1. Equilibrium Reactions LeChatelier’s Principle Equilibrium Constants
Chapter 19, part II Notes
Equilibrium Reactions
LeChatelier’s Principle
Equilibrium Constants

2. Reactions so far…
Up to now, we have assumed that reactions go one way and stop when all reactants have become products.
This is true sometimes, but not always. There are also…

3. Reversible Reactions
In a reversible reaction, the reaction occurs simultaneously in both directions.
Products are being formed and are also being used to form reactants.

4. 2SO2 + O2 D 2SO3

5. Equilibrium So how much product is formed? It depends on the reaction.
Eventually, the reaction will reach chemical equilibrium, where forward and reverse reactions take place at the same rate.

6. Equilibrium
This does NOT mean that the amounts of product and reactant are the same.
The equilibrium position is the relative percent of products and reactants in a system at equilibrium.

8. LeChatelier’s Principle
Henri LeChatelier studied shifts in the equilibrium of reactions and came up with his principle. He said:
If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

9. LeChatelier’s Principle
Let’s look at how several factors will shift the equilibrium point.

10. Concentration
Increasing the concentration of a product or reactant in equilibrium will shift the equilibrium point away from that substance.
Decreasing the concentration would have the opposite effect.

11. H2CO3 D H2O + CO2 Which way will equilibrium shift if adding: CO2
Which way will equilibrium shift if removing:

12. Temperature
Adding heat to a reaction at equilibrium will shift the equilibrium point towards the side that absorbs heat.
Removing heat will shift the equilibrium point towards the side that produces heat.

13. 2SO2 + O2 D 2SO3 + heat
Which way will equilibrium shift if heat is added?
Which way will it shift if heat is removed?

14. Pressure
Adding pressure to a system at equilibrium will shift the equilibrium point towards the side that has the least moles of gas.
Decreasing pressure shifts equilibrium towards the side with the most moles of gas.

15. N2 + 3H2 D 2NH3
Assuming all the products and reactants are gases, which way does equilibrium shift when pressure is added?
Which way does it shift when pressure is relieved?

17. Equilibrium Constants
Generally, the equilibrium point is expressed as a numerical value, called the equilibrium constant, for any reversible reaction at a given temperature.

18. Given the equation of a general reaction to be:
coefficients
aA + bB D cC + dD

19. The equilibrium constant would be:
[C]c[D]d
[A]a[B]b
Keq =
The brackets around the substances means it is their concentration (molarity).

20. Equilibrium constant
If the equilibrium constant (Keq) is bigger than one, it means the products are favored at equilibrium.
A Keq less than one means the formation of reactants is favored at equilibrium.

22. N2O4 D 2NO2
In the above reaction, at the equilibrium point, dinitrogen tetroxide has a concentration of M and nitrogen dioxide has a concentration of 0.030M. What is the equilibrium constant for the reaction?

23. N2 + 3H2 D 2NH3
Calculate Keq for the reaction if at equilibrium in 1.0L there is 0.15mol H2, 0.25mol N2 and 0.10mol NH3.

24. 2HI D H2 + I2
Keq for the above reaction is 0.020; if the concentration of hydrogen and iodine are both 0.50M, what is the concentration of HI?