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Topic: How chemical reactions produce energy  (Read 5951 times)

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Crystal

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How chemical reactions produce energy
« on: March 15, 2005, 04:36:03 PM »
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  

Offline Donaldson Tan

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Re:How chemical reactions produce energy
« Reply #1 on: March 16, 2005, 02:34:09 PM »
according to thermodynamics,

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


W = v dp
« Last Edit: March 16, 2005, 10:54:21 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline hmx9123

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Re:How chemical reactions produce energy
« Reply #2 on: March 18, 2005, 04:31:03 AM »
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).
« Last Edit: March 26, 2005, 06:50:02 PM by hmx9123 »

charco

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Re:How chemical reactions produce energy
« Reply #3 on: March 20, 2005, 07:25:52 PM »
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.

Offline hmx9123

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Re:How chemical reactions produce energy
« Reply #4 on: March 26, 2005, 06:51:35 PM »
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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