1.
CHAPTER OVERVIEW:
- Introduction
- Electrochemistry
- Galvanic cell
- Standard Electrode Potential
- Standard Hydrogen Electrode
- Nernst equation
- Conductance of Electrolytic cell
- Measurement of conductivity
- Variation of conductivity with concentration
- Electrolytic cell and Electrolysis
- Product of electrolysis
- Batteries
- Primary Battery
- Secondary Battery
INTRODUCTION TO ELECTROCHEMISTRY
- The branch of chemistry which deals with interconversion of
electrical energy and chemical energy.
- Electricity to generate a redox chemical reaction or a redox
reaction to generate electricity.
- Redox reaction: the loss and gain of electrons.
ELECTRICIT Y FROM REDOX REACTION:
- Galvanic cell or voltaic cell – is used to generate electricity from
a redox reaction .
- Electricity: is the flow of electrons.
- Zinc has higher tendency to lose 2 electrons while the copper
has higher tendency to gain electrons.
- 𝑍𝑛(𝑠) → 𝑍𝑛2+ + 2𝑒 − (Oxidation) and 𝐶𝑢2+ + 2𝑒 − → 𝐶𝑢(𝑠) (reduction)
- Cathode: Where the reduction takes place and Anode: where the
oxidation takes place
- The process is a spontaneous process.
REDOX REACTION FROM ELECTRICIT Y:
- Electrolytic cell is used to generate redox reaction by passing
electric current.
- In molten NaCl at 8010C, Cl- has one extra electron while Na+
needs on electrons. However, Cl- will not release electrons by
their own so, a battery is connected to pull the electrons.
- No. of electron loss = No. of electron gain .
- It is a non-spontaneous reaction , as it cannot be occurred by
their own .
ELECTROCHEMICAL CELL
- A combination of cell which converts chemical energy to
electrical energy and vice-versa , is called electrochemical cell .
- Combination of Galvanic cell and Electrolytic cell . (not
simultaneous)
- Galvanic Cell : Converts chemical energy into electrical energy.
- Electrolytic Cell : Converts electrical energy into chemical energy.
GALVANIC CELL
- Cell that converts chemical energy into electrical energy.
- It is also known as Voltaic cell
- Comprises two-half cell , voltmeter and a salt bridge
- Involves oxidation and reduction reactions in separate vessels
(indirect redox reaction)
- Figure above reflects the pictorial representation of Daniell Cell
- In electrochemical Series – Zn is more reactive than Cu. i .e ., Cu
has stronger reduction potential i .e . Cu has the higher tendency
of gaining electrons.
- Selection of Electrolyte: Electrolyte must the solution , that can
spontaneously react with the electrode . For example , if the
electrode is Fe than the electrolyte must be iron salt solution .
For instance , FeSO4.
- In Daniell Cell , Zn releases two electrons by reacting with
ZnSO4 undergoing oxidation rxn .
- 𝑍𝑛(𝑠) → 𝑍𝑛2+ + 2𝑒 −
- On the other half cell , Cu2+ ion in the solution will gain the
two electrons undergoing reduction reaction .
- 𝐶𝑢2+ + 2𝑒 − → 𝐶𝑢(𝑠)
- The salt bridge will complete the circuit allowing continues flow
of charges.
SALT BRIDGE:
- A U-shaped tube , containing concentrated solution of an inert
electrolyte like KCl , KNO3, etc. in agar-agar jelly or gelatin .
- agar-agar is a gelatinous derived from sea weed .
- The warm agar-agar is used fill the tube and on cooling, it sets
to form jelly.
- Electrolyte to be used in the salt bridge should be inert, else it
will interfere with the redox reaction .
- Functions of Salt bridge:
1. It completes the electrical circuit by allowing movement of
ions from one solution to another without mixing of the two
solutions.
2. Salt bridge helps I maintaining the electrical neutrality of
the solution in the two-half cell .
- Zn2+ will increase in the solution resulting to stop the flow of
current. Using salt bridge the Accumulation of charges will stop.
DANIELL CELL
- It is a special type of a galvanic cell in which the anode is made
up of Zinc and cathode is made up of copper.
- At anode: 𝑍𝑛(𝑠) → 𝑍𝑛2+ + 2𝑒 − (oxidation)
- At cathode: 𝐶𝑢2+ + 2𝑒 − → 𝐶𝑢(𝑠) (reduction)
- 𝑍𝑛(𝑠) + 𝐶𝑢2+ → 𝐶𝑢(𝑠) + 𝑍𝑛2+ (redox)
Half Cells:
- Oxidation half-cell or Anode (Zn) - 𝑍𝑛(𝑠) → 𝑍𝑛2+ + 2𝑒 −
- Reduction half-cell or Cathode (Cu) - 𝐶𝑢2+ + 2𝑒 − → 𝐶𝑢(𝑠)
Note: Oxidation of Zn produces excess of Zinc ions.
Reduction of Zn produces excess of Cu
Salt bridge is used to maintain electrical neutrality.
- With time , the concentration of copper sulphate solution
decreases as copper ions gets reduced .
- Thus, current falls with time
- The weight of the Cu rod increases while that of Zn rod decreases
as the cell operates.
- The electrons flow from the anode to cathode and the current
flows from cathode to anode .
- If the electrodes of a cell are connected together, the electrons
will flow from negative terminal to the positive terminal .
- In this case , the cell potential is measured as 1.1V provided that
[Zn+2] = [Cu+2] =1M.
