Water Entropy [ENDORSED]

[Lusin_Yengibaryan_3B]

I'm curious about how water's entropy can decrease when it freezes without violating the second law of thermodynamics. Can someone also explain what happens to the entropy of its surroundings?

[Anirudh Mahadev 1G]

Water freezing is an exothermic process, so it releases heat into its surroundings. This increases the entropy of the surroundings. It would not violate the second law because the law can be extended to account for the changes in the entropy of the surroundings, not just of the system. The favorability would heavily depend on the current temperature. If the temperature is low, by Le Chatelier's Principle the reaction would shift to the right, freezing more water to release more heat. The forward reaction therefore becomes more favorable in lower temperatures.

[Chem_Mod]

The entropy of the surroundings increase as heat left the system (water) and entered the surroundings.

Since heat goes from T(higher) to T(lower), in this case from water to surroundings, the increase in entropy of the surroundings is a larger positive value which results in delta S(total) > 0.

[Thomas Gimeno]

When water freezes it goes from a not very orderly liquid to a relatively orderly solid, this means entropy has decreased. However, when water freezes it also looses heat energy which enters the surroundings. This heat released into the surrounding increases the entropy of the surroundings. This increase is often far greater than the decrease in the entropy of the freezing water meaning the overall change in the entropy of the universe is still positive.

[705383815]

Consider what happens in the universe (system + surroundings) as a whole when the exchange occurs.

[Chem_Mod]

Correct.

To determine a favorable process need:

Total entropy > 0
or
Gibbs free energy system < 0

[Thomas Gimeno]

The second law essentially says that the total entropy is always increasing, in the case of freezing water, the water itself is becoming more ordered and therefore loosing entropy. However, freezing is an exothermic process meaning that heat from the water has entered the surroundings and therefor increased that entropy. When you and the negative entropy change of the ice to the positive entropy change of the surroundings you'll always get a positive value showing the universe is still gaining entropy.