Question

(a) How spontaneity of the reaction is determined based on entropy?
(b) What is enthalpy?

Answer 1

When the formation of a product is favoured in a chemical reaction under given conditions, then the reaction is said to be spontaneous. These reactions are generally exothermic and release free energy which can be used to do work. In simple words, the products have comparatively lower energy than reactants in spontaneous reactions.

Complete answer:
(a) Entropy: It is a thermodynamic quantity which is a measure of disorder or randomness of the particles in a system. According to the second law of thermodynamics, entropy of the system predicts the direction of process or reaction and determines whether the reaction is irreversible or not. The observations formulated from the second law of thermodynamics that for a reaction to be spontaneous, the entropy of the universe must always increase.
Therefore, $\Delta {S_{universe}} > 0$
Or it can be represented as $\Delta {S_{system}} + \Delta {S_{surrounding}} > 0$
(b) Enthalpy: It is the thermodynamic quantity which is the amount of heat absorbed or released in process when the pressure of the system is constant. It can also be defined as the total amount of heat of a system equivalent to the summation of internal energy of the system and product of pressure and volume.
It is expressed as follows:
$H = U + PV$
If the sign of enthalpy is negative, then it indicates that the process is exothermic i.e., removal of heat is taking place during the process whereas if the sign of enthalpy is positive, then it indicates that the process is endothermic i.e., heat is gained by the system during the process.

Note:
It is important to note that for a process to be spontaneous, it must be exothermic i.e., the value of enthalpy of reaction must be less than zero for the process. Also, an increase in the entropy of the system always accounts for the irreversibility of the processes which means a spontaneous process must always be an irreversible process. If in case, a reversible process is given then the entropy of the system is considered constant.