Chem 2 - Chemical Equilibrium X: Le Chatelier's Principle and Temperature Changes

1. Chemical Equilibrium (Pt. 10)
Le Chatelier’s Principle and
Temperature Changes
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. 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

4. 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.

5. When the reaction temperature changes, the
equilibrium constant K no longer has the same
value,
but the value for Q remains the same.
The equilibrium shifts so that Q = K at the
new temperature.
Temperature Changes as a Stress

6. The easiest way to think about this is to
describe “heat” as a reactant or product.
An endothermic reaction absorbs heat from
the surroundings (H) in the forward
direction.
Heat is a reactant
Temperature Changes and H

7. An exothermic reaction releases heat into the
surroundings (H) in the forward direction.
Heat is a product
So how can we use this concept?
Temperature Changes and H

8. Suppose we increase the temperature… Which
way will the equilibrium shift?
2 NO2 N2O4
Temperature Changes as a Stress
Hrxn =  57 kJ/mol

9. Suppose we increase the temperature… Which
way will the equilibrium shift?
Temperature Changes as a Stress
Hrxn =  57 kJ/mol
2 NO2 N2O4 + heat

10. If the temperature goes up, we are
adding heat (a product), so the reaction
shifts toward reactants (in reverse)
2 NO2 N2O4
Temperature Changes as a Stress
Hrxn =  57 kJ/mol
2 NO2 N2O4 + heat

11. Now, reverse the reaction and decrease
the temperature…
Which way will the equilibrium shift?
Decreasing the Temperature-
Endothermic Reactions
N2O4 2 NO2
Hrxn = + 57 kJ/mol

12. If the temperature goes down, we are
removing heat, so the reaction shifts
toward reactants (in reverse).
Reducing Temperature
N2O4 2 NO2
Hrxn = + 57 kJ/mol
Heat + N2O4 2 NO2

13. The temperature is increased…
Which direction will the
equilibrium shift?
Mini Quiz
A + B 2 C
Hrxn = + 105 kJ/mol

14. If the temperature is increased, we are
adding heat, so the reaction shifts
toward products (in the forward
direction).
Mini Quiz Solution
A + B 2 C
Hrxn = + 105 kJ/mol
Heat + A + B 2 C