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Author Topic: Energy needed to break bonds in reactions  (Read 2699 times)

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Bel-p

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Energy needed to break bonds in reactions
« on: May 12, 2006, 11:48:47 PM »

This may be a really stupid question, but I've just started reading about how energy is needed to break bonds of reactants in a reaction, and there's something I don't understand. 

I've read "In a chemical reaction you need to put energy in to break bonds in the reactants, you get energy out when new bonds are formed to make the products".

example:
Example 1. Hydrogen + Chlorine ==> Hydrogen Chloride
The symbol equation is: H2(g) + Cl2(g) ==> 2HCl(g)
but think of it as: H-H + Cl-Cl ==> H-Cl + H-Cl
(where - represents the chemical bonds to be broken or formed)
the bond energies in kJ/mol are: H-H 436; Cl-Cl 242; H-Cl 431
Energy needed to break bonds = 436 + 242 = 678 kJ taken in
Energy released on bond formation = 431 + 431 = 862 kJ given out
The net difference between them = 862-678 = 184 kJ given out (92 kJ per mole of HCl formed)
More energy is given out than taken in, so the reaction is exothermic.


So this reaction needs 678 kJ of energy to make it happen, this energy I understand can come from the sun so if you put hydrogen & chlorine in the sun they'll react and produce Hydrogen chloride.  OK I'm happy with that, and some reactions only happen if you heat them with a bunsen burner or something, so that's ok too, but what about all the reactions that happen with no energy (apparently) being put in?  Eg CaCO3 & HCl, you don't need any heat, you just pour a bit of acid onto chalk and it reacts, so where's the energy coming from to break the bonds?  Or am I misunderstanding it all?
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Yggdrasil

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Re: Energy needed to break bonds in reactions
« Reply #1 on: May 13, 2006, 10:58:19 PM »

The energy comes from the thermal energy in the air.  At room temperature, the CaCO3 and HCl molecules have sufficient kinetic energy to overcome the relatively low activation energy required for the reaction to take place.  However, if you lower the temperature significantly, the reaction will proceed very slowly because very few of the molecules will have sufficient kinetic energy to react.
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Bel-p

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Re: Energy needed to break bonds in reactions
« Reply #2 on: May 14, 2006, 11:41:23 PM »

Hm, there is so much I don't understand about all this energy stuff that I'm not even sure what question to ask.  Can someone suggest a good website where I can read about this?  Also how do you know how many Kj are needed to break a bond, are there tables somewhere or do you work it out? 

I think this is all about "enthalpy change", am I right?
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