Chapter 5 – Thermodynamics (JEE)
1. What is Thermodynamics?
Thermodynamics is the branch of chemistry that deals with the study of:
- Energy changes
- Heat transfer
- Work done
- Feasibility of chemical reactions
2. System, Surroundings and Universe
- System: Part of the universe under study
- Surroundings: Rest of the universe
- Universe: System + Surroundings
Types of Systems
- Open system – exchange of matter and energy
- Closed system – exchange of energy only
- Isolated system – no exchange
3. State of a System
The state of a system is defined by measurable properties such as:
- Pressure
- Volume
- Temperature
- Composition
4. State Functions and Path Functions
| State Functions | Path Functions |
|---|---|
| Depend only on initial and final state | Depend on path followed |
| Examples: $U, H, S, G$ | Examples: $q, w$ |
5. Internal Energy ($U$)
Internal energy is the total energy contained in a system due to:
- Molecular motion
- Intermolecular forces
$\Delta U = U_{final} - U_{initial}$
6. Work Done ($w$)
$w = -P \Delta V$
Negative sign indicates work done by the system.
7. Heat ($q$)
Heat is the energy transferred due to temperature difference.
Heat absorbed → $q > 0$
Heat released → $q < 0$
8. First Law of Thermodynamics
$\Delta U = q + w$
Energy can neither be created nor destroyed, it can only be transformed.
9. Enthalpy ($H$)
$H = U + PV$
$\Delta H = \Delta U + \Delta(PV)$
10. Enthalpy Change for Reactions
- Exothermic reaction: $\Delta H < 0$
- Endothermic reaction: $\Delta H > 0$
11. Heat Capacity
$C = \frac{q}{\Delta T}$
$q = mC\Delta T$
12. Hess’s Law of Constant Heat Summation
Total enthalpy change of a reaction is independent of the path followed.
13. Enthalpy of Formation
Enthalpy change when 1 mole of a compound is formed from its elements.
14. Bond Enthalpy
$\Delta H = \sum E_{bonds\,broken} - \sum E_{bonds\,formed}$
15. Entropy ($S$)
Entropy is the measure of randomness or disorder of a system.
$\Delta S = \frac{q_{rev}}{T}$
16. Second Law of Thermodynamics
Entropy of the universe always increases for a spontaneous process.
$\Delta S_{universe} = \Delta S_{system} + \Delta S_{surroundings}$
17. Gibbs Free Energy ($G$)
$G = H - TS$
$\Delta G = \Delta H - T\Delta S$
18. Criteria of Spontaneity
| $\Delta G$ | Process |
|---|---|
| $<0$ | Spontaneous |
| $=0$ | Equilibrium |
| $>0$ | Non-spontaneous |
19. Relation Between $\Delta G$ and Equilibrium Constant
$\Delta G^\circ = -RT \ln K$
20. Common JEE Traps
- Confusing sign conventions
- Forgetting temperature in Kelvin
- Mixing $\Delta U$ and $\Delta H$
- Incorrect application of Hess’s law
21. Final Revision Checklist
You have mastered Thermodynamics if you can:
- Apply first law confidently
- Calculate $\Delta H$, $\Delta S$, $\Delta G$
- Predict spontaneity
- Solve numerical problems accurately