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Topic: Why do reactions occur?  (Read 3219 times)

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Offline wanderingmind

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Why do reactions occur?
« on: April 12, 2007, 04:42:28 PM »
I didnt see it anywhere else so i decided to ask it.
Does it have to do with electrogravities or being an ion or anion?

Stange how our chemistry teacher never taught this.

Offline lemonoman

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Re: Why do reactions occur?
« Reply #1 on: April 12, 2007, 06:43:54 PM »
Everything in nature tries to get to a lower 'energy state'.  Water flows downward, to have less gravitational potential energy.  Excited atoms will give off photons to get back to their ground state.  The list goes on.

If two chemicals react into products, it's because the sum of the energies of the products is less than the sum of the energies of the reactants.  The chemicals can get to a lower energy state by reacting, so they do.

"Energy" here refers to "Free energy", which you may or may not have heard of...it's the 'G' in ?G

In early classes, they teach you other tricks to predict if things with 'react' or not...solubitility rules, acidity, stuff like that.

Hope that cleared things up :) ... ask some follow-up questions if you'd like

Offline constant thinker

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Re: Why do reactions occur?
« Reply #2 on: April 12, 2007, 09:36:05 PM »
It's probably worth stating the equation used to calculate the change in Gibbs free energy for a reaction.

?G=?H-T?S
where
?G=change in Gibbs free energy
?H=change in enthalpy
T=temperature
?S=change in entropy
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allanf

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Re: Why do reactions occur?
« Reply #3 on: April 13, 2007, 12:19:15 AM »
Everything in nature tries to get to a lower 'energy state'.  Water flows downward, to have less gravitational potential energy.

To be pedantic, objects move through space and time in such a way as to extremize the action (this is sometimes called Hamilton's principle).  When dealing with systems that are not in (relative) motion (i.e. have no kinetic energy), this is equivalent to saying that the system seeks to minimize its potential energy.  Hence why chemists like to draw potential energy surfaces and talk about how the system would like to move to the lowest valley in the PES.

As for why the system follows that path instead of any others is fairly complicated.  The only explanation I am aware of comes from Feynman's work on quantum electrodynamics (and is described in the wonderfully well written QED), in which case this rather general principle of classical mechanics just "falls out" of the quantum mechanics.

But this is all going way off topic  ;D

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