This document discusses chemical equilibrium. It defines equilibrium as a state where the macroscopic properties of a system (such as pressure, volume, temperature) have stopped changing, even though microscopic reactions continue to occur. For a system to be at equilibrium, the rates of the forward and reverse reactions must be equal. The document provides examples to illustrate these concepts, such as the equilibrium between NO2 and N2O4 gases. It emphasizes that regardless of whether a system starts with only reactants or only products, the equilibrium concentrations or pressures will be the same once equilibrium is reached.

1. Chemical Equilibrium (Pt. 1)
What is Equilibrium?
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. Question:
Which system is at equilibrium?
Water in a sealed
container at 25C
Ice cubes in a sealed
container at 25C

3. How Do We Know Which System is
at Equilibrium?
Water in a sealed
container at 25C
Ice cubes in a sealed
container at 25C
This system is at
equilibrium
This system is
NOT at equilibrium

4. Equilibrium and Macroscopic
Change
A system has reached equilibrium
when its macroscopic observables
have stopped changing.
What are macroscopic observables?
Things we can measure…pressure,
temperature, volume, density, etc.

5. Question:
Will This System Come to Equilibrium
Eventually?
Ice cubes in a sealed
container at 25C
Let time pass…

6. Will This System Come to Equilibrium
Eventually?
YES!
We will have water
in a sealed container
at 25C
Ice cubes in a sealed
container at 25C
Let time
pass…

7. Equilibrium and Microscopic Change
For a system at equilibrium,
macroscopic observables have
stopped changing, but microscopic
processes are still going on!
But what does this mean?

8. Recall: Equilibrium Vapor Pressure
Molecules in the liquid phase
continuously vaporize and
condense in a closed container.
rate of evaporation = rate condensation
The partial pressure of the gas is
constant at equilibrium!
Liquid molecule

9. Example: Gas Phase Equilibrium
For the reaction
2 NO2 (g) N2O4 (g)
The forward reaction is
2 NO2 (g) N2O4 (g)
The backward reaction is
N2O4 (g) 2 NO2 (g)

10. Microscopic Processes at Equilibrium
For the reaction
2 NO2 (g) N2O4 (g)
At equilibrium, both of these processes
are occurring microscopically!
2 NO2 (g) N2O4 (g)
N2O4 (g) 2 NO2 (g)

11. Question
What happens to the partial
pressure of each gas once the
system reaches equilibrium?
2 NO2 (g) N2O4 (g)

12. Answer
The partial pressure of each
gas remains constant!
2 NO2 (g) N2O4 (g)

13. The Equilibrium State
Let’s do two experiments:
2 NO2 (g) N2O4 (g)
Expt 1: Put 1.0 atm NO2(g) in a 1.0 L flask at 298K
Expt 2: Put 0.5 atm N2O4(g) in a 1.0 L flask at 298K
What will the final partial pressures of
each gas be in Experiments 1 and 2?

14. Time (s)
P (atm)
0.75
0.50
0.25
1.0 Let time pass… This reaction takes
about a minute to come to equilibrium.
𝐏 𝐍𝐎 𝟐
𝐏 𝐍 𝟐 𝐎 𝟒
Experiment 1: Start with 1.0 atm NO2 (g)

15. Time (s)
P (atm)
0.75
0.50
0.25
1.0
Let time pass…
Look carefully at the final partial
pressure of each gas.
𝐏 𝐍𝐎 𝟐
𝐏 𝐍 𝟐 𝐎 𝟒
Experiment 2: Start with 0.5 atm N2O4 (g)

16. The Equilibrium State
It didn’t matter if we started with all
reactant (NO2) or all product (N2O4).
The equilibrium partial pressures of
the gases arrived at the same state!
2 NO2 (g) N2O4 (g)
0.50 atm 0.25 atm

17. Central Equilibrium Concept 1
At equilibrium, there is no change in
macroscopic observables.
It looks like nothing is happening!
However, microscopic processes are still
happening. We just can’t see or measure
them macroscopically!

18. Central Equilibrium Concept 2
The final equilibrium concentrations (or
partial pressures) of reactants and
products is completely independent of the
initial concentrations/partial pressures of
reactants and products.
If we start with all reactants or all
products, the reaction still goes to the
same distribution of reactants/products!

19. Next up,
The Relationship Between
Kinetics and Equilibrium (Pt 2)