1. What Thermodynamics Can and Cannot Do
Thermodynamics tells us how heat, work, and energy relate during physical processes. It deals with macroscopic properties like pressure, temperature, and volume.
However, it does not explain what happens at the microscopic particle level. Because of this, thermodynamics has several limitations.
2. Limitation 1: No Information About Microscopic Behaviour
Thermodynamics does not explain how individual atoms or molecules move or interact. It only studies the overall effect of many particles acting together.
2.1. Why This Is a Limitation
Two systems with different microscopic structures can have the same macroscopic state, but thermodynamics cannot distinguish between them.
3. Limitation 2: Rates of Processes Cannot Be Predicted
Thermodynamics tells whether a process is possible or not, but it cannot say how fast the process will occur.
3.1. Examples
- It tells that ice melts at room temperature but not how long it will take.
- It predicts that a chemical reaction may occur but not the reaction speed.
To study rates, separate subjects like kinetics or transport theory are needed.
4. Limitation 3: Cannot Predict the Path of a Process
Thermodynamics only cares about initial and final states. It does not describe the exact path or intermediate steps between them.
4.1. Why Path Is Important
The path can change the amount of work done. But thermodynamics alone cannot determine which path a real system will follow.
5. Limitation 4: No Insight into Molecular Mechanisms
Thermodynamics cannot explain why gases exert pressure, why heat capacity varies, or why different substances behave differently at the microscopic level.
5.1. Example
It cannot explain why some gases deviate from ideal behaviour; for that, molecular theory or statistical mechanics is required.
6. Limitation 5: Assumes Large Systems Only
Thermodynamics is valid only when a system contains a huge number of particles. It breaks down for very small systems (like nanoparticles or a few molecules).
6.1. Reason
Macroscopic properties such as temperature and pressure are not meaningful for extremely small systems.
7. Limitation 6: Cannot Explain Direction of Processes Without Second Law
The First Law alone cannot tell why heat flows from hot to cold or why natural processes are irreversible. The Second Law is required to understand direction and spontaneity.
7.1. Example
The First Law allows heat to flow either way, but the Second Law says only hot to cold is natural.
8. Limitation 7: No Information About Energy Distribution
Thermodynamics tells the total internal energy of a system but not how that energy is distributed among particles.
8.1. Why This Matters
Understanding energy distribution helps explain heat capacity, molecular speeds, and behaviour of gases, which thermodynamics alone cannot.
9. How Modern Theories Extend Thermodynamics
To overcome these limitations, modern physics uses:
- Kinetic theory of gases – explains microscopic motion of particles.
- Statistical mechanics – connects microscopic behaviour with thermodynamic quantities.
- Quantum mechanics – explains energy levels, molecular behaviour, and heat capacity.
9.1. Why These Are Needed
They help explain the details that thermodynamics cannot, especially atomic interactions and microscopic mechanisms.