Question

What is Helmholtz free energy and Gibbs free energy?

Answer 1

Hint: Thermodynamic potentials: It is a scalar quantity which represents the thermodynamic state of a system. These are the functions of thermodynamic variables pressure, volume, temperature, and entropy.

Complete answer:
Helmholtz free energy: It is a thermodynamic potential in which the work is measured when the system is closed and volume and temperature of the system is constant.
It can be represented from the following equation:
$$F=U-TS$$
where,
 $F\to$ Helmholtz free energy
$U\to$ Internal energy
$T\to$ Temperature
$S\to$ Entropy
Derivative form of Helmholtz free energy in terms of thermodynamic potential can be represented as follows:
$dF=-PdV-SdT$
Hence it can be written as function of volume and temperature as $F=F(V,T)$
Application of Helmholtz free energy:
Specific heat at constant pressure can be determined as follows:
$C_V=-T{\left({\displaystyle \frac{{\partial }^{2}F}{\partial T}}\right)}_V$
Gibbs free energy: It is a thermodynamic potential in which the maximum reversible work is measured when pressure and temperature of the system is constant.
It can be represented from the following equation:
$$G=H-TS$$
where,
 $G\to$ Gibbs free energy
$H\to$ Enthalpy
$T\to$ Temperature
$S\to$ Entropy
Derivative form of Gibbs free energy in terms of thermodynamic potential can be represented as follows:
$dG=VdP-SdT$
Hence it can be written as function of pressure and temperature as $G=G(P,T)$
Application of Gibbs free energy:
Can be used to determine spontaneity of the reaction.
Specific heat at constant pressure can be determined as follows:
$C_P=-T{\left({\displaystyle \frac{{\partial }^{2}G}{\partial T}}\right)}_P$

Note:
Helmholtz energy is sometimes represented with the symbol ‘A’, so need not to get confused. For finding relations between thermodynamic potentials and energies, a thermodynamic square method should be used.