Question

The $\Delta G$ in the process of melting of ice at $ - {15^o}C$ is:
A. $\Delta G < 0$
B. $\Delta G > 0$
C. $\Delta G = 0$
D. all of these

Answer 1

Hint: The spontaneous process can be defined as the one in which there is a loss of free energy in order to gain stability by the system. Thus, spontaneity of a reaction only determines whether or not a process can occur. It gives no indication as to whether the process will occur or not. In other words, spontaneity is a necessary, but not sufficient, condition for a process to actually occur.

Complete step by step answer:
For a process that occurs spontaneously, the change is Gibbs free energy of the process should remain negative. In other words, the sign convention for the change in Gibbs free energy is an important criterion for determining the spontaneity of the reaction. The change in Gibbs free energy is used in a reaction when we consider the processes to occur under constant pressure and temperature conditions.
Thus, for a process that occurs at constant pressure and temperature, the change in Gibbs free energy can be determined mathematically as:
 $\Delta G = \Delta H - T\Delta S$
Where, $\Delta G = $ change in Gibbs free energy
$\Delta H = $ change in enthalpy of system
$\Delta S = $ change in the entropy of the system
$T = $ temperature
Now, as we know that melting of ice can only be achieved at a temperature of ${0^o}C$ , this means that below this temperature (even at $ - {15^o}C$), the process will be considered non-spontaneous for melting of ice. Hence, the value of change is Gibbs free energy is positive or simply, greater than zero.


Thus, the correct option is B. $\Delta G > 0$ .

Note:
The various cases defined for change in Gibbs free energy are:
(i) $\Delta G < 0$= This means that the process is spontaneous/ continuous/ irreversible.
(ii) $\Delta G > 0$= This means that the process is nonspontaneous/ discontinuous/ reversible.
(iii) $\Delta G = 0$= This means that the process has achieved equilibrium and no extra free energy is left for it to get released.