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Author Topic: The effect of temperature on equilibrium  (Read 2263 times)

Offline glovesforfoxes

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The effect of temperature on equilibrium
« on: 03/06/2009 17:36:25 »
I've just completed studying A-level chemistry, but there is one thing that I cannot memorise because I don't understand it.

My question is this: why is it than when temperature is increased in a forwardly exothermic reaction, that the l.h.s of the eqn is favoured? The same for temp increase in fwd endothermic rxn favouring the r.h.s. I haven't studied physics, and i'm betting that's at the root of this whole thing. Can anyone help? ???

I'd really like a comprehensive answer to this - you'd really be helping me get into Uni, as this endo/exothermic stuff is central to one of my upcoming exams :)

Thanks,

glovesforfoxes


 

Offline Bored chemist

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The effect of temperature on equilibrium
« Reply #1 on: 03/06/2009 19:12:48 »
I don'tt know about a comprehensive answer but you might want to try this.
Lets take a nice simpel case. Carbon burns in oxygen and gives out CO2 and heat.

If you force heat back into the system by heating it, one way the system can absorb that heat is by reversing the reaction.

Does that make sense?
 

Offline daveshorts

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The effect of temperature on equilibrium
« Reply #2 on: 04/06/2009 11:05:11 »
Think of the reaction as a system that can be in two states, a high energy one - the seperate carbon and oxygen and a low energy state, the carbon dioxide.



Imagine a similar 2 state system - a lot of bouncy balls and a step. Each ball can be at the top of the step or at the bottom. Now give the bouncy balls thermal energy (bouncing around at random). If it is very cold, all the balls will be in the bottom state, but if you make the system hot more and more will move up to the top. But you will always have a slightly higher density of balls at the bottom than the top.

This still won't make an endothermic reaction, but now consider a slightly different setup. A low step, where the top of the step is much larger than the bottom. At low temperatures the balls will all fall down to the bottom again, but as you heat the system up, they will distribute themselves with a slightly lower density on the top step, but because there is so much more space up there there will be a lot more of them at the top.

->

This means that the total potential energy of the system has gone up so the temperature must have gone down to conserve energy.

This is directly analogous to dissolving a salt. The crystalline state is a lower energy, but there is a lot more space in the dissolved state, so ions will dissolve and essentially get lost amongst the water molecules.

This whole idea has been formalised in the concept of entropy. This is a measure of the disorder of things, and the entropy of the universe is always increasing because billions of random interactions tend to make things less ordered (if you have a box full of red and white balls with the red ones at one end, and you shake it , slowly over time they will mix and get more disordered).

The entropy of the universe can increase either by the entropy of your reaction increasing, or by your reaction converting ordered potential energy into disordered heat energy.

There are lots more subtleties if you are interested.
 

Offline glovesforfoxes

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The effect of temperature on equilibrium
« Reply #3 on: 05/06/2009 09:31:17 »
Thanks to both of you for helping - I'll need to reread them both to get my head around them properly! Entropy is a topic I am interested in but know next to nothing about - I read the laws of thermodynamics once or twice and found them pretty fascinating. I had a slight feeling that they had to do with this whole endo/exothermicity problem.

I would be interested in the subtleties, sure dave :) thanks again!
« Last Edit: 05/06/2009 09:33:08 by glovesforfoxes »
 

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The effect of temperature on equilibrium
« Reply #3 on: 05/06/2009 09:31:17 »

 

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