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How chemical reactions produce energy

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Crystal:
Could you help me by explaining how chemical reactions produce energy and how they cause increases in pressure? Is it beacuse they are exothermic and something to do with bond enthalpies?
Thanx
 :)Crystal  

Donaldson Tan:
according to thermodynamics,

dH = Q + W
where dH is the change in enthalpy
Q is the heat
W is the work


W = v dp

hmx9123:
In a more qualitative sense, if you have a chemical reaction, what's happening on a molecular level is you generally have a transfer of electrons.  Depending on the reactants and a few other variables (this is very non-specific and simplified), you can have a reaction that absorbs heat from the surroundings (endothermic) or gives off heat to the surroundings (exothermic).

As for your question about pressure, you have to keep a few things in mind in addition to thermo.  From only thermo, you find that pressure and temperature are related--for a constant volume, if temperature goes up, so does pressure.  So, in that respect, if you have an exothermic reaction that gives off heat, it will accordingly raise the pressure.  This kind of makes sense when you think about it--it's why you don't heat closed containers (e.g., throw hair-spray bottles on a campfire...or maybe you do, but you know the result from the rapidly increased pressure).

Now, the other factor contributing to an increase in pressure is the release of gas.  Perhaps more than heat alone, the release of a gas can increase the pressure incredibly in a system.  Ever see the baking soda volcano when you were a kid?  If you add vinegar to baking soda, what happens?  You get a lot of CO2 (carbon dioxide).  If one of the products of your reaction is a gas, you will get an increase in pressure much faster than just an exotherm.

BTW, this all assumes you have a closed system.  If you have an exothermic reaction that gives off gas, but it's open to the atmosphere (like in an open beaker), you get no pressure increase (but I assumed you'd have figured that out).

charco:
I'm afraid that you have got it compl3etely wrong hmx9123

If the bond energies of the products are greater than those of the reactants then there is an EXOthermic reaction

It is a common misconception amongst chemistry students that bonds somehow 'contain' energy that can be released by breaking them (like a bottle of energy)

Bonds release energy when they are made NOT when they are broken.

Think about it like a glass. In order to break a glass you have to input energy - conversely (thanks to the law of energy conservation) putting a glass back together again will release energy.

Endothermic - bond breaking, exothermic - bond forming

Also, not all reactions involve a transfer of electrons, for example neutralisation.

hmx9123:
Charco, you're totally right.  I wasn't thinking when I wrote that the first time.  Sorry about that.  I was getting bond energies mixed up with heats of formation... man, I shouldn't post late at night.  I corrected the post.

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