Second Law of Thermodynamics
Definition and meaning of Second Law of Thermodynamics in chemistry.
The Second Law of Thermodynamics states that the entropy of an isolated system always increases with any spontaneous process. Heat flows spontaneously only from hotter to colder bodies, never in the reverse direction.
In more detail
The law establishes that all spontaneous processes are irreversible and increase disorder in the universe as a whole. This principle explains why heat engines cannot achieve 100% efficiency, why ice melts in warm water but does not spontaneously refreeze, and which chemical reactions proceed on their own. The entropy change of the universe (ΔS_universe) must be positive for any spontaneous process to occur, serving as the fundamental criterion for determining chemical and physical feasibility.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | ΔS_universe (entropy change of universe) |
| Core Principle | Spontaneous processes always increase total entropy (ΔS_universe > 0) |
| Practical Consequence | No heat engine can convert all input heat to useful work; some energy must be rejected as waste |
Ice at room temperature melts spontaneously into liquid water. Melting is endothermic, so the ice absorbs heat from the warmer surroundings; this heat loss slightly decreases the entropy of the surroundings. However, the water molecules become significantly more disordered as the solid becomes liquid, and this entropy gain is larger than the entropy lost by the surroundings, so the total entropy of the universe still increases. This spontaneous process would never reverse on its own.
Frequently asked questions
Why can't we build a perpetual motion machine?
The Second Law prevents complete conversion of heat into work. Some energy must always be rejected as waste heat, and systems eventually reach thermal equilibrium where no further useful work is possible.
Can entropy ever decrease in a system?
Yes, a system's entropy can decrease if work is performed on it (such as in refrigeration), but the total entropy of the universe (system plus surroundings) must still increase for any spontaneous process.