Question

Read the passage given below and answer the questions that follow:

Oxidation-reduction reactions are commonly known as redox reactions. They involve transfer of electrons from one species to another. In a spontaneous reaction, energy is released which can be used to do useful work. The reaction is split into two half-reactions. Two different containers are used and a wire is used to drive the electrons from one side to the other and a Voltaic/Galvanic cell is created. It is an electrochemical cell that uses spontaneous redox reactions to generate electricity. A salt bridge also connects to the half-cells. The reading of the voltmeter gives the cell voltage or cell potential or electromotive force. If \[\ce{E^0_{cell}}\] is positive the reaction is spontaneous and if it is negative the reaction is non-spontaneous and is referred to as electrolytic cell. Electrolysis refers to the decomposition of a substance by an electric current. One mole of electric charge when passed through a cell will discharge half a mole of a divalent metal ion such as Cu2+. This was first formulated by Faraday in the form of laws of electrolysis. The conductance of material is the property of materials due to which a material allows the flow of ions through itself and thus conducts electricity. Conductivity is represented by k and it depends upon nature and concentration of electrolyte, temperature, etc. A more common term molar conductivity of a solution at a given concentration is conductance of the volume of solution containing one mole of electrolyte kept between two electrodes with the unit area of cross-section and distance of unit length. Limiting molar conductivity of weak electrolytes cannot be obtained graphically.

1. Is silver plate the anode or cathode?  (1)
2. What will happen if the salt bridge is removed?  (1)
3. When does electrochemical cell behaves like an electrolytic cell?  (1)
4. (i) What will happen to the concentration of Zn²⁺ and Ag⁺ when E_cell = 0.   (1)
   (ii) Why does conductivity of a solution decreases with dilution?  (1)
   OR
   The molar conductivity of a 1.5 M solution of an electrolyte is found to be 138.9 S cm²mol^-1. Calculate the conductivity of this solution.  (2)

Answer 1

a. Silver plate acts as the cathode.

b. By completing the circuit, salt bridge both preserves the charge balance between anode and cathode by the mobility of electrons and lets current flow. The voltage will drop to zero and no current will enter the circuit if the salt bridge is taken out.

c. When an external opposite potential is applied on the galvanic cell and reaction is not stopped until the opposing voltage reaches the value of 1.1 V, an electrochemical cell operates like an electrolytic cell. At this point, no current passes through the cell; so the reaction will operate in the opposite direction when one increases the external potential any more.

E_ext > E_cell

d. (i) When E_cell = 0, an equilibrium condition is reached and the concentration of Zn²⁺ and Ag⁺ remains the same.

(ii) Conductivity of a solution defined as the conductance of ions present in a unit volume of the solution. On dilution, the number of ions per unit volume decreases. Thus, the conductivity of the solution decreases on dilution.

OR

Molar Conductivity (k) = `"Conductivity"/"Concentration"`

∧_m = `k/c`

= `(138.9 " S cm"^2"mol"^-1 xx 1.5 " mol"//"L")/(1000 " cm"^3//l)`

Conductivity = ∧_m × C

= 0.208 Scm^–1