Sunday
August 31, 2014

Homework Help: chem

Posted by Mira on Saturday, March 17, 2007 at 1:13pm.

How do these things shift the direction of a chemical reaction?:

-adding/removing reactants
-adding/removing products
-adding/removing energy

This is how I think I understand it:
-adding more reactants pushes it to the right
-removing reactants pushes it to the left
-adding more products pushes it to the left
-removing products pushes it to the right
-adding more energy to and endothermic pushes it to the left
-adding more energy to an exothermic reaction pushes it to the left

I don't know. I'm pretty confused. I need some sort of rule or something!!!

By the way, if you are replying,DrBob222, I actually do need your help.. :)

You need to rethink the last two. I wouldn't expect adding more energy to shift a reaction the same way for BOTH an endothermic AND an exothermic reaction. What is it you don't understand about Le Chatelier's Principle?

Oh, sorry, I typed that wrong. I meant energy for endothermic to the right.

But are the first four rules the same whether it is an endo or exo?

I think I understand it.

Yes and yes. Adding energy to an endothermic reaction shifts the equilibrium to the right. Adding energy to an exothermic reaction shifts the equilibrium to the left. But the "rules" are not somthing that must be memorized if you understand what is going on. Le Chatelier's Principle just says that an equilibrium, whcn placed under a stress, will react in such a way so as to reduce the stress. In the A + B ==> C + D reaction at equilibrium, adding more A stresses the equilibrium; it must react to the stress by shifting so as to remove some of what has been added. Shifting to the right uses up some of the added A. Same thing for B. Added B means the reaction will shift to the left in order to use up some of the added B. For endothermic and exothermic reactions, I think of the endothermic raaction as this.
A + B + heat ==> C + D.
Adding heat moves the equilbrium to the right because that uses up heat.
For an exothermic reaction I think of it as
A + B ==> C + D + heat.
Adding heat moves the equilibrium to the left to use up the heat.

So when you add something to one side, it wants to shift the other way? Like if you add to the right side, it wants to shift to the left to get rid of the pressure?

That's right but with pressure there IS a rule to remember. Increased pressure will shift the equilibrium to the side with the fewer mols of gas. For example,
2H2(g) + O2(g) ==> 2H2O(g)
Increasing pressure will shift the equilibrium to the right because there are only two mols of gas on the right and three mols of gas on the left. So the shift is to occupy the smaller volume since increased pressure makes a gas occupy a smaller volume.

Okay, thanks so much!!!

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