Why is frozen water exothermic

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Entropy S.


[Greek: en tropos = alternating]

Enthalpy cannot be the only cause that triggers a spontaneous reaction. There are also reactions that take place on their own and are endothermic. For example, some salts dissolve in water when it is cooled. These reactions are therefore endothermic. An example of this is ammonium chloride, which dissolves into ammonium ions and chlorine ions in water:

There are also reactions that not voluntarily expire, although they exothermic are. The freezing of water, for example, is an exothermic process and yet it does not happen spontaneously.
What is the driving force behind these facts?
In 1850, R. Clausius introduced a new quantity that describes the state of a system in thermodynamics. He called her Entropy S.. It is a measure of the `` disorder ''. If there is great disorder, its numerical value is also great. However, the more orderly a system is, the smaller the numerical value of the entropy.
In order to be able to explain this clearly, a vessel is considered that is filled with water. A salt crystal is placed in this vessel:


The salt crystal has a very high degree of order. The individual ions are in a strictly ordered scheme. If you bring this crystal into contact with water, the outer ions are first detached and distributed in the water:

Now the ions are much more disordered than in the crystal.
The reaction itself is endothermic, since the lattice formation energy has to be overcome again in order to dissolve the crystal lattice. The crystal draws this required energy from the water, which cools down as a result. However, the level of disorder increases, which makes the process happen spontaneously.
The drive to strive for greater disorder comes from probability. Disorder is just a lot more likely than order. If you look at a card game, there is only one possibility of order, in which the cards are arranged according to the sequence of numbers, but there are many more possibilities to combine the cards in a non-ordered sequence:



The list of unordered examples could go on and on.

The value of entropy in an isolated system is also called Measure of internal stability to understand. The larger the numerical value, the more stable the system and the smaller the possibility for spontaneous changes.
Just like enthalpy, entropy is an equation of state. Every change in state also leads to a change in entropy:

Entropy cannot be measured at one point, but only the difference to another state. It is measured in J / kmol.
In a spontaneous reaction, the entropy always increases, no substance will move into an ordered state by itself. A gas, for example, is always distributed evenly in a container and is not concentrated in one corner, which would be more orderly. The entropy in one system can only decrease if at the same time the entropy increases more in value in another place. It follows from this that the total entropy must always increase. According to Clausius, because of this fact, the universe tends towards so-called "heat death". By this he understands the final state of equilibrium at which the entropy has reached its highest value and systems are no longer capable of spontaneous reactions. Arthur Eddington called entropy the "arrow of time". If the entropy of the universe is determined at two different points in time, the point in time with the larger value is always later than the one with the smaller. Thus, the direction of time is determined by the increase in entropy in the universe.

Quotes from scientists:L.Boltzmann recognized: "The kinetic energy distributed to the individual molecules of a body always changes from a less probable distribution state to a more probable one, but not vice versa. If, for example, all air molecules are initially in one corner of a room, they are evenly distributed in this room: the entropy increases. However, it is practically impossible that, conversely, the evenly distributed molecules all collect in one corner of the room. "


Enthalpy
entropy
free enthalpy



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