Question

EMF of the cell $Pt.{H_2}(1\,atm)|{H^ + }(aq.)||AgCl|Ag$ is 0.27 V and 0.26 V at 25°C and 35°C. Heat of reaction occurring inside the cell at 25°C is:
A. -54.8 kJ
B. 26.05 kJ
C. -26.05 kJ
D. +54.8 kJ

Answer 1

Hint- In order to solve this question we need to have the basic understanding of an electrochemical cell. We will use direct results to calculate the heat of reaction or Gibb’s energy change, and reach the answer of this question.

Complete step-by-step answer:
In an electrochemical cell, for a spontaneous redox reaction to produce an electric potential in the cell, the change in Gibbs free energy must be negative. This is the opposite of cell potential, which is positive when electrons flow spontaneously through the electrochemical cell.
We know the heat dissipated in an electrochemical cell comes out as the Gibbs free energy. This energy is related to the electrode potential of the cell as:
$\Delta G = - nF{E^0}_{cell}$
Where F is 1 faraday= 96500C
n is the number of moles of electrons gained or lost. For the given cell, n=1
Electrode potential of the cell = 0.27V (Given)
Putting the value in the energy equation:
$
  \Delta G = - 1 \times 96500 \times 0.27 \\
  \Delta G = - 26.05kJ \\
$
Hence -26.05 kJ is the required heat of reaction. And the correct option is C.

Note- The change in Gibbs free energy for an electrochemical cell is related to the cell potential. This energy change tells us if the reaction will be spontaneous or not. If the Gibbs energy change is negative, the reaction is spontaneous. Whereas the reaction becomes nonspontaneous, if the energy change is positive.