combine to give the molecules.
Cl(g) + Cl(g)→Cl2
Overall equation
2Cl-(aq)→ Cl2(g) + 2e-
Chlorine gas is obtained at the anode.
3.Electrolysis Of Copper (II) Tetraoxosulphate (VI) Solution Using Different Anode:
With carbon or platinum electrodes.
Cations Anions
CuSO4 → Cu2+(g) + SO42-(aq)
H2O → H+(aq) + OH-(aq)
At the Cathode: Cu2+ and H+ ion migrate to the cathode where Cu2+ being lower than and less electropositive than H+ is preferentially discharged as metallic copper on the cathode.
Cu2+(aq) + 2e-→ Cu(s)
At the anode: SO42- and OH- ions migrate to the anode where OH- ions lose their electrons to become a neutral - OH group.
OH-(aq) → OH + e-
OH + OH → H2O(l) + O(g)
O(g) + O(g)→O2(g)
Overall equation
4OH-(aq) →2H2O(l) + O2(g) + 4e-
Electrolysis of CuSO4 using different electrodes
1. Using Pt. or C- electrodes: - Pt. and C-electrodes as we know are inert or passive electrodes and do not determine the product of the electrolysis
Uses Of Electrolysis
1. Extraction of metals like Na, K, Mg, Ca and Al.
2. Purification of metals copper,
3. Electroplating
4. Preparation of some elements like Cl2 as sodium hydroxide, hydrogen and chlorine from electrolysis of brine using cathode.
OBJECTIVE QUESTIONS
1. Electrolysis is the process of
(a) producing electricity from chemicals
(b) using electricity to cause chemical change
(c) breaking compounds by heating
(d) mixing acids and bases
2. The substance that is decomposed during electrolysis is called the
(a) electrode
(b) electrolyte
(c) conductor
(d) catalyst
3. Which of the following is NOT an electrolyte?
(a) Molten sodium chloride
(b) Dilute sulphuric acid
(c) Distilled water
(d) Sodium hydroxide solution
4. In electrolysis, oxidation occurs at the
(a) cathode
(b) anode
(c) electrolyte
(d) battery
5. Reduction takes place at the
(a) anode
(b) cathode
(c) electrolyte
(d) circuit
6. The positive electrode in an electrolytic cell is the
(a) cathode
(b) anode
(c) electrolyte
(d) salt bridge
7. Which particle moves to the cathode during electrolysis?
(a) Anion
(b) Cation
(c) Electron
(d) Neutron
8. During the electrolysis of molten sodium chloride, the product at the cathode is
(a) chlorine gas
(b) sodium metal
(c) hydrogen gas
(d) oxygen gas
9. During electrolysis of acidified water, the gas collected at the anode is
(a) hydrogen
(b) oxygen
(c) chlorine
(d) nitrogen
10. Which of the following statements is correct?
(a) Oxidation occurs at the cathode
(b) Reduction occurs at the anode
(c) Anions move to the anode
(d) Cations move to the anode
11. A substance that allows electric current to pass through it by movement of ions is called a
(a) conductor
(b) semiconductor
(c) electrolyte
(d) insulator
12. The electrodes used in electrolysis may be
(a) active or inert
(b) solid or liquid
(c) metals only
(d) non-metals only
13. Which of the following is an example of an inert electrode?
(a) Zinc
(b) Copper
(c) Platinum
(d) Iron
14. The process of coating an object with a thin layer of metal using electricity is called
(a) electrolysis
(b) electroplating
(c) electrolysis reaction
(d) electrorefining
15. In the electrolysis of copper(II) sulphate using copper electrodes, the anode
(a) dissolves
(b) gains mass
(c) remains unchanged
(d) produces hydrogen
16. Which law of electrolysis states that the mass of a substance deposited is proportional to the quantity of electricity passed?
(a) Ohm’s law
(b) Faraday’s first law
(c) Boyle’s law
(d) Charles’ law
17. The electrolyte in a dry cell is usually
(a) sodium chloride
(b) potassium hydroxide
(c) ammonium chloride paste
(d) sulphuric acid
18. In electrolysis, electrons flow through the
(a) electrolyte
(b) external circuit
(c) electrode
(d) salt solution
19. Which of the following products is obtained at the cathode during electrolysis of dilute sulphuric acid?
(a) Oxygen
(b) Hydrogen
(c) Chlorine
(d) Sulphur
20. The main purpose of electrorefining is to
(a) produce alloys
(b) extract metals from ores
(c) purify metals
(d) coat metals