1. Equilibrium Law & the Equilibrium Constant

2. Equilibrium Law
Equilibrium law – the mathematical description of a chemical system at equilibrium
aA + bB ⇌ cC + dD
Equilibrium constant (K) – a constant numerical value defining the equilibrium law for a given system (units are typically not reported for this constant)

4. Remember: for this K no units are needed!

5. K varies with temperature

6. Write the equilibrium law equation

7. Equilibrium law equation:
At 500 °C
What is K?

8. Equilibrium constants for forward and reverse reactions are reciprocally related
At 500 °C
K = 6.49x10-2
At 500 °C
K = 15.4 = 1/6.49x10-2

9. The relationship between equilibrium constant and reaction rate
Consider the following reaction at equilibrium
Assume the reaction is composed of elementary steps. Then we
Can use our understanding of rate laws to write:

10. At equilibrium the forward and reverse reaction rates are equal
Notice that this relationship is the same one you would derive
From the equilibrium law if the coefficients are all 1:
Caution – This only works if the reaction is an elementary process!!!

11. Heterogeneous Equilibria
The equilibria to this point have been homogenous systems (gas and solution)
Homogeneous equilibrium – a chemical equilibrium in which all reactants and products are in the same state of matter
Heterogeneous equilibrium – a chemical equilibrium in which the reactants and products are present in at least two different states (eg gas and solid)

12. The quantity of pure materials is irrelevant since their
concentration cannot change!
-The only concentration that changes over the course of
The reaction is that of CO2 gas.

13. Instead of writing:
We can write:
Since C1 and C2 are constants they can be included in
K resulting in an equilibrium equation of:

14. If pure solids or liquids are involved in a chemical equilibrium system, their concentrations are not included in the equilibrium law equation for the reaction system

15. Using the magnitude of the equilibrium constant to predict the direction of a reaction
At 25 °C
At equilibrium the reaction lies far to the right with high
Concentration of CO2

16. At equilibrium the products and reactants are roughly
equal in concentration (K ~ 1)

17. At equilibrium the reaction lies far to the left. The smaller
the K value the higher the concentration of reactants.