Question Re: Entropy and Gibbs "Free" Energy

[Anti]

Hello,

I have a question that I hope those of you who have more knowledge of the subject than I do can help answer.

The other day I was in a discussion about Entropy with a friend of mine. I admittedly know very little of this subject since my background is in a totally different field; but I have heard it said many times that:

"In any system, Entropy always increases..."

I asked him (a PhD chemist) if this was true. He said it was not, and when I asked for an explanation he said "Gibbs Free Energy, and then blurted the equation. I kinda felt like I was wasting his time with such moronic questions, to be honest. So I went off to the internet to see if I could dig up my own info and I'm not totally clear on what I've read.

Some things say that Entropy always increases and this is an inviable law of Physics as we know it. This is kinda what I always thought was the case. Even when some system shows a decrease in Entropy, another system had a larger increase in Entropy to offset it.

However, other things I've read are more ambiguous and seem to suggest that there are cases in nature where the above is not the case; but they never really give examples.

So... all you Physics gurus out there, anyone care to help me out?

Thanks in advance!

 

[lrkun]

What is entropy? What is its effect?

What is Gibbs Free Energy? What is its effect?

 

[nasher168]

In a closed system (a system into which nothing is being added) I think entropy tends to increase. One highly simplified analogy might be a bedroom's tidiness. Unless you go out of your way to tidy your room every now and again (put in energy) it will tend towards a state of disorder.
With something like Earth, it gets more complicated. Earth is not a closed system, as we have the sun putting in energy, allowing for localised areas of low entropy (as in organisms) at the cost of greater entropy elsewhere. Organisms take in energy, allowing themselves to be ordered, but they also increase the entropy of their surroundings (excreting) slightly more than they decrease it within themselves, as they are not perfect systems. In a perfectly efficient system, the order created locally would exactly equal the disorder created in the surroundings.

I'm sure I probably got something wrong there, but that's my very basic understanding of it.

 

[Deleted member 619]

In a closed system, entropy tends to increase or stay the same. Entropy can decrease locally, but always at the cost of increasing entropy overall.

Andromeda'sWake gives a beautiful and simple treatment of entropy in the thread on Science, Laymen and Language.

 

[Undeath]

To give a quick expansion on the concept of Gibb's Free Energy, it's a way to measure how a chemical system will behave. Gibb's energy will always attempt to fall, and to understand the implications of that, we need to quickly recap what it is.

For a system, the change in Gibb's Energy, is defined as deltaG = deltaH - T deltaS, where deltaS is the change in entropy, T is the temperature, and deltaH is the change in enthalpy. At constant pressure, deltaH is the same as the change in heat of the system - a negative deltaH implies the system is giving off heat, while a positive deltaH implies the system is absorbing heat.

Given this definition, we can see that the Gibb's Energy can fall either by lowering the enthalpy, aka. giving off heat, or raising the entropy. Thus the entropy can be lowered in the system if it gives off heat.

Now, as others have pointed out, entropy will always rise for a closed system (actually, as hackenslash says, it will either rise or stay the same, but the latter only applies to a perfectly reversible system). However, a system that gives off heat is not a closed system, since it interacts with the surroundings. More importantly, giving off heat will actually cause an increase in the entropy of the surroundings. Thus, the entropy of a local system can be lowered by increasing the entropy of the surroundings.

 

[AndromedasWake]

Hello,

I have a question that I hope those of you who have more knowledge of the subject than I do can help answer.

The other day I was in a discussion about Entropy with a friend of mine. I admittedly know very little of this subject since my background is in a totally different field; but I have heard it said many times that:

"In any system, Entropy always increases..."

I asked him (a PhD chemist) if this was true. He said it was not, and when I asked for an explanation he said "Gibbs Free Energy, and then blurted the equation. I kinda felt like I was wasting his time with such moronic questions, to be honest. So I went off to the internet to see if I could dig up my own info and I'm not totally clear on what I've read.

Some things say that Entropy always increases and this is an inviable law of Physics as we know it. This is kinda what I always thought was the case. Even when some system shows a decrease in Entropy, another system had a larger increase in Entropy to offset it.

However, other things I've read are more ambiguous and seem to suggest that there are cases in nature where the above is not the case; but they never really give examples.

So... all you Physics gurus out there, anyone care to help me out?

Thanks in advance!

Speaking as generally as possible, the second law of thermodynamics says that systems in contact with each other, or an isolated system in a non-equilibrium state will try to reach thermodynamic equilibrium, and as they do so energy will be lost as heat. The energy lost is called entropy, and it can't be recovered.

Consider a hot cup of tea in a room at room temperature. If we treat this as a closed system (ie. no mass-energy can enter or leave the room from the outside) the entropy of the system will increase as the tea cools down and the room warms up.

Speaking in the context of unusable energy, it is only correct when applied to an isolated system with a true adiabatic border. The Universe is the only system considered isolated, because no matter how much heat any system on Earth exchanges with it, its temperature and pressure goes effectively unaltered. In this case, entropy always rises.

Applying entropy to life processes is always a bad idea. They are just too complicated. Energy is being exchanged all over the place, entropy is increasing and decreasing here and there depending on how you define your system, but you will never define a closed system in which entropy decreases. If you could, you'd be on to a perpetual motion machine, a Nobel prize and a lot of money.

Gibbs free energy and its equation are not an answer here, so I'm not sure what your friend is getting it. The Gibbs energy is a measure of the maximum thermodynamic potential energy of a particular type of system, of which the temperature and pressure don't change, and is proportional to the system entropy, but for closed and isolated systems, it's derived from the second law, so it can't possibly violate it.

Your friend might just have objected to the first part of you question, "In any system..."

This isn't true. The law is only true of closed systems (the system and its surroundings combined) and isolated systems (the Universe) but almost every process on Earth occurs in an open system, exchanging matter and energy with other systems.

 

[TheFlyingBastard]

I think I'll retell the story in layman's terms as far as I understand it. It tends to be confusing with all the big words flying around. It's kinda like how if there a system with energy, for example heat, at some point the energy will be spent.

Taking AndromedasWake's cup of tea above - it inevitably loses its heat to the rest of the room. This is a closed system, and the longer you wait, the more the tea will cool down until it's lukewarm/cold. This is because all the energy is spread around the system, which we call "entropy". When there's no more energy to move around, it's "total entropy".

Since energy has this tendency to spread itself out to achieve a balance you can say that everything tends towards total entropy. But you can fix this.

Now you take the cup of tea and you put it on the stove. Energy from outside the room (fire burning from gas) will bring more heat into the tea, causing the available amount of energy to increase (and thus entropy - spent energy - to decrease). This is an "open system".

The earth is an example of such an "open system". The sun keeps sending us warmth, energy, fighting the entropy our planet - like all other things - tends to go towards.

That's how I understand it, but since I'm not exactly a physicist, I might be totally off the mark here and I hereby invite scrutiny from other posters.

 

[Master_Ghost_Knight]

(...)
Speaking in the context of unusable energy, it is only correct when applied to an isolated system with a true adiabatic border.
(...)
Gibbs free energy and its equation are not an answer here, so I'm not sure what your friend is getting it. The Gibbs energy is a measure of the maximum thermodynamic potential energy of a particular type of system, of which the temperature and pressure don't change, and is proportional to the system entropy, but for closed and isolated systems, it's derived from the second law, so it can't possibly violate it.

Your friend might just have objected to the first part of you question, "In any system..."

This isn't true. The law is only true of closed systems (the system and its surroundings combined) and isolated systems (...)

My taughts (almost) exactly.

 

[DeathofSpeech]

Applying entropy to life processes is always a bad idea.

Complex yes. Painfully difficult to sort, yes... It is also the only elegant explanation for biological processes, credible theories of abiogenisis, DNA transcription errors, and the chemical basis for evolution.

 

[AndromedasWake]

Applying entropy to life processes is always a bad idea.

Complex yes. Painfully difficult to sort, yes... It is also the only elegant explanation for biological processes, credible theories of abiogenisis, DNA transcription errors, and the chemical basis for evolution.

Nevertheless, I believe it is still much more favourable to describe life processes in terms of Gibbs energy than entropy. As for abiogenesis, I'd like to look into that if you have any resources.

 

[DeathofSpeech]

Nevertheless, I believe it is still much more favourable to describe life processes in terms of Gibbs energy than entropy. As for abiogenesis, I'd like to look into that if you have any resources.

Okay, that makes more sense. I think I misunderstood your actual meaning that Gibbs was the most utilitarian application of thermodynamics for biological systems.
I think I took you out of context.