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Topic: Why does a reaction happen?  (Read 5972 times)

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Offline Agnostical

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Why does a reaction happen?
« on: September 24, 2009, 07:24:27 AM »
Hi,

I know this may be an ambiguous questions but i want to know the main reason why a bond forms. Like how come sometimes when u put 2 reactants together they will form another product rather than stay in the same form- how come sometimes it reacts instead of stay the same? Is it to do with enthalpies of holding bonds together?

My current knowledge level: Year 12 Cambridge Advansed Supplementary level Chemistry

Thanks

Offline Borek

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Re: Why does a reaction happen?
« Reply #1 on: September 24, 2009, 07:43:58 AM »
ΔG = ΔH - TΔS

So it is either change in enthalpy or change in entropy that makes reaction happen.

In most cases new bonds are stronger - that is, products have lower energy than rectants. Sometimes it is growing entropy that prevails, perhaps the most spectacular example being explosion of acetone peroxide.
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Offline Agnostical

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Re: Why does a reaction happen?
« Reply #2 on: September 24, 2009, 11:41:30 PM »
hmm using that rule wouldnt all reactions be exothermic?? why wud there be endothermic reactions if the products gain more energy?

Offline Mitch

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Re: Why does a reaction happen?
« Reply #3 on: September 25, 2009, 01:23:10 AM »
Look at it from this point of view. If you supply the energy for a molecule to form when it wouldn't energetically want to form in the first place, then in essence you are making it exothermic.

However, don't really think of it like this. Everything in that upper paragraph would get me laughed at from all chemists, I was just trying to convey how one could perceive endothermic reactions happening.
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Offline Borek

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Re: Why does a reaction happen?
« Reply #4 on: September 25, 2009, 02:44:48 AM »
hmm using that rule wouldnt all reactions be exothermic?? why wud there be endothermic reactions if the products gain more energy?

Exatly 50% of my post was devoted just to that.
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Offline Agnostical

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Re: Why does a reaction happen?
« Reply #5 on: September 25, 2009, 08:11:15 AM »
Look at it from this point of view. If you supply the energy for a molecule to form when it wouldn't energetically want to form in the first place, then in essence you are making it exothermic.

However, don't really think of it like this. Everything in that upper paragraph would get me laughed at from all chemists, I was just trying to convey how one could perceive endothermic reactions happening.

Did you mean "in essence you are making it endothermic" because if you supply energy the system will have more energy=endo?


Offline Agnostical

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Re: Why does a reaction happen?
« Reply #6 on: September 25, 2009, 08:15:51 AM »
ΔG = ΔH - TΔS

So it is either change in enthalpy or change in entropy that makes reaction happen.

In most cases new bonds are stronger - that is, products have lower energy than rectants. Sometimes it is growing entropy that prevails, perhaps the most spectacular example being explosion of acetone peroxide.

oh thanks. Does the same apply for whether something dissolves in water or not?

Offline Borek

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Re: Why does a reaction happen?
« Reply #7 on: September 25, 2009, 08:22:52 AM »
It applies to any process.
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