Lecture Notes Ė LeChatelierís Principle and the Equilibrium Lab††††††† ††††††††††††††† Std 9a

 

LeChatelierís 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?Letís 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

 

IfCorDincrease, you must increase eitherAorBfor theKto 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 LeChatelierís 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 Ė 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.