The Second Law of Thermodynamics states that in any spontaneous process, the total entropy of the universe increases. It implies that natural processes tend to move towards a state of greater disorder or randomness.
Entropy is a measure of the disorder or randomness in a system. It is a state function that quantifies the number of possible configurations a system can have. Higher entropy indicates more disorder.
The Second Law implies that no heat engine can be 100% efficient. Some energy will always be lost as waste heat, and complete conversion of heat into work is impossible.
No, heat cannot flow from a colder body to a hotter body on its own. This process requires external work, which is why refrigerators and air conditioners consume energy to move heat against its natural direction.
Join ALLEN!
(Session 2026 - 27)
Choose class
Choose your goal
Preferred Mode
Choose State
Second Law of Thermodynamics in Chemistry
The Second Law of Thermodynamics states that the entropy of the universe increases in the course of every spontaneous process. Entropy, often referred to as a measure of disorder, is a state function that is directly defined by the second law.
1.0What is Second Law of Thermodynamics
Have you ever noticed that once you mix milk into coffee, it never separates back on its own? Or that once you scramble an egg, it can’t be unscrambled? Why does your phone's battery always seem to drain, but never charge itself? These everyday occurrences all illustrate the concept of the "arrow of time." This principle dictates that certain processes only happen in one direction—forward. In physics, this is closely tied to the thermodynamic arrow of time, or entropy, which measures the level of disorder within a system. Represented by ΔS, changes in entropy highlight that over time, isolated systems naturally evolve towards greater disorder and randomness, making the progression of time asymmetrical and irreversible.
In any spontaneous (irreversible) process, the entropy of the universe always increases. The change in entropy (ΔS) of a system can be determined by the formula:
ΔS=∫Tdqrev
where dqrev is the heat exchanged in a small reversible step and T is the absolute temperature at which the heat is absorbed.
2.0Types of Entropy Changes
ΔSsystem: Entropy change of the system.
ΔSsurrounding: Entropy change of the surroundings.
ΔStotal: Entropy change of the universe, which is the sum of the entropy changes of the system and surroundings.
For a spontaneous process:
ΔSuniverse = ΔSsystem+ΔSsurrounding > 0
For a reversible process:
ΔSuniverse = 0
This implies that during an irreversible process, the entropy of the universe increases, while during a reversible process, it remains constant.
3.0Physical Significance of Entropy (ΔS):
Entropy (ΔS) is a measure of disorder or randomness in a system. To understand this, imagine heating a substance. If you supply the same amount of heat at two different temperatures, say T1 (low) and T2 (high), the increase in disorder will be more significant at the lower temperature (T1). This is because, at lower temperatures, the system is already in a relatively ordered state, so adding heat increases the kinetic energy and disorder more compared to adding the same heat at a higher temperature, where the particles are already moving faster and are more disordered.
4.0Determining Entropy Change of the Surroundings
For irreversible processes, calculating the entropy change (ΔS) directly is difficult due to their fast and unpredictable nature. To address this, we use a hypothetical reversible path where temperature and heat exchange are known.
The change in entropy is given by - Exact temperature at which heat is absorbed Heat absorbed by an entity
Reversible Process: In reversible processes, changes occur in a controlled way. Entropy change (ΔS) for the system and surroundings is calculated as:
ΔS=∫Tdqrev
where dqrev is the heat exchanged reversibly and T is the absolute temperature.
Irreversible Process: In irreversible processes, the surroundings, being large and stable, absorb heat predictably. The entropy change of the surroundings is given by:
ΔSsurr =−Tqirr
where qirr is the actual heat exchanged. This method helps estimate the overall entropy change even without knowing the system's exact temperature.
5.0Second law (in terms of Engine)
No Perfect Heat Engine:
It is impossible for a cyclic engine to convert all the absorbed heat into work without affecting the surroundings. There will always be some heat loss.
Heat Flow:
Heat cannot spontaneously flow from a colder body to a hotter body without external work or intervention.
Heat and Work Conversion:
Complete conversion of heat into work in a cyclic process is not possible, although the reverse (work into heat) can be achieved. This indicates that heat (disordered energy) and work (ordered energy) are not equivalent in thermodynamics.
6.0Heat Engine
A heat engine is a device that converts heat into work while operating in a cyclic manner. It absorbs heat from a high-temperature source, performs work, and rejects some heat to a low-temperature sink.
Image 1
QH = Heat gained by engine per cycle
QC = Heat rejected by engine per cycle
The efficiency (η) of a heat engine is given by:
Efficiency of engine: η=QHQH−QC=QHwby
7.0Solved Questions on the second law of thermodynamics
Q. A heat pump uses 300 J of work to extract 400 J of heat from a low-temperature reservoir. How much heat is delivered to the high-temperature reservoir?
Solution:
The heat pump transfers energy by doing work to move heat from a cooler area (low-temperature reservoir) to a warmer area (high-temperature reservoir). The total heat delivered to the high-temperature reservoir is the sum of the heat extracted from the low-temperature reservoir and the work done by the pump.
Given:
Work done (W) = 300 J
Heat extracted from the low-temperature reservoir (Qlow) = 400 J
The heat delivered to the high-temperature reservoir (Qhigh) is calculated as:
Qhigh=Qlow+W
Substitute the given values:
Qhigh = 400 J + 300 J = 700 J
Q.2 Three moles of an ideal gas are expanded isothermally from 2 L to 10 L at 27°C. Calculate the changes in entropy for the system (ΔSsys), surroundings (ΔSsurr), and the universe (ΔSuniv) for the following cases:
Reversible Expansion
Irreversible Expansion against a constant external pressure of 1 atm.
(ii) Entropy Change of the Surroundings (ΔSsurr):
For a reversible process, the change in entropy of the surroundings is equal in magnitude but opposite in sign to the heat absorbed by the system. Since the process is isothermal:
ΔSsurr =−Tqrev
The heat absorbed by the system, qrev, is given by: