Question

1.Write any two functions of salt bridge
2. What is cell constant? Write its SI unit.
3. Write the Nernst equation for the cell reaction in the Daniel cell. How will ${{\text{E}}_{{\text{cell}}}}$ be affected when concentration of $Z{n^{2 + }}$ ions is increased?

Answer 1

Hint:To answer this question, you must be familiar with the concept of electrochemistry and the structure of an electrochemical cell. A galvanic cell is an electrochemical cell composed of two half cells joined together by a salt bridge. The Nernst equation contains the logarithmic ratio of the concentration oxidized species by the reduced species.

Complete answer:
1. A salt bridge is an inverted U- tube device which is used in an electrochemical cell for connecting the oxidation and reduction half cells wherein a weak electrolyte is used. In other words, a salt bridge is the junction connecting the anodic and cathodic compartments of a galvanic cell. The salt bridge is a tube which is filled with a strong electrolyte namely silver nitrate, potassium chloride, etc. The functions of a salt bridge are:
Prevention of the diffusion or mixing of the two electrolyte solutions while maintaining the constant flow of electrons.
Prevention or minimization of the liquid-liquid junction potential. It is the potential that arises between two electrolyte solutions when they are in contact with each other.
2. The ratio of the distance of between the electrodes of a cell to the area of the electrode plates is termed as the cell constant of a certain cell. It is given by the expression,
$k = \dfrac{{\text{l}}}{{\text{A}}}$.
Hence, the SI unit of cell constant can be written as ${\text{c}}{{\text{m}}^{{\text{ - 1}}}}$
3. The reaction occurring in a Daniel cell can be given as,
$Zn + C{u^{2 + }} \to Z{n^{2 + }} + Cu$
Therefore, the Nernst Equation for a Daniel cell can be written as
${{\text{E}}_{{\text{cell}}}} = {\text{E}}_{{\text{cell}}}^{\text{o}} - \dfrac{{0.059}}{2}\log \dfrac{{\left[ {{\text{Z}}{{\text{n}}^{2 + }}} \right]}}{{\left[ {{\text{C}}{{\text{u}}^{2 + }}} \right]}}$
When the concentration of $Z{n^{2 + }}$ ions is increased, the ${{\text{E}}_{{\text{cell}}}}$decreases. This relation can be easily determined using the Nernst Equation for the cell.

Note:
A Daniel cell is an example of a Galvanic cell and it converts chemical energy into electrical energy. A Daniel cell consists of two electrodes of different metals zinc and copper. It uses the spontaneous redox reaction between zinc and copper to produce electric current.