This document discusses how temperature changes act as a stress on chemical equilibriums according to Le Chatelier's principle. It explains that increasing the temperature of an endothermic reaction shifts the equilibrium toward reactants, while decreasing the temperature shifts it toward products. For exothermic reactions, increasing the temperature shifts the equilibrium toward products, while decreasing the temperature shifts it toward reactants. This is because temperature changes are adding or removing heat, which acts as a reactant in endothermic reactions and a product in exothermic reactions.

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