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bfr


Member


Joined: 13 Feb 2006
Posts: 108

Posted: 01 Nov 2008 08:16:10 pm    Post subject:

Just a few simple chemistry questions:

Out of solids, liquids, and gases, which has the most potential energy?

Also, exothermic chemical reactions convert potential energy to kinetic energy, and endothermic reactions covert kinetic energy to potential energy..?


Last edited by Guest on 01 Nov 2008 08:17:45 pm; edited 1 time in total
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luby
I want to go back to Philmont!!


Calc Guru


Joined: 23 Apr 2006
Posts: 1477

Posted: 01 Nov 2008 08:23:18 pm    Post subject:

Usually what ever is more dense has more potential energy, so usually solids.

Not really. Exothermic produce heat and Endothermic suck heat from the surroundings.
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alexrudd
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Joined: 06 Oct 2004
Posts: 2335

Posted: 01 Nov 2008 09:04:15 pm    Post subject:

Converting potential energy into kinetic energy = raising average speed of molecules = raising temp. = exothermic. bfr was correct.
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DigiTan
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Joined: 10 Nov 2003
Posts: 4468

Posted: 01 Nov 2008 11:31:20 pm    Post subject:

The first question's a little tricky. There's lot of ways to look at a liquid, solid, or gas having "potential energy." But in heating terms, solids tend to have more chemical energy because of their high density. In engineering, this usually gets labeled as energy density, especially if you're talking combustion.

The...cool thing about endothermic reactions is they take in ambient heat and store that energy as new chemical bonds. For hydrocarbons and others, those bonds get broken and release that stored energy with some amazing energy density.


Last edited by Guest on 02 Nov 2008 12:44:34 am; edited 1 time in total
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bfr


Member


Joined: 13 Feb 2006
Posts: 108

Posted: 02 Nov 2008 03:40:20 pm    Post subject:

That makes sense, thanks

EDIT/BUMP: I have another kind of related question: if a reaction were exothermic or endothermic, how would this be explained in terms of entropy? Would entropy surroundings increase and entropy of the system decrease in both cases?

Also, if I mixed room temperature water with hot water, how could the energy change be explained there in terms of entropy?

EDIT AGAIN: Actually...I thought I remember something about when a solid is converted to a liquid, its potential energy increases (Hfus and stuff like that). So maybe for the same volume of gas/solid, solids have more potential energy, but for the same amount of moles, gas has more potential energy?


Last edited by Guest on 02 Nov 2008 06:42:37 pm; edited 1 time in total
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nitacku


Advanced Member


Joined: 23 Aug 2005
Posts: 408

Posted: 02 Nov 2008 10:04:11 pm    Post subject:

From my understanding entropy increases as the total randomness in a system increases. Looking at it in terms of temperature, entropy would increase as the temperature of the system increases. However, this does not mean that temperature can always be used as a means to derive entropy. As entropy is only a measurement of randomness, a solid will always have a lower entropy than a gas regardless of either temperature since the randomness of the atoms in the solid is always less than that of the gas.

When applied to states of matter, entropy increases from solid to liquid to gas, since there is an increase in the disorder of atoms.

An exothermic process in the system causes heat to flow into the surroundings, increasing the random motions and thus the entropy of the surroundings. For this case, Change in Entropy of the surroundings is positive. The opposite is true for an endothermic process in a system at constant temperature (Change of Entropy of the surroundings is negative).
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