Question

Which of the following is correct for a spontaneous reaction at any temperature?
A. $\Delta H = - ve,\;\Delta S = + ve,\;\Delta G = - ve$
B. $\Delta H = - ve,\;\Delta S = - ve,\;\Delta G = - ve$
C. $\Delta H = + ve,\;\Delta S = + ve,\;\Delta G = - ve$
D. $\Delta H = - ve,\;\Delta S = + ve,\;\Delta G = + ve$

Answer 1

Hint: Spontaneous reaction is a process which has a natural tendency to occur either by its own or after proper initiation under a given set of conditions.

Complete step by step answer:
Enthalpy change and spontaneity: All physical and chemical changes are accompanied by energy changes. If we examine the dropping of a stone from a building, flowing of water down a hell, a net decrease in potential energy in the direction of change takes place. As we know that the stone or water possessed more potential energy at higher level than at lower level. This suggests that macroscopic objects proceed by decrease of energy to undergo a spontaneous process because the state of lowest energy corresponds to maximum stability. Similarly, the chemical reactions which are exothermic in nature and spontaneous. For example:

${H_2}(g) + \dfrac{1}{2}{O_2}(g) \to {H_2}O(l)\;\Delta H = - ve$

Entropy change and spontaneity:
Entropy: It is a thermodynamic term which measures the randomness or disorder of the system.
Entropy of the substance increases from solid to liquid and liquid to gas.
When the substance is heated in a system and heat is absorbed by the system, the molecules undergo vigorous movement due to increased kinetic energy. Thus, we can say that heat has a randomizing influence of the system and then the system is spontaneous $(i.e.\;\Delta S = + ve)$.
For a spontaneous reaction at any temperature:

$\Delta H = - ve,\;\Delta S = + ve,\;\Delta G = - ve$

For spontaneous processes, the change in Gibbs free energy is negative.
According to relation, $\Delta G = \Delta H - T\Delta S$
Where, $\Delta G$ is Gibbs free energy change, $\Delta H$ is change in Enthalpy and $\Delta S$ change in Entropy. This is possible at all temperatures when Enthalpy change is negative and Entropy change is positive. Gibbs free energy: It is a thermodynamic term, whose decrease in value during a process which is useful work done by the system.

Hence, the correct option is (A).

Note: A spontaneous process does not mean that the process should be instantaneous. The rate of process may vary from extremely slow to extremely fast.