1. (4) For this reaction: NO 2. + H 3 AsO 3 D H 2 AsO 3

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1 Exam 1 Chem 1B, Spring 2017 Fossum Name: There are 6 points of extra credit built in to this exam. Always show your work and explain your reasoning, include units, and check significant figures. Note: always check to see if your answer makes sense. If not, check your work. If you can t find the mistake, please comment on why you think your answer doesn t make sense. Extra points will be taken off for answers that are obviously impossible and not noticed by you. 1. (4) For this reaction: NO H 3 AsO 3 D H 2 AsO HNO 2 a. Label each substance (in the reactants and in the products) as either an acid or a base. b. This equilibrium lies to the left. Which is the stronger acid? How can you tell? c. What can you say about the value of K c for the reaction? Explain briefly how you can tell. 2. (2) a. What is the conjugate acid of (CH 3 ) 2 NH? b. What is the conjugate base of HC 2 O 4 -? 3. (3) If a solution has a hydroxide concentration of M, what is the concentration of hydronium ion in this solution? Is the solution acidic, basic, or neutral? 4. (2) Calculate the ph of M Ba(OH) 2 (aq). 1

2 5. (4) a. For a hypothetical reaction, if you double the concentration of the reactant X, the rate of the reaction is 8 times faster. What does this tell you? b. For the same reaction, if you triple the concentration of the catalyst Y, the rate triples. What does this tell you? c. What is the rate law of this reaction? d. If you were to triple the concentration of X, what would happen to the rate of the reaction? 6. (2) For the equilibrium: CH 4 (g) + 2 H 2 S (g) D CS 2 (g) + 4 H 2 (g) If you wanted to maximize the yield of the products, would you use a high or low pressure for the reaction? Why? Explain. 7. (2) For the equilibrium: CO (g) + 2 H 2 (g) D CH 3 OH (g) H = kj Which way would this equilibrium shift if you were to increase the temperature of the mixture? Why? Explain the underlying reason. 8. (6) Given the following proposed mechanism: 1. NH 2 NO 2 + OH - - D H 2 O + NHNO 2-2. NHNO 2 N 2 O + OH - (slow) (fast) a. Identify any intermediates and/or catalysts in this mechanism. b. Determine the rate law that is consistent with this mechanism. Show your work. 2

3 9. (4) Given the following three graphs for the same reaction: a. What is the order of this reaction with respect to the reactant A? Explain clearly how you can tell. b. Is it possible to determine the value of k (the rate constant) for this reaction? Explain. If not, why not? If so, explain how you would go about determining the k value. 3

4 10. (2) What is the frequency factor in the Arrhenius equation (A), and what influences the size of the frequency factor? Explain. 11. (5) What is the effect of temperature on the rate of a reaction? Explain the underlying reason(s) at the molecular level. Include a sketch of the Boltzmann distribution to support your answer. Make sure your explanation is clear and complete for full credit. 12. (6) If a first order reaction takes 32.5 minutes for the reactant concentration to decrease to 30.0% of its initial value, what is the value of k (the rate constant) for this reaction? b. Calculate the half life of this reaction. c. Explain why this half-life makes sense. 4

5 13. (4) For a reaction that is second order in the reactant A with a rate constant of M -1 hr -1 : if you start with M A and allow it to react, how much A remains after 2.50 hours? 14. (5) Given the following reactions and their corresponding equilibrium constants at 25 C, 2 H 2 O (l) D H 3 O + (aq) + OH - (aq) K c = Ca(OH) 2 (s) D Ca 2+ (aq) + 2 OH - (aq) K c = a. Calculate the equilibrium constant for this reaction: Ca(OH) 2 (s) + 2 H 3 O + (aq) D Ca 2+ (aq) + 4 H 2 O (l) K c =? b. Comment on the physical meaning of the size of the K value that you calculated above. What does this mean specifically for this overall reaction? 15. (6) Given the attached graph of ln k vs. 1/T, a. Determine the x and y values of both of the points indicated on the graph. b. Determine the slope of this graph. c. Determine the activation energy of this reaction in kj/mole. 5

6 16. (6) For the following equilibrium: CH 4 (g) + 2 H 2 S (g) D CS 2 (g) + 4 H 2 (g) K c = 3.6 at 900. C. a. What does the size of the equilibrium constant tell you about this reaction? Please be specific. b. Which way will this reaction go (forward, reverse, or neither) if you mix the substances together so that you have 1.0 M CH 4, 1.0 M H 2 S, 1.0 M CS 2, and 1.0 M H 2 to start with? Explain/show work. c. Which way will this reaction go (forward, reverse, or neither) if you mix the substances together so that you have 0.20 M CH 4, 0.30 M H 2 S, 3.5 M CS 2, and 1.5 M H 2 to start with? Explain/show work. d. Compare your answers to parts b and c. What are the differences between the two situations? 17. (8) a. Calculate the ph of 0.27 M HClO. b. Calculate the percent ionization of this compound under these conditions. 6

7 18. (9) For the reaction: 3 H 2 (g) + N 2 (g) D 2 NH 3 (g) a. When moles of N 2 and moles of H 2 are combined at 450. C in a L container and allowed to come to equilibrium, moles of NH 3 is present in the container at equilibrium. Determine the value of K c for this reaction at this temperature. b. Using your answer from part a, determine the value of K p for this reaction at this temperature (450. C). 19. (7) A M solution of a weak base has a ph of when it comes to equilibrium. Determine the value of K b for this weak base. 7

8 20. (10) Calculate the ph and the concentrations of all ions in M H 3 PO 4. (Calculate the ph and [H 3 PO 4 ], [H 2 PO 4 - ], [HPO 4 2- ], and [PO 4 3- ] at equilibrium.) 8

9 If you do better on the following problem than you did on the similar problem on quiz 3, you will earn extra points on your quiz 3 score. (Don t skip this problem, though if you don t do it, it s minus 9 points.) This 9 points will replace the corresponding 9 points on the quiz. It is not extra credit, it s more like a re-do A (g) + 3 B (g) D 5 C (g) + 2 D (g) K c = (9) If you mix 5.0 mole of A, 4.0 moles of B, and 4.0 moles of D in a 10.0 L container, calculate the equilibrium concentrations of A, B, C, and D. 9