DO NOT TURN THE PAGE UNTIL INSTRUCTED TO DO SO!

Transcription

1 CHEMISTRY 121 WINTER 2004 Midterm Exam #2 Wednesday, March 17 Name Student Number Signature DO NOT TURN THE PAGE UNTIL INSTRUCTED TO DO SO! Make sure you have all 7 pages (including this one). Make a note of the point value of each question, and allocate your time accordingly. Carefully read each question before answering. Where required, work must be shown to receive full credit. Explanations in point form are acceptable and encouraged. No aids or notes of any kind are permitted or required, except a calculator. I m looking over a three-leafed clover that I ve overlooked bethree. Total Points: 60 Total Time: 80 minutes Potentially Helpful Data, Equations, and Constants Constants absolute zero = ºC 1 atm = 760 torr = kpa = bar 1 Latm = J 1 ev = J/mol N A = mol 1 R = Latm/molK = J/molK K w (298K) = Equations [base] ph = pk a + log [acid] PV = nrt H = E + PV E = q + w w = P V q = mc T H = H final H initial S = q rev /T G = H T S Gº = RTlnK G = Gº + RTlnQ K ln K 2 1 Hº 1 1 = R T1 T2 Acid Dissociation Constants at 25ºC acid K a H 2 SO HClO H 3 PO HF HNO CH 3 CO 2 H H 2 S H 2 PO HSO HClO H 3 BO HCN HPO H 2 O Base Dissociation Constants at 25ºC base K b CH 3 CH 2 NH (CH 3 ) 2 NH CH 3 NH (CH 3 ) 3 N NH C 5 H 5 N C 6 H 5 NH H 2 O

2 2 Chem 121, Midterm#2, Winter NOW I KNOW MY ABCS (12 POINTS, 2 EACH) Circle the one best answer for each question. a) Two bars of equal mass, one of Fe and one of Cu, start at a temperature of 150ºC and lose 500 J of heat. The heat capacity of Fe is greater than that of Cu. Which statement is true? a) The iron has a lower final temperature than copper. b) The iron has a higher final temperature than copper. c) The two bars have the same temperature, since they lose the same amount of heat. d) The work for both bars is +500J, because energy is conserved. b) Which acid-base pair would be the best choice to prepare a buffer solution with ph = 8? a) H 3 PO 4 and NaH 2 PO 4 b) H 3 BO 3 and NaH 2 BO 3 c) [C 5 H 5 NH]Cl and C 5 H 5 N d) NaH 2 PO 4 and Na 2 HPO 4 c) Two aqueous solutions are mixed in a flask and an exothermic reaction takes place. Which of the following is true? a) The products have more enthalpy than the reactants. b) The mixed solution gets colder, because the reaction loses heat. c) The mixed solution gets warmer, because the reaction loses heat. d) The mixed solution stays the same temperature, because energy is conserved. d) A gas absorbs 20 kj of heat, and E of the gas is 15 kj. Which of the following is true? a) q = 20 kj b) The gas expands, doing work on the surroundings. c) The system is at constant volume. d) q = 5 kj e) The standard heat of formation of N 2 (l) is a) equal to the NN bond strength. b) zero, because N 2 is an element. c) a positive value, because N 2 is stable as a gas under standard conditions. d) a negative value, because heat must be removed from N 2 (g) for it to condense. f) Water freezes at 5ºC. Which of the following statements regarding this change is false? a) The change is spontaneous, because the entropy of the universe increases. b) The entropy lost from the water equals the entropy gained by the surroundings. c) The entropy of the surroundings increases, because the change is exothermic. d) The entropy of the water decreases, because the molecular motion becomes less random.

3 3 Chem 121, Midterm#2, Winter ENEMY BASES HAVE BEEN NEUTRALIZED (8 POINTS) Consider ml of an aqueous solution that contains M of aniline (C 6 H 5 NH 2 ) and M of the anilinium ion ( [C 6 H 5 NH 3 ] + ). a) What is the ph of the solution? (2) b) What is the resulting ph after moles of HCl are added to the solution? (3) c) What is the resulting ph if more HCl is added, to a total of moles? (3)

4 4 Chem 121, Midterm#2, Winter HESS AND HERS (6 POINTS) A long-sought achievement in synthetic chemistry is the catalytic and selective oxidation of methane to form methanol: CH 4 (g) + ½ O 2 (g) CH 3 OH(l) Given the thermochemical data at below, calculate Hº rxn for the reaction above. 2 H 2 O(g) 2 H 2 (g) + O 2 (g) Hº = kj 2 H 2 O(l) 2 H 2 (g) + O 2 (g) Hº = kj 3 H 2 (g) + CO(g) CH 4 (g) + H 2 O(l) Hº = kj CO(g) C(s) + ½ O 2 (g) Hº = kj Hº f (CH 3 OH(l)) = kj/mol

5 5 Chem 121, Midterm#2, Winter FICKLE NICKEL (8 POINTS) The dissolution and reaction of Ni(OH) 2 (s) in water has K = at 25ºC. Ni(OH) 2 (s) + 2 H 3 O + (aq) 4 H 2 O(l) + Ni 2+ (aq) a) What is Gº for the above reaction of Ni(OH) 2 in water at 25ºC, in kj? (2) b) What is the value of Q for the reaction of Ni(OH) 2 in a solution of M Ni(NO 3 ) 2 (aq) at ph 7.80? (3) c) Does Ni(OH) 2 dissolve under these conditions? Justify your answer quantitatively. (3)

6 6 Chem 121, Midterm#2, Winter AN EQUAL AND OPPOSITE REACTION (8 POINTS) Hº f Tin tetrachloride was once called the Fuming Liquor of (kj/mol) Libavious, because it reacts with atmospheric moisture to give a visible white smoke. SnCl 4 (l) + 2 H 2 O(g) SnO 2 (s) + 4 HCl(g) Consider the thermochemical data given at right and answer the questions below. a) Calculate Hº and Sº for this reaction of SnCl 4 with atmospheric water. (4) Sº (J/molK) SnCl 4 (l) SnO 2 (s) H 2 O(l) H 2 O(g) HCl(g) HCl(aq) b) Calculate K of this reaction at 35ºC (2) c) Under what range of temperatures is this reaction reactant-favoured? Justify your answer. (Possible answers might include things like all temperatures, no temperatures, or above 78ºC ). (2)

7 7 Chem 121, Midterm#2, Winter MERCEDES BENZOATE (8 POINTS) Consider the titration curve shown at right, for the titration of ml of a benzoic acid (C 6 H 5 CO 2 H, K a = ) solution with M NaOH(aq). a) What is the ph of the solution when ml of NaOH(aq) has been added? (3) ph 6 4 equivalence point = ml volume added NaOH (ml) b) Explain briefly why the ph at equivalence is not that of neutral water. (Preferably, your explanation will be in words, although I suppose a numerical calculation of the ph would suffice.) (2) c) Qualitatively sketch on the graph the corresponding titration curve of an equimolar weak acid with K a of Indicate clearly whether the ph values at 0, 35, and 60 ml added NaOH are greater, lesser, or equal to those on the benzoic acid curve. (3) Question Points Score TOTAL 50