Introduction
Thermodynamics is a branch of physical chemistry that deals with the study of energy changes, particularly heat and work, in chemical processes. It is a crucial topic for NEET Chemistry as it lays the foundation for understanding various physical and chemical phenomena. In this study note, we will break down the key concepts of thermodynamics, making them easier to understand and apply.
Key Concepts of Thermodynamics
System and Surroundings
- System: The part of the universe under study.
- Surroundings: Everything else outside the system.
- Boundary: The real or imaginary surface that separates the system from its surroundings.
Always clearly define the system and surroundings to avoid confusion in thermodynamic calculations.
Types of Systems
- Open System: Can exchange both matter and energy with the surroundings (e.g., an open beaker of water).
- Closed System: Can exchange energy but not matter with the surroundings (e.g., a sealed container of gas).
- Isolated System: Cannot exchange either matter or energy with the surroundings (e.g., a thermos flask).
State Functions and Path Functions
- State Functions: Properties that depend only on the state of the system (e.g., pressure, volume, temperature, internal energy).
- Path Functions: Properties that depend on the path taken to reach a particular state (e.g., work, heat).
State functions are independent of the path, while path functions are dependent on the path taken.
First Law of Thermodynamics
The first law of thermodynamics, also known as the Law of Energy Conservation, states that energy cannot be created or destroyed, only transferred or converted from one form to another.
$$ \Delta U = q + w $$
Where:
- $\Delta U$ = Change in internal energy
- $q$ = Heat added to the system
- $w$ = Work done on the system
If 50 J of heat is added to a system and 20 J of work is done by the system, the change in internal energy is: $$ \Delta U = q + w = 50 , \text{J} + (-20 , \text{J}) = 30 , \text{J} $$
Work and Heat
- Work (w): Energy transfer due to a force acting over a distance.
- For expansion/compression work: $w = -P \Delta V$
- Heat (q): Energy transfer due to temperature difference.
Do not confuse heat (q) with temperature. Heat is energy transfer, while temperature is a measure of thermal energy.
Enthalpy (H)
Enthalpy is a state function defined as the total heat content of a system at constant pressure.
$$ H = U + PV $$
Where:
- $H$ = Enthalpy
- $U$ = Internal energy
- $P$ = Pressure
- $V$ = Volume
Second Law of Thermodynamics
The second law states that the total entropy of an isolated system can never decrease over time. It can remain constant for a reversible process but increases for an irreversible process.
$$ \Delta S_{universe} = \Delta S_{system} + \Delta S_{surroundings} \geq 0 $$
Where:
- $\Delta S$ = Change in entropy
Entropy measures the disorder or randomness of a system. Higher entropy means greater disorder.
Gibbs Free Energy (G)
Gibbs free energy is a state function that combines enthalpy and entropy to predict the spontaneity of a process at constant temperature and pressure.
$$ G = H - TS $$
Where:
- $G$ = Gibbs free energy
- $H$ = Enthalpy
- $T$ = Temperature
- $S$ = Entropy
A process is spontaneous if $\Delta G
< 0$.
Third Law of Thermodynamics
The third law states that the entropy of a perfect crystal at absolute zero temperature is zero.
Thermodynamic Processes
Isothermal Process
- Isothermal: Occurs at constant temperature.
- For an ideal gas, $q = -w$.
Adiabatic Process
- Adiabatic: Occurs without heat exchange with the surroundings.
- $\Delta U = w$
Isochoric Process
- Isochoric: Occurs at constant volume.
- $w = 0$, so $\Delta U = q$
Isobaric Process
- Isobaric: Occurs at constant pressure.
- $\Delta H = q$
Calculate the work done when 2 moles of an ideal gas expand isothermally and reversibly from 10 L to 20 L at a temperature of 300 K.
$$ w = -nRT \ln \left( \frac{V_f}{V_i} \right) $$
$$ w = -2 \times 8.314 \times 300 \ln \left( \frac{20}{10} \right) $$
$$ w = -4988.4 , \text{J} $$
Conclusion
Understanding thermodynamics is essential for mastering NEET Chemistry. By breaking down the concepts into manageable sections and using clear definitions, equations, and examples, you can develop a solid foundation in this topic. Remember to practice problems regularly to reinforce your understanding and application of these principles.
