Entropy (THINK: That would be Chem then?)

I have my Unit 6 paper on Tuesday and I was wondering if anyone could explane entropy to me and how that helps you know wether something will disolve. I would be really greatfull

Reply 1

selkie222

Ok entropy... a measurement of the degree of disorder. That's really all you probably need to know, but I'll try to explain. This might get quite long.

A simple analogy is the "messy room". A room that is tidy has low entropy. As time goes by, if we don't put particular effort into tidying it, we end up with a messy, disordered room. The messy room will have a higher entropy than the tidy room.

To return the room into a state of tidyness, we need to put in effort to tidy the room. However, this means that we will stir up more disorder in the form of air molecules moving, or breathing out carbon dioxide molecules... in other words, the overall entropy in the universe will have increased. That is the general principle--- entropy always increases.

It evidently requires less effort to maintain a messy room (ie high entropy) than a tidy room (ie low entropy). Therefore, a high entropy state is more favourable than a low entropy state.

Therefore, in any process, if the entropy after the reaction is higher than the entropy before the reaction, the process/change is likely to occur. Such changes are said to be "entropy-driven".

Now bring this back to chemistry. Think back to module one, structure, bonding, all that sort of thing. Solids have a regular, tightly packed structure, with particles that vibrate about a fixed point. Liquids have a looser, but still closely packed structure, with particles that are mobile to a degree. Gases have no regular structure at all, and its particles are totally free to move around at high speed. Comparing their entropy, we end up with
solid < liquid <<< gas
Gases have the highest entropy in all three, and can be several times the entropy of the other two states.

Therefore, if a reaction has more moles of gas after the reaction than before it (eg decomposition of CaCO3, solid->solid+gas), its entropy will be higher after the reaction than before it. This reaction will also be said to be "entropy-driven".

However, the fact that a reaction is "entropy-driven" and likely to happen doesn't mean that it will happen automatically. In the case of decomposition of CaCO3, you need to heat it to a certain temperature.

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When you do entropy questions in the exam, it is usually coupled with the Gibbs free energy equation
[delta]G = [delta]H - T[delta]S

[delta]G: Gibbs free energy
[delta]H: standard enthalpy change for the reaction
T: temperature (in K)
[delta]S: standard entropy change for the reaction

which is really just plugging in numbers (with appropriate change in units for [delta]H or [delta]S).

Negative [delta]G means that a reaction has a tendency to occur. This is the thermodynamical side of the situation, but you still have to consider the activation energy, etc, which is the kinetic side of the situation.

You will need to be very clear that the thermodynamics and the kinetics are different and have very little to do with each other.
e.g. sugar burns very exothermically and releases CO2 and H2O, so thermodynamically it has a huge tendency to occur. However, kinetically it has a very high activation energy, so we don't normally see our sugar pots spontaneously igniting! (which is a good thing :biggrin:

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I hope I don't sound too much like a textbook :wink:

(I honestly didn't look at a textbook when I wrote all that stuff above).

If there is anything you are unsure, keep asking.

selkie

Reply 2

Emma18

also, (just to be helpful!!!) remeber the stuff about chelating ligands!!! like when EDTA replaces all 6 water ligands, you end up with more 'free' molecules on the product side, ie an increase in entropy (and an overall increase in entropy = a big positive Delta S value....unless im worng, in which case tell me!!!!)

Reply 3

selkie222

Whoops didn't answer why something will dissolve... you could actually ignore entropy for dissolving things in this exam...

The quick way of course is that ionic solids dissolve, small covalent ones dissolve to an extent, and large covalent ones do not. Just be careful of Al, which doesn't form totally ionic solids and behave abnormally. Al2O3 doesn't dissolve in water, but AlCl3 undergoes hydrolysis to form an acidic solution.

Covalent compounds that could form hydrogen bonds (eg alcohols, amines, etc) are also pretty good at dissolving into water.

I can't really think of questions where they link entropy and dissolving solids directly.... sometimes they link ligand substitution with entropy
[Cu(H2O)6]2+ + EDTA4- --> [Cu(EDTA)]4- + 6H2O)6
In this case, it goes from 2 moles of particles to 7 moles of particles, so is entropy-driven.

I think otherwise most entropy questions are calcuations with the Gibbs free energy... :biggrin:

selkie