Chem 2 - Chemical Equilibrium VIII: Le Chatelier's Principle- Concepts and Reaction Quotient Q

1. Chemical Equilibrium (Pt. 8)
Le Chatelier’s Principle-
Concepts and
Reaction Quotient Q
By Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0
International License.

2. “Stressing” a Chemical System
Chemical reactions at equilibrium
like to stay at equilibrium.
Any factor that disrupts the
equilibrium state is considered a
“stress.”

3. Le Chatelier’s Principle
Le Chatelier’s principle is a
qualitative rule for predicting
the direction a chemical
reaction (at equilibrium) will
shift in response to a stress.
So what is considered a stress?

4. Stresses and Le Chatelier’s Principle
A stress is some change to the
system, which can include
• Adding/removing product or
reactant
• Changing the pressure on a system
• Changing the volume of a system
• Changing the temperature

5. Le Chatelier’s Principle
When a chemical reaction at
equilibrium is stressed, it
will shift the equilibrium
point in the direction that
counteracts the stress
applied.

6. Effects on Equilibrium: Adding/Removing
Reactants and Products
For example, if more reactant is
added,
Q will be less than K (Q<K)
The reaction will shift toward
products (which “uses up”
reactants to make product).
𝐊 ∝
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

7. Add N2O4 (g) to the reaction flask.
Which direction will the equilibrium
shift?
2 NO2 N2O4
Example Problem: Adding N2O4(g)

8. Add N2O4 (g) to the reaction flask. Which
direction will the equilibrium shift?
Q is now greater than K
(“too many products”)
Product gas was added to the flask, so
the reaction shifts toward reactants.
2 NO2 N2O4
Example Problem Solution: Adding N2O4(g)
𝐐 ∝
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

9. Next up,
Le Chatelier’s Principle and
Pressure-Volume Changes
(Pt 9)