Thermodynamics forms a crucial part of Physical Chemistry for JEE. It deals with energy changes, spontaneity, and equilibrium in chemical systems. This guide covers the fundamental laws of thermodynamics, important thermodynamic functions such as internal energy, enthalpy, entropy, Gibbs free energy, and their applications. We also discuss important equations, derivations, and tips to solve JEE problems confidently.
Thermodynamics studies the relationship between heat, work, energy, and how these influence chemical reactions and physical changes.
Key Terminologies:
Also known as the Law of Conservation of Energy, it states:
Energy can neither be created nor destroyed, only transformed.
Mathematically,
ΔU = Q - W
Where:
Internal energy is the total energy contained within a system, including kinetic and potential energies of molecules.
For work done by an ideal gas during expansion or compression,
W = \int P dV
Enthalpy is a state function useful for processes at constant pressure:
H = U + PV
Change in enthalpy ΔH is heat absorbed or evolved at constant pressure:
ΔH = ΔU + PΔV = Q_P
The total enthalpy change for a reaction is the same regardless of the path taken, provided initial and final states are the same.
This helps calculate enthalpy changes indirectly using known reactions.
It states that the entropy of the universe always increases for spontaneous processes.
\textit{No process is possible whose sole result is the transfer of heat from a cooler to a hotter body.}
Entropy is a measure of randomness or disorder of a system.
Change in entropy for a reversible process at temperature T:
\Delta S = \frac{Q_{\text{rev}}}{T}
Combines enthalpy and entropy to predict spontaneity at constant temperature and pressure:
G = H - TS
Change in Gibbs free energy:
ΔG = ΔH - TΔS
Used at constant volume and temperature:
A = U - TS
Derived from the fundamental thermodynamic equations, these relate different partial derivatives:
| Relation | Equation |
|---|---|
| \(\left(\frac{\partial T}{\partial V}\right)_S = -\left(\frac{\partial P}{\partial S}\right)_V\) | Relates temperature & pressure changes |
| \(\left(\frac{\partial T}{\partial P}\right)_S = \left(\frac{\partial V}{\partial S}\right)_P\) | Relates temperature & volume changes |
| \(\left(\frac{\partial S}{\partial V}\right)_T = \left(\frac{\partial P}{\partial T}\right)_V\) | Entropy & pressure dependence on temperature and volume |
| \(\left(\frac{\partial S}{\partial P}\right)_T = -\left(\frac{\partial V}{\partial T}\right)_P\) | Entropy & volume dependence on temperature and pressure |
Relates pressure and temperature changes during phase transitions:
\frac{dP}{dT} = \frac{\Delta H_{\text{trans}}}{T \Delta V_{\text{trans}}}
Describes vapor pressure dependence on temperature:
\ln P = -\frac{\Delta H_{\text{vap}}}{RT} + C
Or differential form:
\frac{d \ln P}{dT} = \frac{\Delta H_{\text{vap}}}{RT^2}
Standard values of ΔH°, ΔS°, ΔG° are measured at 1 atm and 298 K.
Heat absorbed or released under standard conditions.
Change in entropy under standard conditions.
Change in Gibbs energy under standard conditions.
ΔG° = -RT \ln K
This connects thermodynamics to chemical equilibrium.
Idealized thermodynamic cycle with maximum efficiency operating between two temperatures \(T_H\) (hot) and \(T_C\) (cold).
\eta = 1 - \frac{T_C}{T_H}
Thermodynamics combines theory and problem-solving skills. Strong command on this topic ensures high scoring in JEE Chemistry and deepens your understanding of energy transformations in chemical processes.