NEET Chemistry Notes Chemical Thermodynamics – Second Law of Thermodynamics
Second Law of Thermodynamics
Second Law of Thermodynamics
In terms of entropy second law of thermodynamics is defined as:
“Entropy of the system and surrounding remains constant in a reversible equilibrium process, while it increases in an irreversible process.”
In nature, all the process are irreversible (spontaneous) going to completion. Hence, entropy of the universe increases and tends to be maximum.
Gibbs Energy Change
Gibbs free energy of a system is a thermodynamic quantity. It is the decrease in the value of energy which during the process is equal to the useful work done by the system 
. It is given by the relation G = H -TS
It is a state function hence, change in free energy 
for a process is independent of path.
This equation is also known as Gibbs Helmholtz equation, i.e.
Criteria of Spontaneity
All spontaneous processes are quantitatively decided by temperature, enthalpy change and entropy change.
Spontaneity and enthalpy change 
The value of enthalpy change describes the spontaneity of process Qualitatively as
Spontaneity and total entropy change 
The value of total entropy change describes the spontaneity of process quantitatively as
Spontaneity and Gibbs free energy (G) In most of the chemical reactions, change in enthalpy and entropy occurs simultaneously. Thus, we can’t decide the spontaneity of process alone on the basis of decrease in enthalpy or increase in entropy.
Thus, a new thermodynamic function is introduced to define spontaneity of chemical process known as Gibbs free energy (G).
This equation is also known as Gibbs Helmholtz equation.
The spontaneity of process is related to Gibbs free energy change as
Third Law of Thermodynamics
This law was proposed by German chemist Walther Nemst. According to this law, “The entropy of a perfectly crystalline substance approaches zero as the absoKite zero of temperature is approached”. It forms the basis from which entropies at other temperatures can be measured, 