## lizcody1 3 years ago Thermodynamics questions: I'm struggling with enthalpy, entropy and Free energy. Can you help clarify for positive and negative values of ∆H and ∆S what is an example process? I'd like to have 4 simple examples I can think about, and understand how various state functions will affect these processes. (∆H +, ∆S -) - endothermic, decreases entropy of the system, ∆G is always positive (∆H +, ∆S +) - endothermic, increases entropy of the system (∆H -, ∆S -) exothermic, decreases entropy of the system (∆H -, ∆S +) exothermic, increases entropy of the system, ∆G is always negative

1. xmelodymusic

Are you by any chance in FLVS? (:

2. lizcody1

no idea what that is

3. xmelodymusic

Ah, nevermind then. Sorry! Thought I had found someone similar struggling through FLVS Chem, haha.

4. lizcody1

No - I'm studying for the mcat, but chem is totally my kryptonite :)

5. xmelodymusic

Oh, is that for medical college? O: It's my krypotnite too, worst subject ever, haha. Good luck on your test! :)

6. lizcody1

So far I have Combustion (∆H -, ∆S+) and boiling H2O (∆H+, ∆S+) and freezing of water

7. agentx5

"H" refers to the change in heat energy. If it's negative energy is being released from the system into the environment (i.e.: fire), what is called exothermic. Endothermic is the exact opposite (i.e.: photosynthesis). Entropy is a much more complex concept. In the most simplified theoretical sense, it means "choices" or "options". In chemistry terms, it really means all the atoms want to be a gas but they don't have enough kinetic energy (i.e.: temperature) to break free into the gas state. Even better would be a plasma (i.e.: lightning) because then you have even more options.

8. lizcody1

so what would be an example for each? is freezing of water ∆H -, ∆S- (since bond formation releases energy, but there is less dispersal of energy in a solid than liquid) ...what would be an example of ∆H+ and ∆S-, and what about some non-phase change examples?

9. agentx5

an example of +∆H and -∆S : photosynthesis creating sugars. It's endothermic, meaning heat is absorbed FROM the environment TO the system. The chloroplasts absorb light. Thus, +∆H Gases and liquid have more "options" on where to move around than a solid (and gasses more then liquids). Creating aqueous or solid sugar decreases entropy. If \(\Delta S_{system}\) > 0, if it's +, the system becomes more disordered through the course of the change If \(\Delta S_{system}\) < 0, if it's -, the system becomes more ordered through the course of the change Thus, -∆S Fire is -∆H and +∆S , and ∆H is a large negative value for fire so the reaction (once it gets going) is self-sustaining (i.e.: chain reaction) and because of the magnitude of the difference the rate tends to accelerate until it reaches a limiting reagent.

10. agentx5

Glucose is more structured than the gases or liquids used to make it. Think about it. :-)

11. lizcody1

Thanks, I wondered about photosynthesis. Also, I like the idea of entropy as "choices" but can you explain this more specifically, because In my mind, (this may be a mistake), I prefer to think of entropy as "certainty" or "boringness" since when entropy is low there is a lot of uncertainty about what will happen next because when entropy is low there is still a jackpot of dense energy waiting to be dispersed, but when entropy is at its maximum, all the energy in a system has been equally dispersed and "everyone's been paid" so it's like the system is discharged or flat-lined. To demonstrate, imagine a group of atoms in a closed container, (say in the form of an egg). They are tightly bound together, and the entropy of the system is low (you can say either that there is only a limited number of options for these atoms to be all bound together) or you could say that the uncertainty of what is going to happen next is high (since there's still a nearly infinite number of ways that the molecules could disperse). But when we wait a few billion-trillion years (or whatever), our egg has decomposed, and is now floating in the container as a haze of evenly distributed atomic particles, relatively uniform in their average kinetic and potential energy, and distribution. At this point, the entropy has increased (to a maximum?), and the system's certainty has increased to the point of boringness, because there is no way it can do anything exciting without being acted on, or breaking a law of conservation.

12. agentx5

Entropy is most certainly not "boringness" and saying "certainity" will get you confused easily because certainty depends on a frame of reference and values to compare to. In chemistry entropy increases means there are more moles and typically in more free-flowing states (listing the states of matter, and not just the typical three, in order of entropy): solid < gel < aqueous < liquid < gas < plasma So decomposition reactions? Increasing entropy. Turning something from solid to gas (i.e.: pyrolysis), increasing entropy. Makes sense? The law of conservation has it's limits you know... Many concepts while still remaining true in the macro-level or on an electron-exchange level hold true, but be careful when generalizing entropy. Example of something where the law of conservation of mass gets bypassed? Nuclear reactions. In those reactions mass is converted into energy which is carried via a mediating particle (photons, neutrons, and electrons typically, and the anti-particles for each) and the relationship of this I'm sure you've seen before: E = mc\(^2\) , where E is energy in joules, m is mass in kilograms, and c is maximum speed of a particle not affected by the Higgs field (i.e.: photons, light)

13. agentx5

Matter and energy are interchangeable related. Matter will loose potential energy until it reaches an equilibrium, and this trend of existence to go from areas of high energy or high concentration to lower energy is a fundamental concept of entropy and thus, time itself (which is observed change).

14. agentx5

It takes a LOT of energy (in the form of temperature typically) to get matter into the plasma state lol (although you do see it on CSI a lot with those mass spectroscopy machines, and everytime you see lightning flashes from a storm) PS: Where does the energy come from that powers lightning?

15. agentx5

1. Fusing hydrogen & helium in the sun's core generates massive energy for a slight mass loss 2. The sun's radiation evaporates water (endothermic) 3. Water vapor rises due to buoyancy 4. Water vapor radiates thermal energy out into space, gradually cooling. (cools faster at night, hence effects like fog that appear at night) 5. When it condenses into liquid state it forms droplets. 6. Droplets get enough mass they fall out of suspension as rain 7. Falling rain creates fluid friction 8. The friction creates an imbalance in electrons and protons, when the potential difference (aka. voltage) get's high enough to overcome the resistivity of air, a spark jumps the gap to restore the equilibrium. In short, entropy says nature likes choices, and nature likes balances. Why? That's just the way our universe works. :-)

16. lizcody1

okay that's a lot to think about thanks! I guess I imagine entropy at the universal level, since energy is being dispersed in the form of particles then net outward motion will disperse the universe until it's a bunch of floating sparse particles with constant inertia and nothing to ever collide into. But maybe that's incorrect. Anyways, I am still getting stuck when I think about entropy in terms of other concepts like bond energies and the conservation of energy (and with concepts of potential and kinetic energy). Since energy is released (from the system, to the universe) when bonds are formed, and energy is absorbed (from the universe, by the system) when bonds are broken, can I think of the bond formation energy is like the Universe's "entropy tax" on the system? It is rewarding an increase in entropy, and demanding a payoff for decreases in energy? Maybe I'm being too creative here.

17. agentx5

You can't say that about covalent bonds or ionic bonds and entropy and heat, there are exceptions to the rule. The universe goes the oppose direction of what you described, things cool off, not become more gaseous. The difference with something like photosysnthesis is that glucose and free oxygen gas are at a higher potential energy than carbon dioxide and water. In fact elements between Iron and Lead are the most stable, low-energy nuclei in the universe from a nuclear reaction standpoint (exception being Technetium), and a chemical like Carbon Dioxide is one of the most low-energy molecules in existence. Again the trend is to go from high entropy and high potential energy to lower. Everything may want to be a gas but as it cools off (radiates and looses heat) it will start to have intermolecular forces cause them to stick together and eventually just vibrate in place (that's what a solid is, molecules just vibrating in place)

18. agentx5

There are few fundamental truths about nature: Equilibriums are the norm High concentration to low concentration High potential energy to low potential energy Time only goes one-way Higher entropy with more "options". etc.

19. agentx5

Low entropy = more structure.

20. lizcody1

So when we say the energy of the universe is constant, but that it is expanding in size and increasing increasing in entropy, and its temperature is cooling, what are we really saying about the universe is that over time particles are probabilistically colliding in directions that send them more toward a path of infinite separation from all other particle of mass (where potential energy is zero) and all the energy in the universe will be converted into kinetic energy, but this kinetic energy will be so relatively uniformly distributed (and there will be no other particles around to transfer heat energy) so that temperature of the whole system will approach 0k? Also, I've heard that mass and energy are interchangeable, but how does this apply to a particle in isolation? When the particles (of matter) of the universe spread infinitely far apart, then does it matter whether they are mass or energy, if there is nothing to collide with, or be attracted or repelled by?

21. lizcody1

btw thanks for helping explain this, i really appreciate it, you are very clear, but I've been banging my head against these ideas for a while now...

22. agentx5

The universe as a whole is expanding and "possibly" cooling (again, exceptions to the rule), but it is not increase in entropy. Gravity pulls matter together matter cooling off by radiation is going to mean a decrease in entropy. And before you think structure is bad, we need structure no only to survive but for anything to exist past an amorphous blob. Structure creates boundaries, symmetry, and predicable phenomena. Particles are not in isolation, they exist in space-time and this is where things get really confusing and we have a gap between general relativity (big stuff) and quantum mechanics (small stuff). We now know that every particle as a related field to it. The only one we were missing from the set was the Higgs and finally it's been experimentally proven (no longer just mathematical expressions and such). In fact all particles behave also as waves on a subatomic level. It's kind of a paradox, or rather probably something that our little brains can't quite comprehend and so we just perceive it as being contradictory when it's not. Also, the reason matter doesn't just become an amorphous nebula of gas and has structure is thanks to the supermassive black holes at the center of galaxies such as ours. They are amazingly powerful and gravity doesn't have a range limit. In fact some physics believe that gravity may just be an effect of space-time and not actually a field at all. In order to be a force field you need a mediation particle, examples: Field, Light; Particle, Photon Field, Magnetism; Particle, Electron Field, Higgs; Particle, Higgs Boson etc. Be very thankful we live in a galaxy with an active black hole :-D (sounds crazy, but there wouldn't be a solar system or solid inner planets without its help)

23. agentx5

One scary note though, our black hole hasn't devoured anything in awhile but it possible that it could sometime soon. If it does, it will start glowing again. Luckily we're just out of range if it does, or we would be irradiated with gamma and X-rays as matter crosses the event horizon >_<

24. agentx5

(when I say glowing I mean round the edges, the center viewed head on is simply, well, black. Nothing coming back)

25. lizcody1

Okay, that's great stuff, I'm trying to put this together in my own thoughts... since we got on the subject of space, there is a really nice video on Vega by Professor Klemperer on the subject of the chemistry of interstellar medium (it's from 1995) but it's very good and definitely flies by in way less than an hour if you watch it late enough at night (how's that for relativity). http://vega.org.uk/video/programme/64?fb_action_ids=10100892175939197&fb_action_types=og.likes&fb_source=aggregation&fb_aggregation_id=288381481237582 I'm still trying to understand how the thermodynamics that I'm reading about in my textbook extend to these phenomenon...

26. lizcody1

Agentx5 - just wanted to say thanks for your help (and patience) - I think entropy finally clicked for me! I stopped overthinking things and trying to unravel the universe and got on with it -also the explanation of lightning was awesome!