Chapter 19, part II Notes

1. 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 + O2D 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. H2CO3D H2O + CO2 • Which way will equilibrium shift if adding: • CO2 • H2CO3 • H2O Which way will equilibrium shift if removing: • CO2 • H2CO3

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 + O2D 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 + 3H2D 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: Keq = [C]c[D]d [A]a[B]b 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. N2O4D 2NO2 • In the above reaction, at the equilibrium point, dinitrogen tetroxide has a concentration of 0.0045M and nitrogen dioxide has a concentration of 0.030M. What is the equilibrium constant for the reaction?

23. N2 + 3H2D 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?