Chemistry Sen. Sec

ELECTRO CHEMISTRY: Electrolytic cell, Faraday’s first law, Electrolytic conductor

Purpose

The chapter that deals with the relations between electricity and chemical species.
Types of cell

  1. Electrolytic cell

  2. Galvanic cell

  3. Conductivity cell

Electrolytic cell
Electrical energy is converted into chemical energy.
Galvanic cell
Chemical energy is converted into electrical energy.
Conductivity cell

Measure the conducting properties of electrolytic solution.


Electrolytic cell

Any cell is made up of two half cells.

There are two types of half cell

  1. Anode

  2. Cathode


Conductors are mainly of two types

  1. Electronic: - Electron carry current.  e.g. any metal wire, Cu, Ag etc.

  2. Electrolytic: - Ions carry current. e.g. any solution which has ions.

Both conductors obey ohm’s law i.e. V = IR. In electronic conductor with increase in temperature resistance increases whereas in case of electrolytic conductor with increase in temperature resistance decreases

When an electronic conductor comes in contact with an electrolytic conductor, then it is called electrode.


There are two types of electrode

  1. Anode: - It can be +ve or –ve. Always oxidation reaction occurs at anode.

  2. Cathode: - It also can be +ve or –ve. Here reduction reaction takes place.

In electrolytic cell cathode is negatively charged and anode is positively charged. At cathode cations get discharged whereas at anode anion get discharged.
For example in the electrolysis of molten NaCl
At cathode
Na+ + e-    Na (s)
At anode
2Cl-  Cl2 + 2e-
Electrolysis is defined as the migration of ions towards their respective electrodes and getting discharged on application of potential difference.
Platinum electrode is inert electrode. It does not participate in reaction. It only transfer electrons.


 

Faraday’s first law

It states that the amount of anodic or cathodic product is directly proportional to the charge being passed.
W (amount of substance)   Q (charge)
W  It (where Q = It)
W = ZIt 
Z is electrochemical equivalent.


Faraday’s second law
For n numbers of cells connected in series no. of equivalents of products obtained at any anode will be constant.
Here current is same and potential get divided.
Fig: -


Types of products
Nature of products obtained during the electrolysis depends on the following factors: -

  1. Electrolytic conductor

  2. Electronic conductor

  3. Current density

  4. Concentration of electrolyte


Electrolytic conductor

Electrolysis of molten NaCl will produce sodium at cathode and chlorine at anode.
Whereas the electrolysis of aqueous sodium chloride will produce hydrogen at cathode and chlorine at anode.
When more than one types of anion is competing for discharge, the ions with lower discharge potential will get discharge first.


Discharge potential order

SO4-2  NO3-   OH-   Cl-   Br-   I-   K+   Na+   Ca+2   Mg+2   Al+3   Zn+2   H+   Cu+2   Ag+   Au+

Alkali and alkaline earth metal can’t be produced by the electrolysis of their aqueous salt solution. They have to be prepared by fused or molten salt solution. Most of the time in order to reduce the fusion temperature impurities are added.


What are the electrolysis product of CuSO4 using platinum electrode and using copper electrode?

Platinum is a non- attacked type electrode
In this case at cathode
Cu+2 + 2e-  Cu
At anode
4OH-  2H2O + O2 () + 4e-
In this case concentration will change.
Copper electrode is an attacked type electrode
In this at cathode
Cu+2 + 2e-  Cu 
At anode
Cu  Cu+2 + 2e-
In this case concentration of copper sulphate will not change.
2nd process is used in metallurgy. Size of the anode will decrease as it dissolves and the size of cathode will increase.
Electrolysis is also used for electroplating. Electroplating of iron with zinc is called galvanization. It gives uniform thin layer cotting of few atomic diameter thickness.