Question

The effect of temperature on the spontaneity of reactions are represented as:  Which of the following is correct?

• (B) and (D) only
• (A) and (D) only
• (B) and (C) only
• (A) and (C) only

Hint:

The spontaneity of a reaction depends on both the enthalpy and the entropy change, and the effect of temperature can either make a reaction spontaneous or non-spontaneous.

Answer 1

The Correct Option is C

The spontaneity of a reaction as influenced by temperature is evaluated using the Gibbs free energy change equation:

\(\Delta G = \Delta H - T\Delta S\)

Definitions:

• \(\Delta G\) represents the change in Gibbs free energy.
• \(\Delta H\) represents the change in enthalpy.
• \(T\) is the absolute temperature (in Kelvin).
• \(\Delta S\) represents the change in entropy.

Reaction spontaneity is determined by the signs of \(\Delta H\) and \(\Delta S\):

1. If \(\Delta H > 0\) and \(\Delta S < 0\):
   • The reaction is non-spontaneous at all temperatures, as both terms contribute to an increase in \(\Delta G\).
2. If \(\Delta H > 0\) and \(\Delta S > 0\):
   • The reaction is spontaneous at high temperatures, as the \(T\Delta S\) term can exceed the positive \(\Delta H\).
3. If \(\Delta H < 0\) and \(\Delta S < 0\):
   • The reaction is spontaneous at low temperatures, as the negative \(\Delta H\) is dominant over the negative \(T\Delta S\).
4. If \(\Delta H < 0\) and \(\Delta S > 0\):
   • The reaction is spontaneous at all temperatures, as both terms lead to a decrease in \(\Delta G\).

Options (B) and (C) are identified as correct because they correspond to cases 2 and 3, respectively, where temperature influences reaction spontaneity.