NEET Chemistry Notes Chemical Thermodynamics – Hess’s Law of Constant Heat Summation
Hess’s Law of Constant Heat Summation
Hess’s Law of Constant Heat Summation
Total enthalpy change during complete course of reaction in a single step and several steps are equal. Therefore enthalpy change during several steps conversion is equal to enthalpy change during single step conversion.
Applications of Hess’s Law
- In determination of heat of reaction.
- In determination of heat of formation.
- In determination of heat of transition.
Heat Capacity
Heat capacity (C) of a system is defined as the amount of heat required to raise the temperature of the system by 1°C.
If the system consists of a single substance or a solution and weighs 1 mole, the heat capacity of the system is referred as molar heat capacity.
If the system consists of a single substance or a solution and weights 1 g, the heat capacity of the system is referred as specific heat of the system.
where, m = mass of substance,
C = specific heat capacity.
Molar Heat Capacity in Different Cases
- Molar heat capacity at constant pressure
Molar heat capacity, at constant voncne, Cv =ey x M(M = molecular weight of gas )
(Cp and Cv are specific heats at constant pressure and volume respectively and M is molecular weight of gas.) - Cp-Cv =R[R is the molar gas constant.)
- The molar heat capacity at constant volume,
- The molar heat capacity at constant pressure
- Poisson’s ratio,
Kirchhoff s Equation
According to this equation, the partial derivatives of the change of enthalpy (or of internal energy) during a reaction, with respect to temperature, at constant pressure (or volume) equals the change in heat capacity at constant pressure (or volume)
Entropy Change 
Entropy is the measurement of randomness or disorder of the molecules.
“For a spontaneous process in an isolated system, the change in entropy is positive this statement is known as second law of thermodynamics”. A process, which proceeds of its own accordance without any’ outside help is termed as spontaneous process. Entropy is a state function and depends only on initial and final states of the system, i.e.
Unit of entropy is joule per kelvin per mol.
Entropy is the measure of unavailable energy.
Unavailable energy = entropy x temperature,
For a reversible change at constant temperature
=heat absorbed or evolved at absolute temperature, T
If 
, heat is absorbed and if 
, heat is evolved. The change of matter from one state to another is called phase transition.
Spontaneous Process
A physical or chemical change that occurs by its own, i.e. without the help of an external source is called spontaneous process.
The entropy changes at the time of phase transition are as follows:
