Electrochemical cells

The set up in which chemical energy is converted to electrical energy is called an Electrochemical cell. It consists of two half cells, an oxidation half cell reaction and a reduction half cell reaction. The overall redox reaction results in a flow of electrons i.e. an electric current.

An example of electrochemical cell is a zinc electrode dipping into a solution of zinc tetraoxosulphate (VI), connected to a copper electrode dipping into a solution of copper (II) tetraoxosulphate (VI).

Types of Electrochemical cells

1. Daniel cell: this type of cell consists of a copper vessel filled with a saturated solution of copper (II) tetraoxosulphate (VI) and porous pot filled with a zinc tetraoxosulphate VI solution. Immersed in zinc tetraoxosulphate VI solution is a zinc rod.

*Daniel cell

Source: www.commons.wikimedia.org 

2. Leclanche’ cell: leclanche’ cell consists of a zinc metal container as the anode and a graphite rod surrounded by manganese IV oxide as the cathode.

*Leclanche Cell

Source: www.commons.wikimedia.org 

3. Secondary cell: cells which can be recharged are known as secondary cells. They can be recharged by passing a direct current through them. An example of a secondary cell is the lead accumulator, used in motor cars.

Electrolysis

Electrolysis is the chemical decomposition of a compound which takes place when an electric current is passed through either a solution or the molten form of the compound.

Terms used in electrolysis

Electrolyte: this is a compound either in solution or molten form which conducts Electric current and is decomposed at the electrodes in the process.e.g acids, Alkalis and soluble salts. A non electrolyte like sugar, alcohols and trichloro methane can’t conduct electricity.

Electrodes: these are conductors, (pole of carbon or metal) through which an electric current enters or leaves the electrolyte.

Anode: this is the positive electrode, by which the current enters the electrolyte or by which electrons leave the electrolyte.

Cathode: this is the negative electrode by which the current leaves the electrolyte or by which electrons enter the electrolyte.

Electrolyte cell: this consists of a container of electrolyte with two electrodes connected to a suitable direct current supply.

*A simple electrolytic cell

Source: www.commons.wikimedia.org 

Ionic theory

Ionic theory is of the view that when an electrolyte is melted or dissolved in water, some, if not all the substance breaks up (dissociates) into freely moving charged particles called ions.

Electrolytes and non- electrolytes

An electrolyte conducts electricity only when molten or in solution as a result of the movement of its ions to the electrodes and their subsequent reactions at the electrodes.

Nonelectrolytes do not conduct electricity since they do not ionize. They are covalent, and are mainly organic compounds.e.g sugar, ethanol, benzene, ether and trichloroethane.

Preferential discharge of ion during electrolysis depends on;

1. Position of ions in the electrochemical series

2. Concentration of the ions and 

3. Nature of the electrodes.

Laws of electrolysis

Michael Faraday in 1833, after performing series of electrolytic experiment put forward two laws:

Faraday’s first law of electrolysis: the law states that the mass (m) of a substance liberated at an electrode during electrolysis is directly proportional to the quantity of electricity (Q) passing through the electrolyte.

   Mathematically; M=Q

Quantity of electricity= current (I) × time(t)

i.e Q= It

:. m=It

Or m=EIt 

(Where E is a constant, known as the Electrochemical equivalent of the substance). The coulomb is the unit of the quantity of electricity, while the current is in amperes and time in seconds.

*Verification of first law

Source: www.toppr.com 

*Graph of gain in mass of copper against time

Source: www.researchgate.net 

*Graph of gain in mass against magnitude of current

Source: www.researchgate.net 

Faraday’s second law of electrolysis

The law states that when the same quantity of electricity is passed through different electrolytes, the relative number of moles of the elements deposited are inversely proportional to the charges on the ions of the elements.

*Verification of Faraday’s second law of electrolysis.

Source: www.qsstudy.com 

Uses of Electrolysis

1. Extraction of metals and non metals

2. Purification of metals 

3. Electroplating of one metal by another

4. Industrial preparation of certain chemicals.

*Electrolytic extraction of copper

Source: www.commons.wikimedia.org 

Electroplating

Electroplating is the electrical coating of one metal with another metal to secure improved appearance and greater resistance to corrosion.e.g the coating of tableware, cutleries, jewelries, iron or steel with copper, silver, chromium, nickel or gold.

*Silver plating

Source: www.pikist.com 

Electrolysis of brine 

This process is used for the industrial preparation of hydrogen, chlorine and concentrated solution of sodium-hydroxide.

*Industrial electrolysis of brine

Source: www.en.wikipedia.org 

Electrode potential

Electrode potential is the potential difference set up between an element and a solution of its ions. It is a measure of the tendency of an element to form ions.

*Standard hydrogen electrode

Source: www.commons.wikimedia.org 

*Electrode potential of copper

Source: www.commmons.wikimedia.org 

*Measuring standard electrode potential

Past questions

1. An arrangement of two different metals in aqueous solutions of their salts to produce an electric current is known as? (Wassce 1996)

A. Electrochemical cell 

B. Activity series

C. Thermocouple

D. Voltammeters

E. Galvanometer

Answer: A

2. Which of the following statements is not correct about electrolysis? (Wassce 1996)

A. Reduction occurs at the anode

B. Anions migrate to the anode

C. Positive ions migrate to the cathode

D. Concentration affects the discharge of ions

E. Electrolytes conduct electric current

Answer: A

3. If two metallic ions of the same concentration migrate to a graphite cathode, the one that would be preferentially discharged is the one that has the? (Wassce 2002)

A. Smaller mass 

B. Lower electrical charge

C. Greater stability in solution

D. Stronger tendency to accept electrons

Answer: D

4. When concentrated sodium chloride solution is electrolyzed using inert electrodes, the products are? (Wassce 2002)

A. Oxygen and hydrogen

B. Hydrogen and chlorine

C. Sodium and oxygen

D. Sodium and chlorine

Answer: C

5. Crude copper could be purified by the electrolysis of concentrated copper II chloride if the crude copper is? (Jamb 1994)

A. Made both by anode and cathode

B. Made by cathode

C. Made by anode

D. Dissolved in the solution

Answer: C

6. Given that M is the mass of substance deposited in an electrolysis and Q the quantity of electricity consumed, then Faraday’s law can be written as? (Jamb 2000)

A. M=Z/Q

B. M=Q/Z

C. M=Z/2Q

D. M=QZ

Answer: D

7. State the ionic theory?

Answers:

Ionic theory is of the view that when an electrolyte is melted or dissolved in water, some, if not all the substance breaks up (dissociates) into freely moving charged particles called ions.

8. Name the types of cell we have?

Answers:

i. Daniel’s cell

ii. secondary cell

iii. leclanche cell