# What is entropy?

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#1
We started doing entropy in chemistry last week. The lesson started off with a wonderful explanation about how thing tend to disorder, like an ice cream melting, and why you don't get a building by dumping a truckload of bricks on the floor.
Then all of a sudden BAM entropy is measured in joules, here is the entropy of a bunch of molecules and here's how to calculate the entropy change of a reaction.
Although it would get me through the exams, this didn't seem to tie in with the first part of the lesson, and my teacher couldn't even tell me how entropy is measured (he mumbled something about computers).
So, how is entropy measured, why is it in joules and how do you attach an entropy to a compound/element?
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7 years ago
#2
Well as you suggested, entropy can be seen as a measure of "disorder" of a system.

Firstly, it doesn't have the correct dimensions to be measured in joules, it's measured in joules per kelvin.

Why it has the dimension of unit energy per unit temperature can be seen from the thermodynamic definition of entropy:

Which means that an infinitessimal change in entropy is equal to the corresponding infinitessimal heat supplied to the system (supplied reversibly) divided by the temperature at which the heat was supplied.

Assuming you're an A level student, this is is presumably more than you need to know anyway, but at least shows why entropy has the dimensions it does, it's equal to an energy divided by a temperature.

Entropies can be measured in a number of ways, for standard entropy tables, use of the third law of thermodynamics (which states that entropy of a perfect crystal at T=0 is 0) and how entropy varies with temperature can provide standard entropies.

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7 years ago
#3
(I'm an A level student still and these are my thoughts about it)

entropy comes continually throughout A Level Chemistry.

I picture entropy as "inverse of energy density" that is "volume/energy or "volume/enthalpy". If 2 systems have the same energy content the one with the larger volume has more entropy. When 2 systems have the same volume the one with the lesser energy content has more entropy.

Obviously less energy density means more stability. So more entropy a system has the more stable it is.

When we dissolve salt in water the particles spread out. Matter wants to spread out. That is it wants the mass density to decreased. The same with energy - it wants to spread out.

The more disorder a system is the more entropy it has. The changes in entropy gives rise to a time axis (a direction or arrow for time). The universe is increasing in size because it wants the energy to spread out and out and out increasing the entropy.

A system wants to have LESS ENTALPY/ENERGY and MORE ENTROPY.

As the above poster mentioned the rate of change of entropy is related to the absolute temperature of the system. Don't be serious about it - as you go deeper into A2 Chemistry it'll be more and more clear.

Entropy is defined (if i remember correctly) as the measure of the random disorder of a system that is the dispersal of molecules and the energy quanta between them

The study of entropy changes is important to judge the feasibility of a reaction. For a reaction to be satisfactorily feasible the entropy of the products should be greater than the entropy of the reactants. That is the products should be more gaseous or having a higher number of molecules.
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