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Definition: If you apply a stress to an equilibrium problem, the system will try to get rid of that stress in order to go back to the original equilibrium.
Mr. McLeod addition to definition:
a. System can only make more reactant or more product, nothing else.
b. System will never get back to original equilibrium (since you add something in that has matter and energy) but comes to a new equilibrium. This is what the shift means, compare the original equilibrium material (ratio of reactant to product) compared to the new equilibrium material (ratio of reactant to product)
How you answer LeChatlier's Principle problems for Mr. McLeod (see blank answer form below)
1. What is stress
2. When you are reducing stress, do you make more reactant or product (for pressure you need to indicate # of total reactant particles and product particles)
3. Favors the forward or backward reaction (for change in temp must add endo or exo)
4. Shift equil to the Right or Left.
a. Shift equilibrium to the Left means that the new equilibrium position has a higher ratio of reactants to product than in the original equilibrium position.
b. Shift equilibrium to the Right means that the new equilibrium position has a higher ratio of product to reactant than in the original equilibrium.
For AP students (how actual amount of material change or is it actual concentrations of reactant to product or the ratio of reactant to product for original to new equilibrium position??):
Here is a blank answer form you will fill out for each LeChatelier's Principle problem.
Stress: _______________________________________________________________
How rxn tries to remove stress: _________________________________________________
Circle: Make more product or more reactant (fill in for pressure changes)__________________
Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes) _____________
Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)
There are 4 Types of Stresses
a. Adding more reactant/products
b. Changing Temperature (really means increase Energy).
c. Pressure change (there are more than one way to do this)
d. Add a catalysts
I will use the following chemical reaction (called the Harbor process, 1st process to fix nitrogen to ammonia industrially) for examples of LeChatelier's Principle
1 N2 (g) + 3 H2 (g) <==> 2 NH3 (g) + Energy (i.e. exothermic rxn)
I will use the above blank answer form but will also include explanations of why. When you do your homework , you do not have to include explanations (I will italicize the words).
a. Adding more reactant/products:
Stress: Adding in product (increasing concentration of products)
How rxn tries to remove stress: _________________________________________________
Circle: Make more product or more reactant (fill in for pressure changes)__________________
Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes) _____________
Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)