Second law of thermodynamics
In any spontaneous process, the total entropy of the system plus its surroundings increases: ΔS(universe) > 0. The system's entropy can fall if the surroundings gain more. Heat released raises the surroundings' entropy by ΔS(surr) = −ΔH/T.
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Analogy
A room never tidies itself. Mess is far more likely than order, so changes lean toward mess overall.
Example
System: ΔS < 0 (water orders into ice)Surroundings: heat released, ΔS > 0Total ΔS(universe) > 0So freezing is spontaneous.
In practice
Water freezing below 0 °C lowers the water's entropy, but the heat it releases raises the entropy of the cold air even more.
Practice
Flashcards
- What does the second law say?The total entropy of the universe increases in any spontaneous process.
Questions people ask
What does spontaneous mean in chemistry?
A spontaneous process happens by itself under the given conditions, without ongoing outside help. It does not tell you how fast it happens. Diamond turning into graphite is spontaneous but so slow that you will never see it. Speed is a matter of kinetics.
Why is heat given out good for the universe's entropy?
Heat released into cooler surroundings makes particles there move more, which raises the surroundings' entropy. The effect is larger at low temperature, because ΔS(surr) = −ΔH/T. This is why exothermic reactions tend to be spontaneous, especially when it is cold.