Question

In a reaction $\Delta H$ and $\Delta S$ both are positive. In which of the following cases, the reaction would be spontaneous?
A. $\Delta H>T\Delta S$
B. $\Delta H=T\Delta S$
C. $\Delta S=\dfrac{\Delta H}{T}$
D. All of these

Answer 1

Spontaneous reactions are generally those reactions which favors the formation of products at the conditions under which the reaction goes on. These types of reactions are exothermic in nature; the main example of spontaneous reaction is bonfire.

Complete Solution :
- Spontaneity of any reaction is based on a term called Gibbs free energy; it can be described as the quantity that is used to measure the maximum amount of work done in a thermodynamic system when the temperature and pressure are kept constant. Gibbs free energy can be represented by the symbol G and it can also be known by Gibbs function or Gibbs free enthalpy. The equation for Gibbs free energy is given by:
$\Delta G=\Delta H-T\Delta S$
- Where $\Delta G$ represents change in Gibbs free energy, $\Delta H$ changes in enthalpy, T shows temperature and $\Delta S$ shows entropy and its value is generally expressed in terms of Joules or Kilojoules.
- For the reaction to be spontaneous the value of $\Delta G$ should be less than zero i.e. $\Delta G<0$
From the above statement we can conclude that reaction will be spontaneous only if $\Delta H>T\Delta S$
So, the correct answer is “Option A”.

Note: Spontaneous reaction can also be defined as the reactions which are considered to be natural because these types of reactions occur by itself without any external action towards it and all spontaneous processes are thermodynamically reversible.