DO NOT OPEN UNTIL INSTRUCTED TO DO SO. CHEM 111 Dr. McCorkle Exam #3A KEY. While you wait, please complete the following information:

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1 DO NOT OPEN UNTIL INSTRUCTED TO DO SO CHEM 111 Dr. McCorkle Exam #3A KEY While you wait, please complete the following information: Name: Student ID: Turn off cellphones and stow them away. No headphones, mp3 players, hats, sunglasses, food, drinks, restroom breaks, graphing calculators, programmable calculators, or sharing calculators. Grade corrections for incorrectly marked or incompletely erased answers will not be made.

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3 Multiple Choice Choose the answer that best completes the question. Use an 815-E Scantron to record your response. [2 points each] 1. Which of the following combinations is the best choice for creating a buffer solution with a ph of 3.50? A) HNO2/KNO2 B) HCl/NaCl C) NH3/NH4F D) HCHO2/NaC2H3O2 E) HClO2/NaClO2 2. If S is positive and H is positive, when is the reaction spontaneous? A) high temperatures B) low temperatures C) all temperatures D) never 3. Which of the following is more soluble in acidic solution than in pure water? A) AgCl B) MgCO3 C) CaBr2 D) Ba(NO3)2 E) NaI 4. If the pka of HCHO2 is 3.74 and the ph of an HCHO2/NaCHO2 solution is 3.11, which of the following is true? A) [HCHO2] < [NaCHO2] B) [HCHO2] = [NaCHO2] C) [HCHO2] << [NaCHO2] D) [HCHO2] > [NaCHO2] E) It is not possible to make a buffer of this ph from HCHO2 and NaCHO2. 5. The plot at right illustrates which type of titration? A) a weak acid titrated with a weak base B) a weak acid titrated with a strong base C) a strong base titrated with a weak acid D) a weak base titrated with a strong acid E) a weak base titrated with a weak acid 6. Without doing any calculations, which of the following processes would you expect to be spontaneous? A) 2 KCl(s) + 3 O2(g) 2 KClO3(s) B) 2 H2S(g) + 3 O2(g) 2 H2O(g) + 2 SO2(g) C) HCl(g) + NH3(g) NH4Cl(g) D) NaCl(s) Na(s) + ½ Cl2(g) E) N2(g) + 3 H2(g) 2 NH3(g) 3

4 7. The Law of Thermodynamics states that for any spontaneous process, the entropy of the universe increases. A) Zero B) First C) Second D) Third E) Fourth 8. A process is always spontaneous under which conditions? A) positive ΔS and positive ΔH B) negative ΔS and positive ΔH C) positive ΔS and negative ΔH D) negative ΔS and negative ΔH E) no process is always spontaneous 9. Place the following in increasing order of molar entropy at 298 K: NO, CO, SO A) NO < CO < SO B) SO < CO < NO C) SO < NO < CO D) CO < SO < NO E) CO < NO < SO 10. A reaction that is spontaneous as written. A) has an equilibrium position that lies far to the left B) is also spontaneous in the reverse direction C) will proceed without outside intervention D) is very rapid E) is very slow 4

5 Calculations Write your initials in the upper-right corner of every page that contains work. For full credit show all work and write neatly; give answers with correct significant figures and units. Place a box around your final answer. 11. Use the Henderson Hasselbalch equation to calculate the ph of a solution that is 10.0 g of HC2H3O2 and 12.0 g of NaC2H3O2 in ml of solution. (Ka = ) [4 points] [HC2H3O2] = g 1 mol HC 2H 3 O g [NaC2H3O2] = g 1 mol HC 2H 3 O g ph = pka + log [base] [acid] ph = log ph = M 1.11 M = L = M = L = M 12. What mass of sodium benzoate (NaC7H5O2) should be added to ml of a 0.16 M benzoic acid (HC7H5O2) solution in order to obtain a buffer with a ph of 4.25? (Ka = ) [5] ph = pka + log [base] [acid] 4.25 = log [base] [acid] 0.06 = log [base] [acid] [base] [acid] = = [base] = 1.15 [acid] = 1.15 (0.16) = M [base] = L mol L g 1 mol = 4. 8 g 5

6 13. Calculate the molar solubility of calcium hydroxide in a solution buffered at ph = (Ksp = ) [5] ph + poh = 14 poh = = 5.00 [OH ] = = M Ca(OH)2(s) Ca 2+ (aq) + 2 OH (aq) I M C +S +2S E S S Ksp = [Ca 2+ ][OH ] = (S)( ) 2 S = M 14. Will a precipitate of MgF2 form when 300. ml of M MgCl2 solution are added to 500. ml of M NaF? (MgF2, Ksp = ) [5] MgF2(s) Mg 2+ (aq) + 2 F (aq) [Mg 2+ ]initial = M M2 = M 1V 1 V 2 = ( )(300. ml) (800. ml) [F ]initial = M = M M2 = M 1V 1 V 2 = ( )(500. ml) (800. ml) = M Q = [Mg 2+ ][F ] 2 = ( )( ) 2 = Q < Ksp so a precipitate will NOT form 6

7 15. A g sample of an unknown monoprotic acid was titrated with M KOH. The equivalence point was determined to be 30.5 ml. What is the molar mass of the unknown acid? [3] L mol KOH L = mol KOH At equivalence point, moles acid = moles base Molar mass = g = g/mol mol ml of M Zn(NO3)2 is mixed with ml of M NH3. After the solution reaches equilibrium, what concentration of Zn 2+ (aq) remains? ([Zn(NH3)4] 2+, Kf = ) [7] Zn 2+ (aq) + 4 NH3(aq) [Zn(NH3)4] 2+ (aq) [Zn 2+ ] = ( L M) ( L L) [NH3] = = M ( L M) ( L L) = M Zn 2+ (aq) + 4 NH3(aq) [Zn(NH3)4] 2+ (aq) I Kf is large and [NH3] >> [Zn 2+ ] so assume most Zn 2+ is consumed. Let x represent the amount of Zn 2+ left. C ( ) 4( ) ( ) E x K f = [Zn(NH 3) 4 2+ ] [Zn 2+ ][NH 3 ] 4 = x = (x)( ) (K f )( ) 4 = ( )( ) 4 = M 7

8 17. Calculate the ph after mol HCl is added to ml of a buffer solution that is 0.10 M ethylamine and 0.15 M ethylammonium nitrate? (ethylamine, Kb = ) [7] CH3CH2NH2 + H3O + CH3CH2NH3 + + H2O CH3CH2NH2 = L 0.10 M = mol CH3CH2NH3 + = L 0.15 M = mol CH3CH2NH2 H3O + CH3CH2NH3 + Before mol mol mol Change mol mol mol After mol mol Ka = Kw / Kb = / = pka = ph = pka + log [base] [acid] ph = log ph = Consider the reaction: 2 Hg(g) + O2(g) 2 HgO(s) ΔG = kj Calculate ΔGrxn at 25 C under these conditions: PHg = atm, PO2 = atm [5] Q = 1 2 = P O2 P Hg 1 (0. 025) 2 = (0. 037) G = G RT lnq G = ( kj 103 J J ) + ( kj ΔG = J = kj mol K 298 K) ln( ) 8

9 19. Consider the titration of ml of M acetic acid (HC2H3O2, Ka = ) with M NaOH. Calculate the ph of the resulting solution after the following volumes of NaOH have been added. [15] a) ml HC2H3O2(aq) + OH (aq) C2H3O2 (aq) + H2O(l) HC2H3O2 OH C2H3O2 Before mol mol 0 mol Change mol mol mol After mol 0 mol mol ph = pka + log [base] [acid] ph = log ( ) ( ) ph = 5.04 b) ml mol HC2H3O2 & mol C2H3O2 so at equiv. point HC2H3O2 OH C2H3O2 Before mol mol 0 mol Change mol mol mol After 0 mol 0 mol mol Only weak base left, Kb = [C2H3O2 ] = mol ( L L) = M C2H3O2 (aq) + H2O(l) HC2H3O2(aq) + OH (aq) = [HC 2H 3 O 2 ][OH ] x 2 [C 2 H 3 O = 2 ] x = = [OH ] poh = log( ) = 5.26 ph = 8.74 c) ml Past equiv. point so strong base dominates HC2H3O2 OH C2H3O2 Before mol mol 0 mol Change mol mol mol After 0 mol mol mol [OH ] = 3 mol = mol = M ( L L) L poh = log[oh ] = log(0.016) = 1.80 ph = =

10 20. Using the data provided, calculate ΔH, ΔS and ΔG at 298K for the following reaction. Also, show that ΔG = ΔH TΔS. [8] 3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g) Substance ΔH f (kj/mol) ΔG f (kj/mol) S (J/mol K) H2O(l) HNO3(aq) NO(g) NO2(g) H rxn = n( H prod) n( H reac) H rxn = ( ) ( ) = kj S rxn = n(s prod) n(s reac) S rxn = ( ) ( ) = J/K G rxn = n( G prod) n( G reac) G rxn = ( ) ( ) = 51.0 kj ΔG = ΔH TΔS ΔG = kj (298 K J K 1 kj ) = kj 10 3 J Same via both methods (when considering significant digits) 10

11 21. Challenge Question: Consider the following reaction: 2 SO2(g) + O2(g) 2 SO3(g) Using the information below, solve for the ΔH f of SO3. [10 points] S(s, rhombic) + O2(g) SO2(g) ΔG rxn = kj S(s) + 3/2 O2(g) SO3(g) ΔG rxn = kj Substance ΔH f (kj/mol) S (J/mol K) O2(g) S(s, rhombic) SO2(g) SO3(g)??? SO2(s) 2 S(s, rhombic) + 2 O2(g) ΔG rxn = 2 ( kj) = kj 2 S(s) + 3 O2(g) 2 SO3(g) ΔG rxn = 2 ( kj) = kj 2 SO2(g) + O2(g) 2 SO3(g) ΔG rxn = kj kj ΔG rxn = kj S rxn = n(s prod) n(s reac) S rxn = ( ) ( ) = J/K ΔG = ΔH TΔS ΔH = ΔG + TΔS ΔH = kj + (298 K J K H rxn = n( H prod) n( H reac) H rxn = [2 ΔH (SO3)] [2 ΔH (SO2) + ΔH (O2)] kj = [2 ΔH (SO3)] [ ] 2 ΔH (SO3) = kj ΔH (SO3) = kj 1 kj ) = kj 10 3 J Extra Credit: Consider two flasks that are joined together, one evacuated and one containing 3 molecules of a gas. When the flasks are allowed to mix, how many microstates are possible? [2 points] 8 possible microstates 11

12 Scratch Page (to be handed in) 12

13 Formulas & Constants M = mol solute liters solution P A = χ A P A o m = mol solute kg solvent R = L atm mol K χ A = mol A total moles R = J mol K ΔTf = m Kf ΔTb = m Kb Π = MRT ΔTf = i m Kf ΔTb = i m Kb Π = i MRT K = C atm = 760 torr = 760 mmhg Sgas = kh Pgas ΔHsol n = ΔHhydration ΔHlattice f = e Ea/RT k = Ae Ea/RT ln k 2 k 1 = E a R ( 1 T 1 1 T 2 ) ln k = E a R (1 T ) + ln A 1 V = 1 J/C Kp = Kc(RT) Δn Kw = Ka Kb = Kw Kw = [H3O + ][OH ] ph = pka + log [base] [acid] ph = log[h3o + ] poh = log[oh ] ΔG = ΔG + RT ln Q ΔG = ΔH TΔS S rxn = n(s prod) n(s reac) ΔG = nfe H rxn = n( H prod) n( H reac) K = e ΔG /RT G rxn = n( G prod) n( G reac) F = 96,485 J/V mol S = k ln W k = J/K 1 A = 1 C/s E cell = E cathode E anode E = E (0.0592/n) log Q E = E (RT/nF) ln Q Order in [A] Rate Law Integrated Form, y = mx + b Straight Line Plot Half-Life t1/2 zero-order (n = 0) first-order (n = 1) rate = k [A] 0 = k [A] t = kt + [A] 0 [A] t vs. t t ½ = [A] 0 2k rate = k [A] 1 ln[a] t = kt + ln[a] 0 ln[a] t vs. t t ½ = ln 2 k = k secondorder (n = 2) rate = k [A] 2 1 [A] t = kt vs. t t [A] 0 [A] ½ = t 1 k[a] 0 13

14 Various Constants at 25 C Substance Formula Formic acid HCHO2 Ka = Chlorous acid HClO2 Ka = Nitrous acid HNO2 Ka = Ammonia NH3 Kb = Ethylamine CH3CH2NH2 Kb = Magnesium fluoride MgF2 Ksp = Tetraamminezinc(II) ion [Zn(NH3)4] 2+ Kf =