Entropy => Energy?

Originally posted by dannyUchiha
Of course it doesn't have a direct relationship with the number of bonds or with the complexity of the molecule. That's not what I said.

What I said was that it involved breaking of bonds. As you may know, entropy is the increase in disorder of a system. This may or may not liberate energy, but I do believe it involves some sort of interaction between ...[text shortened]... atoms (breaking and/or formation of bonds)

Can you give examples when this does not apply?

All heat transfer is a change in entropy. When heat is transfered no chemical reactions are taking place.

If entropy is the increase in disorder of a system then is crystal formation a violation of the second law?

Originally posted by twhitehead
All heat transfer is a change in entropy. When heat is transfered no chemical reactions are taking place.

If entropy is the increase in disorder of a system then is crystal formation a violation of the second law?

The Second Law does not apply to open systems such as crystal formation.

Originally posted by AThousandYoung
The Second Law does not apply to open systems such as crystal formation.

Sure it does, you just have to take the outside system (the rest of the universe) into account as well.

Originally posted by AThousandYoung
The Second Law does not apply to open systems such as crystal formation.

The Second Law applied to everything. Obviously you must take into account the total entropy.
Are you claiming that crystals cannot form in a closed system? Or are you saying it takes work to form crystals? I disagree on both points.

To give another example, the effects of gravity frequently results in a system becoming more ordered. Simply take a glass of muddy water and allow it to settle and you will see it for yourself.
This is true even in an isolated system. If the earth was floating alone in space it would still have mostly iron at its core and water on its surface, it would not slowly turn into a perfectly uniform mud ball.

Originally posted by KazetNagorra
It's a bit inaccurate to view entopy as a measure of disorder. Although it usually comes down to this, you must remember that disorder on a microscopic scale can result in ordering on a macroscopic scale. For example, the demixing of a fatty liquid in water results in an increase in entropy (also note that no chemical bonds are involved in this process).

I don't think it's inaccurate, but rather simplistic.

I agree with you that in a macroscopic view, entropy might lead to order. That's why I specified it was on a molecular level that I was talking about on my first comment.

Also, the demixing of fatty liquid in water does not involve molecular bonds, but it does involve hydrogen bonds. The lipids force water molecules into a more "rigid" state, making interactions withing water molecules less common. This accounts for the increase in entropy when we separate the two, hydrogen bonds increase and so does entropy.

Originally posted by twhitehead
I have gone and read it and I see nothing that contradicts what I said.
Put Hydrogen and Oxygen in a box in an isolated part of space and light it with a spark. Over time it will burn to form water. This either contradicts what you were saying or contradicts the second law.

Of course it will form water.

You have contradicted yourself and reassured my point.

Hydrogen exists as H2 and oxygen as O2. The formation of water needs the breaking of both bonds and the formation of the H-O bonds present in water. This leads to an increase in entropy, just like I suggested.

With this we can see that neither my point nor the second law of thermodynamics are being contradicted here.

Originally posted by twhitehead
[b]All heat transfer is a change in entropy. When heat is transfered no chemical reactions are taking place.

I agree with you on the crystal formation matter, but I'm afraid that fire is indeed product of chemical reaction.

You see, one might argue that the ignition can be caused by no chemcal reactions, but how can you explain the fire? That, my friend, is a chemical process called COMBUSTION.

In its most common case, it breaks down organic compounds into its more oxidated form, CO2. Incomplete combustion reactions can lead to a variety of other organic compounds, but that is still a chemical reaction.

I think we have strayed from the point of the thread, but I think this needed to be clarified before anything else.

Originally posted by dannyUchiha
Of course it will form water.

You have contradicted yourself and reassured my point.

Hydrogen exists as H2 and oxygen as O2. The formation of water needs the breaking of both bonds and the formation of the H-O bonds present in water. This leads to an increase in entropy, just like I suggested.

With this we can see that neither my point nor the second law of thermodynamics are being contradicted here.

OK lets put hydrogen ions and oxygen ions in our hypothetical box. No bonds will be broken.

Originally posted by dannyUchiha
I agree with you on the crystal formation matter, but I'm afraid that fire is indeed product of chemical reaction.

I did not mention fire at all. I said heat transfer. If you put 1 litre of cold air and 1 litre of hot air in a sealed container, the heat will spread throughout the container. This is a change in entropy.
One of the results of the Second law is that heat cannot of itself pass from the cooler to the hotter. (I remember that from a song by Flanders and Swan).

Originally posted by dannyUchiha
I don't think it's inaccurate, but rather simplistic.

I agree with you that in a macroscopic view, entropy might lead to order. That's why I specified it was on a molecular level that I was talking about on my first comment.

Also, the demixing of fatty liquid in water does not involve molecular bonds, but it does involve hydrogen bonds. The lipid ...[text shortened]... increase in entropy when we separate the two, hydrogen bonds increase and so does entropy.

Yes, but a hydrogen bond isn't a chemical bond, it's a physical bond.

Originally posted by KazetNagorra
Yes, but a hydrogen bond isn't a chemical bond, it's a physical bond.

I understand that, but the behavior is similar to a chemical bond, so the same can be said about them.

Originally posted by twhitehead
OK lets put hydrogen ions and oxygen ions in our hypothetical box. No bonds will be broken.

But bonds will form.

I fail to see your point...

Originally posted by dannyUchiha
But bonds will form.

I fail to see your point...

The example twhitehead gave was problematic because it used a chemical reaction as a heat source. An increase in entropy does not require chemical bonds to be formed or broken, If chemical bonds are formed or broken then an increase in entropy will occur, but the converse isn´t true - you can get increases in entropy in other ways.

Here'smy take

When talking about entropy, there are 3 factors.

Total entropy - which must increase! Stotal = Ssystem + Ssurroundings
This is basically the 2nd Law.

All the talk about bond breaking etc is worked out by the fact all substance have absolute entropies nad the entopy of the system can be calculated by

Ssystem = Sproducts - Sreactants

Then the other factor is Ssurroundings = -Hsystem/T


What causes a total entropy increase?

In a chemical reaction
1. An increase in total entropy will always occur if you have an exothermic reaction (as this increases the entropy of the surrounding) and an increase on entropy of the system (e.g if more moles of gas forms). Therefore always a feasible reaction

2.
An decrease in total entropy will always occur if you have an endothermic reaction (as this decreases the entropy of the surroundings) and a decrease in entropy of the system. Therefore a reaction is never feasible.

Temperature has a big factor in other combinations

3.
If you have an endothermic reaction but an increase in entropy of the system then feasible reactions are favoured at high temps (e.g. thermal decompositions).
At higher temperature the decrease in entropy is less at higher temperatures. This is like saying taking £50 per week of a persom earning £1000 per week means less than taking £50 per week of someone earning £10 per week).


4.
If you have an exothermic reaction but a decrease in the entropy of the system the feasible reactions are favoured at lower temperatures.

This is because if you have a reaction at say 300K and the same at 600K and it gives out 50kJ of heat, then there is a bigger increase in entropy in the reaction at a lower temperature. Its like sayng a £50 pay rise means more to someone earning £100 a week rather than a £50 pay rise to someone earning £1000 per week).

No offence to anyone but there is so much nonsence and lack of understanding in this thread I thought I had to chip in (I feel I am qualified as have taught thermodynamics at degree level).

Originally posted by dannyUchiha
If you look at entropy at a molecular level, it is basically the breaking of bonds between atoms to create simpler molecules. Every time you break a molecular bond, energy is liberated because there is a transfer of electrons.

I think this can prove that THEORETICALLY, every increase in entropy can be harnessed into useful energy.

Incorrect
You need to put energy IN to break a molecular bond (i.e it's an endothermic process)

Bond FORMATION liberates energy (i.e all bond formatioms are exothermic)