Question

What are entropy driven reactions? How can a reaction with positive changes of enthalpy and entropy be made entropy driven?

Answer 1

Entropy is a measure of randomness or disorder of the system. The greater the randomness, the higher is the entropy. The crystalline solid state has the lowest entropy and the gaseous state has the highest entropy.

Complete answer:
We know that Gibbs free energy is defined mathematically by the equation;
$\Delta G = \Delta H - T\Delta S$
Where, $\Delta H$ = enthalpy change of the system
$\Delta S$ = Entropy change of the system
If $\Delta G$ is negative , the process will be spontaneous (the process that can take place by itself or has an urge or tendency to take place).
If in a certain reaction;
$\Delta H > 0$ and $\Delta S > 0$ at high temperature. The reaction will be entropy driven.
If both the quantities are positive, $\Delta G$ can be negative, if $T\Delta S > \Delta {\rm H}$ . The entropy factor should dominate over the enthalpy factor. Thus known as entropy driven reactions. There are two ways;
Entropy change ( $\Delta S$ ) should be very large such that even at lower T values , $T\Delta S$ should be greater than $\Delta H$
Temp (T) should be so large that even if $\Delta S$ is small, $T\Delta S > \Delta H$ .

Additional Information:
Gibbs free energy is that thermodynamic quantity of a system, the decrease in whose value during a process is equal to the maximum possible useful work that can be obtained from the system. It should be noted that if $\Delta G$ is zero, the process is in equilibrium.

Note:
We have learnt about entropy driven reactions. According to the laws of thermodynamics the energy of the universe is constant whereas the entropy of the universe is continuously increasing and tends to be maximum.