Question

Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction?
A. Exothermic and decreasing disorder
B. Endothermic and increasing disorder
C. Exothermic and increasing disorder
D. Endothermic and decreasing disorder

Answer 1

Hint:The spontaneity of a reaction is decided by the change in Gibbs free energy (\[\Delta G\] ) associated with it. A negative value of the change in Gibbs free energy indicates that the reaction is spontaneous. Recollect the mathematical relation between Gibbs free energy change (\[\Delta G\]) and the enthalpy (\[\Delta H\] ) and entropy change (\[\Delta S\] ) associated with a reaction.

Formula used:
\[\Delta G = \Delta H - T\Delta S\]
\[\Delta G\] = change in Gibbs free energy (SI unit = Joule (J))
\[\Delta H\] = change in enthalpy (SI unit = Joule (J) or Joule per mole (\[Jmo{l^{ - 1}}\] ))
 T = temperature (assumed to be constant) (SI unit = Kelvin (K))
\[\Delta S\] = change in entropy (SI unit = Joules per Kelvin (\[J{K^{ - 1}}\] ))

Complete step-by-step answer:
A reaction is said to be spontaneous when it takes place on its own without an external addition of energy. Water flowing down a waterfall is an example of a spontaneous process.
The enthalpy of a system (H) is a thermodynamic parameter that is, in essence, a measure of the energy content of the system. During a chemical reaction, as reactants convert into products, the enthalpy associated with the reaction changes. This change in the enthalpy (\[\Delta H\]) is given by the difference in the enthalpies of the products and the reactants.
\[\Delta H = {H_{products}} - {H_{reactants}}\]
A chemical reaction that releases energy is called an exothermic reaction and one that absorbs energy is called an endothermic reaction. The \[\Delta H\]for an exothermic reaction is negative while \[\Delta H\]for an endothermic reaction is positive.
The entropy of a system (S) is a measure of how much the energy of the atoms and molecules of the system becomes spread out over the course of a reaction. It is also commonly described as a measure of the chaos and disorder within the system.
The spontaneity of a reaction can be predicted from the value of the change in Gibbs free energy (\[\Delta G\]) associated with the reaction. \[\Delta G\] is related to \[\Delta H\]and \[\Delta S\]as \[\Delta G = \Delta H - T\Delta S\].
Assuming the temperature to be constant, for a spontaneous reaction, its\[\Delta G\] must be negative. \[\Delta G\] can be negative only when \[\Delta H\] is negative and \[\Delta S\] is positive i.e., only when the reaction is exothermic, and disorder is increasing.


Thus, option C is correct.

Note:The approach to this question hinges on the assumption that temperature is constant throughout. The temperature can be assumed to be constant unless stated otherwise in the question.