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Topic: Why are chemical reactions often hard to reverse?  (Read 16697 times)

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Offline 408

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Re: Why are chemical reactions often hard to reverse?
« Reply #15 on: February 02, 2011, 06:53:43 AM »
Quote from: tamim83 on February 01, 2011, 01:48:05 PM
We can easily get TNT to make water, carbon dioxide, and nitrogen by lighting a fuse. 
 

If by fuse you mean fuse with a detonator, then yes :P
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Offline tamim83

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Re: Why are chemical reactions often hard to reverse?
« Reply #16 on: February 02, 2011, 08:24:37 AM »
Quote
If by fuse you mean fuse with a detonator, then yes

Yep  ;D
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Offline vmelkon

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Re: Why are chemical reactions often hard to reverse?
« Reply #17 on: February 05, 2011, 09:51:42 PM »
TNT is a rather complex molecule. Why not use a simple example like N2 + 3H2 <=> 2NH3
or H2 + Cl2 <=> 2HCl
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Offline AMEDIO

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Re: Why are chemical reactions often hard to reverse?
« Reply #18 on: February 07, 2011, 08:37:48 AM »
Quote from: rabolisk on February 01, 2011, 12:48:45 PM
All irreversible (spontaneous) reactions happen because of entropy.

its not necessary that an irreversible reaction will be spontaneous always... ;)
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Offline rabolisk

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Re: Why are chemical reactions often hard to reverse?
« Reply #19 on: February 07, 2011, 07:21:58 PM »
Quote from: AMEDIO on February 07, 2011, 08:37:48 AM
Quote from: rabolisk on February 01, 2011, 12:48:45 PM
All irreversible (spontaneous) reactions happen because of entropy.

its not necessary that an irreversible reaction will be spontaneous always... ;)

Such as?
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Offline AMEDIO

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Re: Why are chemical reactions often hard to reverse?
« Reply #20 on: February 12, 2011, 02:31:15 AM »
nstance, the decomposition of carbonic acid (H2CO3) into water and carbon dioxide (H2O and CO2) is spontaneous. However, it can be reversed by an enzyme called carbonic anhydrase; it just takes energy.

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Offline rabolisk

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Re: Why are chemical reactions often hard to reverse?
« Reply #21 on: February 12, 2011, 03:11:36 AM »
Enzymes don't change the energetics of the reaction. They only affect the kinetics. Carbonic anhydrase doesn't "reverse" the reaction; the reaction itself is always reversible. The position of the equilibrium is unaffected. The role of carbonic anhydrase is to bring the reaction to an equilibrium rapidly.
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Offline Jorriss

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Re: Why are chemical reactions often hard to reverse?
« Reply #22 on: February 12, 2011, 05:33:22 PM »
He didn't word it this way but I think he means the entropy of a closed system can decrease if heat is added.
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Offline rabolisk

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Re: Why are chemical reactions often hard to reverse?
« Reply #23 on: February 13, 2011, 01:39:50 PM »
Quote from: Jorriss on February 12, 2011, 05:33:22 PM
He didn't word it this way but I think he means the entropy of a closed system can decrease if heat is added.

The entropy of a closed system would increase if heat is added, actually. Regardless, the entropy of the universe cannot decrease. When  :delta: S is negative for a reaction, only the entropy of the system decreases; that of the universe cannot.
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Offline Jorriss

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Re: Why are chemical reactions often hard to reverse?
« Reply #24 on: February 13, 2011, 08:25:49 PM »
Quote from: rabolisk on February 13, 2011, 01:39:50 PM
Quote from: Jorriss on February 12, 2011, 05:33:22 PM
He didn't word it this way but I think he means the entropy of a closed system can decrease if heat is added.

The entropy of a closed system would increase if heat is added, actually. Regardless, the entropy of the universe cannot decrease. When  :delta: S is negative for a reaction, only the entropy of the system decreases; that of the universe cannot.
That's why I said system and not universe. And I take by the heat part and change to some reactions. There are endothermic combination reactions where heat increases the number of products but reduces the entropy. Granted, can't think of any off the top of my head.
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Offline rabolisk

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Re: Why are chemical reactions often hard to reverse?
« Reply #25 on: February 14, 2011, 03:11:54 AM »
An endothermic reaction in which entropy decreases would not be spontaneous.
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Offline Jorriss

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Re: Why are chemical reactions often hard to reverse?
« Reply #26 on: February 14, 2011, 03:26:50 PM »
Quote from: rabolisk on February 14, 2011, 03:11:54 AM
An endothermic reaction in which entropy decreases would not be spontaneous.
Ya know, I've read of endothermic synthesis reactions but I was walking to Ralphs thinking, that doesn't make any sense though...I have to agree with you, I either read something wrong or I need to reread it.
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