Ch. 18 –Equilibrium
Keq general Questions:
1. What is the difference between a reaction in equilibrium and a reaction which “goes to completion”?
2. True or False: (g) and (aq) are found in the Keq expression, but (l) and (s) are not.
3. True or False: H2O(l) ß-à H2O (s) is an example of a physical equilibrium reaction.
4. True or False: O2(g) ßà O2(aq) is an example of a physical equilibrium reaction.
5. True or False: H2O (l) ß--à H3O+(aq) + OH-(aq) is an example of a physical equilibrium reaction.
6. If you were to cool the reaction in question #3 which way would the reaction shift: Left or Right?
7. If you were to cool the reaction in question #4 which way would the reaction shift: Left or Right?
8. What do the [ ]’s mean? What units do they describe?
9. When you write the Keq of an equilibrium reaction, what is divided by what?
10. If the Keq of an equilibrium reaction is less than one, then the reaction favors the Products or Reactants?
11. If the Keq of an equilibrium reaction is greater than one, then the reaction favors the Products or Reactants?
Le Chatelier’s Principle general questions:
Consider the following reaction for questions #13-21: 2NO2(g) ß-à N2O4(g) + 57.2 kJ
Also consider that NO2 is a brown gas while N2O4 is a colorless gas.
13. Write the Keq for this reaction.
14. If you cooled this reaction, which way would the equilibrium shift? Left or Right?
15. If you heated this reaction, which way would the equilibrium shift? Left or Right?
16. If you increased the pressure on this reaction, which way would the equilibrium shift? L or R?
17. If you increased the volume on this reaction, which way would the equilibrium shift? L or R?
18. If the bottle in which this reaction was taking place suddenly turned brown, would you assume the bottle was being warmed or cooled? Increased pressure or less pressure?
19. If the Keq for this reaction was 1.2 x 10-5 at room temperature, what color would you think this bottle of gas would be at room temp?
20. What would the Keq of this reaction be if you were to write it: N2O4(g) + 57.2 kJ ß---à2NO2(g)
21. Is this reaction (as it is written in question #20) endo or exothermic?
Consider the following equation for problems 22-26: PbCl2(s) ß-à Pb+2(aq) + 2Cl-(aq)
22. Write the Keq for this reaction. (no numbers, just symbols and raise it to a power if need be)
23. Lead (II) chloride is mostly insoluble in water. Which side of this equation do you think is favored?
24. Do you think this reaction will have a Ksp greater or less than one?
25. If you added some Pb(NO3 )2 to this reaction, what do you think would happen, would it drive the reaction right, left or no change? (Pb(NO3 )2 is soluble in water and will put more Pb+2 ions into soln.)
26. If you added more PbCl2(s) to this reaction, would that drive the reaction right, left or no change?
1. Reactions in equilibrium have both products and reactants present in the “system”. For example, a bottle of water has both liquid water and gaseous water present in the bottle (system). That is in equilibrium. Reactions which go to completion use up their reactants and turn into products.
5. False – that is a chemical example because bonds have been broken and re-formed
6. shift right
7. shift right. There is more dissolved oxygen (aq) in the cold Arctic ocean than in the warm south seas.
8. concentration in Molarity (moles/L)
14. right – as you cool it, you remove heat. Since it is exothermic, the rxn will go right to make more heat.
15. left - as you add heat, it forces the rxn the other direction to remove the stress.
16. right – increasing pressure will favor the smaller amount of particles present. There is only one mole of gas on the product side. There are two moles of gas on the reactant side.
17. left - increasing volume allows more room (less pressure). There are two moles on reactant side and only one mole on product side. Thus, increasing volume will favor the larger amount of moles.
18. Heated and less pressure. See answers for 14-17 for explanation.
19. Brown. The Keq is less than one so it favors the reactants. The reactants are brown.
20. 1/(1.2 x 10-5) or 8.3 x 104
22. Ksp = [Pb+2] [Cl-]2
24. less than one. It is not soluble in water and therefore favors the reactant side.
25. Left - Pb(NO3)2 will break up into Pb+2 and 2NO3-1 ions. The Pb+2 ions will be free to react with the Cl- ions in solution and since the reaction favors PbCl2,(s) the reaction will proceed to the left.
26. Nothing would happen. The equilibrium would stay the same. Yes, there would be more Pb+2 and Cl- ions created, but they would be in equal amounts. There would be an increase in concentration of all three items, but no overall increase in one item but not the others.