Understanding Dynamic Equilibrium in Chemical Reactions

1. Equilibrium

2. Equilibrium • State of balance. • Condition in which opposing forces exactly balance or equal each other. • Need a 2-way or reversible situation. • Need a closed system.

3. Dynamic Equilibrium • Macroscopic level – looks like nothing is happening. • Microscopic level – lots going on.

4. Equilibrium • Rate of forward process = rate of reverse process. • Hallmark: Looks like nothing is happening. Variables describing system are constant.

5. 3 Kinds of Equilibria • Phase equilibrium – physical • Solution equilibrium – physical • Chemical equilibrium - chemical

6. Phase Equilibrium • Phase changes are reversible processes. • H2O(l)  H2O(g) • H2O(l)  H2O(s) • Same substance on both sides. Phase is different.

7. Examples - Phase Equilibrium • Water & water vapor in a sealed water bottle. • Perfume in a partially full, sealed flask. • Ice cubes & water in an insulated container. • Dry ice & CO2(g) in a closed aquarium.

8. Solution Equilibrium: Solids • Saturated solution = dynamic equilibrium. • Dissolving & Solidification occur at equal rates.

9. Solid in Liquid • NaCl(s)  NaCl(aq) • Favored a little bit by higher temperature.

10. Solution Equilibrium: Gases CO2 in water unopened. CO2(g)  CO2(aq) Favored by high pressure & low temperature.

11. Reversible Reactions • N2(g) + 3H2(g)  2NH3(g) • Forward: N2 & H2 consumed. NH3 produced. • 2NH3(g)  N2(g) + 3H2(g) • Reverse: NH3 consumed. N2 & H2 produced.

12. Reversible Reactions, 1 Equation • N2(g) + 3H2(g)  2NH3(g) • Forward rxn, reactants are on left. Read left to right. • Reverse rxn, reactants are on right. Read in reverse – right to left. • Rxns run in both directions all the time.

13. N2(g) + 3H2(g)  2NH3(g) Why is this point significant? Concentration H2 NH3 N2 Time

14. Reaction Rate • Depends on concentration of reactants. • As concentration of reactants decreases, rate decreases. • As concentration of NH3 increases, rate of reverse rxn increases.

15. Chemical Equilibrium • State in which forward & reverse rxns balance each other. • Rateforwardrxn = Ratereverserxn • Does it say anything about the concentrations of reactants & products being equal? NO!

16. Chemical Equilibrium • Rateforwardrxn = Ratereverserxn • At equilibrium, the concentrations of all species are constant. They stop changing. • They are hardly ever equal.

17. Reversible Reactions vs. Reactions that “Go to Completion” • Ifyour goal is to maximize product yield: • Easier in a reaction that goes to completion. • Use up all the reactants. • Left with nothing but product. • Reversible reactions are different. • Look at Conc/time picture again.

18. N2(g) + 3H2(g)  2NH3(g) Original Equilibrium Point Concentration H2 NH3 N2 Time

19. Reversible Reactions • Once you reach equilibrium, you don’t produce any more product. • This is bad news if the product is what you’re selling. • How can you change the equilibrium concentrations? For example, how can you maximize product?

20. How can you get from here

21. New equilibrium point Lots of product as fast as possible. To here?

22. Affecting Equilibrium • Equilibrium can be changed or affected by any factor that affects the forward and reverse reactions differently.

23. What factors affect rate of rxn? • Concentration/Pressure • Temperature • Presence of a catalyst

24. Catalyst • Has the same effect on the forward & reverse reactions. • Equilibrium is reached more quickly, but the “equilibrium point” is not shifted. • The equilibrium concentrations are the same with or without a catalyst.

25. Concentration, Pressure, Temperature • Changes in concentration, pressure, temperature affect forward & reverse rxns differently. • Composition of equilibrium mixture will shift to accommodate these changes.

26. LeChatelier’s Principle • “If a system at equilibrium is subjected to a stress, the system will act to reduce the stress.” • A stress is a change in concentration, pressure, or temperature. • System tries to undo stress.

27. N2(g) + 3H2(g)  2NH3(g) Original Equilibrium New Equilibrium H2 NH3 N2 Stress: Increased [N2]

28. System • Only 2 possible actions • Shift to the right & form more product. The forward rxn speeds up more than the reverse rxn. • Shift to the left & form more reactant. The reverse reaction speeds up more than the forward rxn.

29. A + B  C + D, at equil. • If I increase the concentration of A, how will the system react? • How does the new equilibrium mixture compare to the original equilibrium mixture? • Use logic. If you increase [A], the system wants to decrease [A]. It has to use A up, so it speeds up the forward reaction.

30. A + B  C + D

31. Changes in Temp • Exothermic rxn: • A + B  C + D + heat • If you increase the temperature, the system shifts to consume heat. So here, it shifts to the left. • Endothermic rxn: • A + B + heat  C + D • If you increase the temperature, the system shifts to consume heat. So here, it shifts to the right.

32. Changes in Pressure • N2(g) + 3H2(g)  2NH3(g) • If you increase pressure, the system shifts to the side with fewer moles of gas. Here, the right hand side has only 2 moles of gas while the LHS has 4. Increasing pressure will cause a shift to the right. • If you decrease pressure, the system shifts to the side with more moles of gas.

33. H2(g) + I2(g)  2HI(g) • This system has 2 moles of gas on the LHS & 2 moles of gas on the RHS. • Systems with equal moles of gas on each side cannot respond to pressure changes.