Lecture Notes – LeChatelier’s Principle and the Equilibrium Lab                        Std 9a

LeChatelier’s Principle tells us that if concentration, volume or temperature change after equilibrium is reached, the position of the equilibrium will shift in a direction that tends to reduce (reverse) that change.

Remember that changes for products and reactants are directly proportional!!!  Why?  Let’s take a simple example with real numbers:

A

--  =  K

B

If A = 2 and   K = 1, then what does B have to be?      Yes, it must be 2.

2

--  =  1

2

Now, if we make A = 4, what does B have to be?      Yes, it must be 4 also.  Notice that when we increased A we also had to increase B so that both numbers will still divide out and become 1.

4

--  =  1

4

So now do you see why products and reactants are directly proportional?

[C]c [D]d

---------  =  K

[A]a [B]b

If  C  or  D  increase, you must increase either  A  or  B  for the  K  to be the same number.  Likewise, if you decrease C or D, you must also decrease A or B.

Here is the chart we did in class to help you remember how things go:

reactants     products      direction of shift

increase            increased          ---->

decrease           decreased         <---

How heat works with LeChatelier’s Principle -------------

 EXOTHERMIC REACTIONS  -  THESE REACTIONS CREATE HEAT, and heat acts like a product .               A  +  B  <-------->  C  +  D  +  Energy (or heat)                         reactants     products      direction of shift               increased          add heat           <----             decreased         reduce heat       ---->

 ENDOTHERMIC REACTIONS – THESE REACTIONS SUCK IN HEAT, and heat acts like a reactant               Energy (or heat)   +   A  +  B  <-------->  C  +  D               reactants     products      direction of shift             add heat           increased          ---->             reduce heat       decreased         <----

So heat either acts like a product (exothermic reactions) or a reactant (endothermic reactions).  Adding heat to a system for an exothermic reaction at equilibrium is like adding more of a product.  Adding heat to a system for an endothermic reaction at equilibrium is like adding more of a reactant.