Question

Cell reaction is spontaneous, when:
(A) ${{E}^{o}}_{red}$ is negative
(B) $\Delta {{G}^{o}}$ is negative
(C) ${{E}^{o}}_{oxid}$ is positive
(D) $\Delta {{G}^{o}}$ is positive

Answer 1

Write down a simple cell reaction which is spontaneous i.e. the reaction is feasible. Now write the Gibbs-Helmholtz equation to find a relation between the electrochemical and thermodynamic factors of a cell. Now the third law of thermodynamics will help us to find if the reaction is spontaneous or not.

Complete step by step answer:
Electrochemistry is the branch of physical chemistry that mainly deals with the relationship between electricity and identifiable chemical change.
Electrochemical reaction is a chemical reaction in which current is externally supplied or produced through a spontaneous chemical reaction.
Chemical reactions where electrons are directly transferred between the constituent molecules or atoms are called oxidation-reduction or rather redox reactions.
A salt bridge is used in chemical reactions where the anode solution is different from a cathode solution. In this case salt bridge is used to complete the circuit so that there is passage of current and hence chemical reaction takes place.
The third law of thermodynamics give the following formula:
$\text{ }\\!\\!\Delta\\!\\!\text{ G = }\\!\\!\Delta\\!\\!\text{ H }-\text{ T }\\!\\!\Delta\\!\\!\text{ S}$

The relation between thermodynamics and electrochemical features is given by the Gibbs Helmholtz equation.
$\text{d(G/T)=}\dfrac{\text{- HdT + TV(dP)}}{{{\text{T}}^{\text{2}}}}$
Where,
G is Gibbs free energy
H is the enthalpy
S is entropy
V is the volume
P is the pressure
T is the temperature
Therefore, the cell reaction is spontaneous or rather takes place when the reaction is feasible. We have already studied that for a reaction to be feasible, Gibbs free energy must be negative.
So, the correct answer is “Option B”.

Note: Gibbs free energy mentioned in the above formula is a measure of unwanted work that happens in a reaction. This is the reason why change in Gibbs energy must be negative for the overall reaction to take place and be thermodynamically feasible.