Electrochemical cell/galvanic/voltaic cell

The device which is used to convert chemical energy produced in a chemical reaction into electrical energy are called electrochemical/galvanic/voltaic cell.

Redox Reaction- (i) Reduction (gain of electrons), (ii) oxidation (loss of electrons)

Direct redox reaction:- The reaction in which both oxidation as well as reduction takes place in same container e.g.- Zinc rod dipped in CuSo₄ solution and redox reaction takes place in which Zn is losing electron and copper gaining electron, and Heat is produced due to transfer of electrons from Zn to Cu.

Zn + CuSO₄ → ZnSO₄ + Cu

Zn + Cu²⁺ → Zn²⁺ + Cu

Observation-

• Zn rod dissolves
• Blue color of CuSO₄ pades
• Brown solid Cu deposited at bottom
• Reduction vessel or beaker hot.
• Exothermic reaction             

Indirect Redox reaction: - In which oxidation and reduction are taking place in separate containers.

Current stops

1. Due to polarity of constitution

2. Circuit is open

Overall reaction

Zn + CuSO₄ → Cu + ZnSO₄

Salt bridge: - Sometime current stops, to overcome this difficulty, we use salt bridge.

Jelly (Agar-Agar) → obtained from seaweeds. It allows a very good medium for flow of ions. Suitable inert electrolytes are one whose ions do not take part in redox reaction and do not react with the electrolytes. Example of this- KNO₃, KCl, K₂SO₄ etc.

Electrolytic used must have cations and anions of approximately same transport number (m) (Fraction of current carried by an ion)

Functions of salt bridge

1. It neutralizes the polarity of two solutions by allowing the ions to move from one half cell to another help cell.

2. It completes the circuits as in outer circuit electrons are the charge carries and in internal circuit (salt bridge) ions are the charge carries.

3. If prevent liquid – liquid function potential.

Electrode potential: - It is the tendency of an electrode to lose electron (oxidation potential) or tendency to gain electron (reduction potential). We always use reduction potential.

The standard reduction potential of various elements are arranged in a series termed as electrochemical series or reactivity series. To compare or calculate we reduction potential of any element, half reduction or oxidation cell of that is joined with the standard hydrogen cell electrode (SHE) or normal hydrogen electrode (NHE) and then potential difference can be measured.