Table of Contents
- Zeroth Law of Thermodynamics
- First Law of Thermodynamics
- Second Law of Thermodynamics
- Third Law of Thermodynamics
- Importance & Applications
- Exam-Oriented Points
- MCQs with Answers
Zeroth Law of Thermodynamics
The Laws of Thermodynamics are fundamental principles that govern energy transfer, heat, work, and the behavior of physical and chemical systems. These laws are extremely important for Class 11 & 12 Chemistry and are frequently asked in JEE, NEET, CUET, IIT-JAM, and university exams. Laws of Thermodynamics
Statement: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
This law introduces the concept of temperature. It forms the basis for the construction of thermometers. Example:
- If body A and body B are separately in thermal equilibrium with body C, then A and B have the same temperature.
Exam Tip:
- Direct theory questions are common.
- Often asked: “Which law defines temperature?”
First Law of Thermodynamics
Statement: Energy can neither be created nor destroyed; it can only be converted from one form to another.
Mathematical expression:
ΔU = Q − W
- ΔU = Change in internal energy
- Q = Heat supplied to the system
- W = Work done by the system
Special Cases:
- Isochoric Process: W = 0 → ΔU = Q
- Isothermal Process: ΔU = 0 → Q = W
- Adiabatic Process: Q = 0 → ΔU = −W
Numerical Importance:
- Most numericals in JEE & NEET are based on this law.
- Sign convention is extremely important.
Second Law of Thermodynamics
This law explains the direction of natural processes. Kelvin–Planck Statement:
It is impossible to convert all the heat absorbed from a reservoir into work without any loss. Clausius Statement:
Heat cannot flow spontaneously from a colder body to a hotter body.
This law introduces the concept of entropy (S).
ΔS = Qrev / T Key Points:
- Entropy of the universe always increases.
- Perpetual motion machine of second kind is impossible.
Third Law of Thermodynamics
Statement: The entropy of a perfectly crystalline substance is zero at absolute zero (0 K).
This law helps in calculating absolute entropy. Limitation:
- Absolute zero temperature cannot be achieved.
Importance & Applications
- Design of heat engines and refrigerators
- Understanding chemical reactions and spontaneity
- Basis of physical chemistry
- Used in metallurgy, engineering, and biology
Exam-Oriented Key Points
- Zeroth law → Temperature
- First law → Energy conservation
- Second law → Direction & entropy
- Third law → Absolute entropy
MCQs (With Answers)
Q1. Which law defines temperature?
A) First law
B) Zeroth law
C) Second law
D) Third law
Answer: B
Q2. ΔU = 0 for which process?
A) Adiabatic
B) Isochoric
C) Isothermal
D) Cyclic
Answer: C
Q3. Entropy of the universe always?
A) Decreases
B) Constant
C) Increases
D) Zero
Answer: C
