1. UNIT 12 REVIEW You Need: Marker Board Marker & Paper Towel Calculator

2. Write the equilibrium expression for the following reaction: 4 HCl(g) + O 2 (g)   2 Cl 2 (g) + 2 H 2 O(g) ANSWER: Keq = [Cl 2 ] 2 [H 2 O] 2 [HCl] 4 [O 2 ]

3. Write the equilibrium expression for the following reaction: SO 3 (g) + H 2 O(l)   H 2 SO 4 (l) Answer: Keq = 1 [SO 3 ]

4. Write the equilibrium expression for the following reaction: Cu(s) + 2 AgNO 3 (aq)   Cu(NO 3 ) 2 (aq) + 2 Ag (s) Answer: Keq = [Cu(NO 3 ) 2 ] [AgNO 3 ] 2

5. CH 4 (g) + 2 O 2 (g)   CO 2 (g) + 2 H 2 O(g) Calculate [O 2 ] if: [CH 4 ] = 0.0150 M, Keq = 3,620 [CO 2 ] = 0.0824 M, [H 2 O]= 0.0603 M Are the reactants or products favored at equilibrium? Answer : Keq = [CO 2 ] [H 2 O] 2 [CH 4 ] [O 2 ] 2 3,620 = (0.0824)(0.0603) 2 (54.3)x 2 = 0.0002996 x 2 = 0.0000552 (0.0150)(x) 2 54.3 54.3 [O 2 ] = 0.00235 M Products are favored because Keq is greater than one.

6. Write the equilibrium expression & calculate [CH 3 OH] for this reaction: CO(g) + 2H 2 (g)  CH 3 OH(g) [CO]=0.025M, [H 2 ]=0.080M, Keq = 394 Answers: Keq = [CH 3 OH] [CO] [H 2 ] 2 [CH 3 OH]: 394 = x = 0.063 M (0.025)(0.080) 2

7. Calculate Keq: PCl 5 (g)   PCl 3 (g) + Cl 2 (g) [PCl 5 ] = 1.00M, [PCl 3 ]=3.16x10 -2 M, [Cl 2 ]=7.18x10 -3 M Are the reactants or products favored at equilibrium? If you change the temperature, would the Keq be the same? Answers: Keq = [PCl 3 ][Cl 2 ] = (3.16x10 -2 )(7.18x10 -3 ) = 2.27x10 -4 [PCl 5 ] (1.00) Reactants are favored because Keq is less than 1. No, because temperature affects concentration.

8. What 2 types of changes cause a reaction to go to completion? If a reaction does not go to completion, what may happen to some of the products? Answers: Forms a Gas (g) or a Precipitate ( ) Can reform reactants (Reverse Reaction)

9. A reaction that does not go to completion is called a __________________ reaction. What part of the equation shows if a reaction goes to completion or not? Answer: Reversible The Arrow (double = reversible, single = completion)

10. Why does the word “dynamic” describe equilibrium? Define reversible reaction. Define completion reaction. Answers: Dynamic describes equilibrium because the reaction does not stop. The forward and reverse reactions continue at equal rates. A reversible reaction can form products and then reform reactants. A completion reaction can only form products and forms a gas or a precipitate.

11. When a system is at ________________________, it will stay that way until something changes. State Le Chatelier’s Principle. List three examples of stresses that can change a system at equilibrium. 1) 2) 3) Answers: Equilibrium When a reaction at equilibrium is disturbed, it will shift to eliminate the stress. Changing temperature, concentration, or pressure/volume (only for gases)

12. You can tell if a reaction is endothermic or exothermic by looking at the equation. If heat is on the reactant side, the reaction is ________________. If heat is on the product side, the reaction is ________________. Determine if these reactions are endothermic or exothermic. a) 2NO 2 ↔ N 2 O 4 + 58.8 Jb) 2AgCl + energy ↔ 2Ag + Cl 2 What direction will the system shift if the temperature in “a” is raised? How could you make reaction “b” produce more products? Add energy or remove energy? Answers: endothermic, exothermic a) exothermic b) endothermic Shift left Add energy

13. When an equilibrium mixture is subjected to a decreased pressure, it will shift to the side with ____________________. Answer: More Moles Decrease Pressure = Increase Volume = Shift to Side with MOST Moles

14. 2 NO 2(g)   N 2 O 4(g) + energy According to Le Chatelier’s Principle, what direction will the system above shift if the following stresses are applied? A. Lower Temperature B. Increase N 2 O 4(g) C. Decrease NO 2(g) D. Increase Pressure E. Increase Volume Answer: A.Right (to replace heat lost on right side) B.Left (to decrease [N 2 O 4 ] C.Left (to replace [NO 2 ]) D.Right (Increase P = Decrease V so shift to side with less moles E.Left (Increase V so shift to side with more moles)

15. What does Ksp stand for? What does it describe? Which lead compound below is the least soluble? Ksp = PbBr 2 = 6.3 x 10 -6 Ksp = PbCl 2 = 1.7 x 10 -5 Ksp = PbCrO 4 = 1.8 x 10 -14 Answers: Solubility Product Constant Ksp describes the solubility (ability to dissolve) of slightly soluble substances. PbCrO 4 because it has the smallest Ksp