CBSE CLASS 12 1.ELECTROCHEMICAL CELLS AND ELECTRODE POTENTIAL TOPIC 1 ELECTROCHEMICAL CELL

TERM-2 CBSE CLASS 12 CHAPTER-WISE & TOPIC-WISE CHEMISTRY QUESTION BANK 

OBJECTIVE + SUBJECTIVE

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CBSE CLASS 12

1.ELECTROCHEMICAL CELLS AND ELECTRODE POTENTIAL

TOPIC 1

ELECTROCHEMICAL CELL

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Electrochemistry is the branch of chemistry which deals with the relationship between electrical energy and chemical energy and inter-conversion of one form into another. Electrochemistry deals with the study of production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non spontaneous chemical transformations.

Electrochemistry holds importance in Production of metals like Na, Mg. Ca and Al, Electroplating, Purification of metals, Batteries and cells used in various instruments.

An Electrochemical cell is a device that generates a potential difference between electrodes by chemical reactions. It comprises two electron conductors which are separated by an ionic conductor and are linked by an electron conductor. The two metallic electrodes are dipped in the electrolytic solution.

An electrochemical cell consists of two metallic electrodes dipped in electrolytic solutions. The cells are of two types:

(1) Electrolytic cells - Electrical Energy → Chemical Energy

(2) Galvanic cells - Chemical Energy → Electrical Energy (Galvanic cell/Daniel Cell /Voltaic cell)

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Daniel cell

An improvement of the galvanic cell is the Daniel cell which converts the chemical energy of a spontaneous redox reaction into electrical energy.

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Construction:

(1) On the LHS there is a plate of zinc is dipped in a solution of zinc sulphate (ZnSO4) in a beaker. 

(2) On the RHS a plate of copper is dipped in a solution of copper (II) sulphate in another container. These plates of metal are called the electrodes of the cell.

(3) These plates of metal are called the electrodes of the cell and they behave as terminal to hold the electrons.

(4) The two electrodes are connected via wire.

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Working

(1) At anode: The Zinc electrode gets oxidized and hence releases electrons that flow through the wire towards the copper electrode. Oxidation loss of electrons.

Zn→ Zn2+ + 2e

(2) At cathode: The copper (II) sulphate solution releases copper ions which gain electrons and are reduced to Copper metal Reduction gain of electrons.

Cu2+ + 2e → Cu

(3) Zinc atoms being more reactive have a greater tendency to lose electrons than that of copper. The electrons in this cell moves from zinc anode to copper cathode through the wire connecting the two electrodes in the external circuit, while metal ions form one half cell to the other through the salt bridge.

(4) Here current flows from copper electrode to zinc electrode that is cathode to anode via an external circuit.

(5) A bulb placed within this circuit will glow and a  voltmeter connected within this circuit will show deflection. 

(6) The net reaction of this cell is the sum of two half-cell reactions.

Zn(s) + Cu2+ (aq) → Zn2+(aq) + Cu(s) 

◆ Cell is represented as

Zn/Zn2+ || Cu2+/Cu

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Salt Bridge

A salt bridge (a glass tube having an inert electrolyte

like potassium chloride or ammonium nitrate in a gelatine form)is a junction that connects the anodic and cathodic compartments in a cell or electrolytic solution.

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Functions of Salt bridge

(1) A salt bridge completes the electric circuit and acts as an electrical contact between the two half cells.

(2) It maintains the electrical neutrality of electrolytic solutions. 

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Working of salt bridge

◆  The oxidation that occurs in anode generate electron and positive ions. Now the electrons flow through the wire leaving the unbalanced positive charge in a beaker. In order to maintain the electrical neutrality the negatively charged (NO3) ion from the salt bridge moves to the positively charged beaker (anodic half cell).

◆ At the cathode the Cu2+ ions are consumed. So for maintaining the electrical neutrality, the K+ ions are migrated into this half cell from the salt bridge.Thus, salt bridge helps in maintaining the charge balance in the two half cells.

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Cell Representation

The following rules are followed for representing an electrochemical cell.

(1) Anode is written on the left hand side and cathode on the right hand side. (2) The anodic cell is represented by writing the

metal first and then the metal ions present in the

electrolyte. These two are separated by a vertical

line or a semicolon.

(3) Salt bridge is represented by two vertical lines. So Daniell cell is represented as:

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If the concentration of both the electrolytes is 1 M. then the cell notation is:

Zn | Zn2+ (1M) || Cu2+ (1M) Cu

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Important

Case 1 when Ecell > External opposite potential

If an external opposite potential is applied and increased slowly, we find that the reaction continues to take place till the opposing voltage reaches the value 1.1 V. Electrons move from Zn to Cu and current moves from Cu to Zn.

Case 2 when Ecell = External opposite potential

When opposing voltage reaches at the value 1.1 V the reaction stops and no further current flows through the cell.

Case 3 when Ecell< External opposite potential

When opposing voltage is more than 1.1 V. The external potential again starts the reaction but in the opposite direction. It now functions as an electrolytic cell, a device for using electrical energy to carry non-spontaneous chemical reactions. Electrons flow from Cu to Zn and current from Zn to Cu.