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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Electro Chemistry - Cell Terminology

Before taking up the study of the electrochemical cells, we should be familiar with a few common terms. Current is the flow of electrons through a wire or any conductor. Electrode is the material : a metallic rod/bar/strip which conducts electrons into and out of a solution.

Electro Chemistry - Cell Terminology

 

Before taking up the study of the electrochemical cells, we should be familiar with a few common terms.

 

Current is the flow of electrons through a wire or any conductor.

Electrode is the material : a metallic rod/bar/strip which conducts electrons into and out of a solution.

 

Anode is the electrode at which oxidation occurs. It sends electrons into the outer circuit. It has negative charge and is shown as (-) in cell diagrams.

 

Cathode is the electrode at which electrons are received from the outer circuit. It has a positive charge and is known as (+) in the cell diagrams.

 

Electrolyte is the salt solution in a cell.

 

Anode compartment is the compartment of the cell in which oxidation half-reaction occurs. It contains the anode.

 

Cathode compartment is the compartment of the cell in which reduction half-reaction occurs. It contains the cathode.

 

Half-cell. Each half of an electrochemical cell, where oxidation occurs and the half where reduction occurs, is called the half cell.

 

Cell diagram or Representation of a Cell

 

A cell diagram is an abbreviated symbolic depiction of an electrochemical cell. For this purpose, we will consider that a cell consists of two half-cells. Each half-cell is again made of a metal electrode in contact with metal ion in solution.

 

IUPAC Conventions. In 1953 IUPAC recommended the following conventions for writing cell diagrams. We will illustrate these with reference to Zinc-Copper cell.

 

(1) a single vertical line (|) represents a phase boundary between metal electrode and ion solution (electrolyte). Thus the two half-cells in a voltaic cell are indicated as

Zn  | Zn2+                       Cu2+ | Cu

ANODE HALF-CELL CATHODE HALF-CELL

 

 It may be noted that the metal electrode in anode half-cell is on the left, while in cathode half-cell it is on the right of the metal ion.

 

           A double vertical line represents the salt bridge, porous partition or any other means of permitting ion flow while preventing the electrolyte from mixing.

 

            Anode half-cell is written on the left and cathode half-cell on the

right.

 

           In the complete cell diagram, the two half-cells are separated by a double vertical line (salt bridge) in between. The zinc-copper cell can now be written as

 

Zn  | Zn2+           | |         Cu2+ | Cu

ANODE HALF-CELL CATHODE HALF-CELL

 

 

(5) The symbol for an inert electrode, like the platinum electrode is often enclosed in a bracket. For example,

 

Mg  | Mg2+ | | H+  | H2 (Pt)

ANODE HALF-CELL    CATHODE HALF-CELL

(6) The value of emf of a cell is written on the right of the cell diagram. Thus a zinc-copper cell has emf 1.1 V and is represented as

Zn     |         ZnSO4         | |       CuSO4        |         Cu     E = + 1.1 V

--- -- > Direction of electron flow

If the emf acts in the opposite direction through the cell circuit it is denoted as a negative value.

Cu     |         CuSO4        ||        ZnSO4  |      Zn     E = - 1.1 V

< -- -- -- Direction of electron flow

The negative sign also indicates that the cell is not feasible in the given direction and the reaction will take place in the reverse direction only. The overall cell reaction for E = -1.1 V of the daniel cell is

Cu(s)  + Zn(aq)2+  -- -- -- - > Cu(aq)2+ + Zn(s)

The reversal of the cell current is accompanied by the reversal of direction of the cell reaction. Thus a reversible cell is defined as that operates by reversal of cell reactions when the direction of flow of current is reversed. The reversible behaviour operates only for infinitesimal changes in e.m.f. of the cell on the positive and negative sides from the balance point which corresponds to the actual emf of the cell.


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