Thermodynamics Basics & Processes
Build the ultimate foundation in physical chemistry. Master the distinction between state and path functions, Intensive vs. Extensive properties, and complex thermodynamic processes.
Module Focus
Thermodynamics deals with the energy changes accompanying chemical and physical transformations. The most common mistake students make in this chapter is skipping the foundational terminology. Misidentifying a property as Intensive or Extensive, or confusing a State function with a Path function, will guarantee a loss of marks. This module bullet-proofs your basics.
1. System, Surroundings & Boundaries
The universe in thermodynamics is divided into two parts: the System (the specific part of the universe under observation) and the Surroundings (everything else). The real or imaginary surface separating them is the Boundary.
Can exchange both matter and energy with the surroundings.
Example: Hot water in an open beaker. Mass escapes as steam, heat escapes through walls.
Can exchange only energy (not matter) with the surroundings.
Example: Hot water in a sealed, conducting metallic container. No mass escapes, but it cools down.
Can exchange neither matter nor energy with the surroundings.
Example: Hot coffee in a perfectly insulated, sealed thermos flask.
2. Intensive vs. Extensive Properties (Highly Tested)
Macroscopic properties of a system are classified based on their dependence on the amount (mass or quantity) of matter present.
Properties whose values are INDEPENDENT of the quantity or size of matter present in the system. (Trick: INtensive = INdependent).
- Temperature ($T$)
- Pressure ($P$)
- Density ($d$)
- Refractive Index
- Standard EMF of a cell ($E^\circ$)
- Boiling point / Freezing point
Properties whose values DEPEND on the quantity or size of matter present in the system.
- Mass ($m$)
- Volume ($V$)
- Internal Energy ($U$)
- Enthalpy ($H$)
- Entropy ($S$)
- Heat Capacity ($C$)
-
The Ratio Rule: The ratio of two extensive properties always gives an Intensive property.
Examples:
$\frac{\text{Mass (Extensive)}}{\text{Volume (Extensive)}} = \mathbf{\text{Density (Intensive)}}$
$\frac{\text{Moles (Extensive)}}{\text{Volume (Extensive)}} = \mathbf{\text{Molarity (Intensive)}}$ -
The "Molar / Specific" Rule: Whenever the word "molar" or "specific" is attached to an extensive property, it becomes Intensive because you are fixing the amount to 1 mole or 1 gram.
Example: Heat Capacity is Extensive. But Molar Heat Capacity or Specific Heat are Intensive!
3. State Functions vs. Path Functions
State Functions (State Variables)
A property whose value depends only on the initial and final states of the system and is entirely independent of the path taken to reach that state. They are represented by capital letters and are exact differentials.
Path Functions
Properties whose values depend on the path or mechanism followed to change the state of the system. They are represented by lowercase letters and are inexact differentials.
Note: While $q$ and $w$ are path functions, their sum ($q + w = \Delta U$) is a state function according to the First Law.
4. Thermodynamic Processes
A thermodynamic process occurs when a system undergoes a change from one state to another. Knowing the mathematical condition for each process is vital for solving numericals.
| Process Type | Defining Condition | Key Consequence (for Ideal Gas) |
|---|---|---|
| Isothermal | Constant Temperature ($\Delta T = 0$) | Since Internal Energy ($U$) of an ideal gas depends only on $T$, $\mathbf{\Delta U = 0}$ and $\mathbf{\Delta H = 0}$. |
| Isochoric | Constant Volume ($\Delta V = 0$) | Work done ($w = -P_{\text{ext}} \Delta V$) is zero. Hence, $\mathbf{\Delta U = q_V}$. |
| Isobaric | Constant Pressure ($\Delta P = 0$) | Heat exchanged at constant pressure is equal to Enthalpy change: $\mathbf{\Delta H = q_P}$. |
| Adiabatic | No heat exchange ($q = 0$) | The system is perfectly insulated. Since $q=0$, the First Law gives $\mathbf{\Delta U = w_{\text{adiabatic}}}$. |
| Cyclic | Initial State = Final State | The change in ALL state functions is zero. ($\mathbf{\Delta U = 0, \Delta H = 0, \Delta S = 0}$). Hence $q = -w$. |
Reversible vs. Irreversible Processes
- Reversible Process: A process carried out infinitesimally slowly such that the system and surroundings are always in near-perfect thermal and mechanical equilibrium. The driving force is only infinitesimally greater than the opposing force ($P_{\text{gas}} \approx P_{\text{ext}}$). It requires infinite time. Maximum work is obtained during a reversible expansion.
- Irreversible Process: A process that occurs rapidly in a single step or finite number of steps. The system is NOT in equilibrium during the transition. The opposing force differs significantly from the driving force ($P_{\text{ext}} \neq P_{\text{gas}}$). All natural, spontaneous processes are irreversible.
NEET Grand Test: Thermo Basics
15 High-Order Thinking Questions testing Intensive/Extensive properties and process mechanics.
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