Chem 1001S Final Exam NAME. August 9, 2011, 2 hrs

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1 Chem 1001S Final Exam NAME August 9, 2011, 2 hrs St # Final answers must be entered in the space provided. The equations used and work must be shown in the space below for credit. Use the reverse for scrap which will not be marked. Part marks are awarded for clearly presented work in cases of math or logical errors. Questions are designed to minimize the time spent on algebra. The space given is indicative of the amount of work required. R = J K -1 mol -1 or L atm K -1 mol -1 at 298 K RT ln (X) = log (X) No = x mol -1 F = 96,485 C/mol at 298 K RT/F ln (X) = log (X) or ln (X) Coulomb X Volt = Joule amp = Coulomb per second electron charge e = X C No X e = F ph, Equilibria kinetics, thermo and E chem. 100 TOTAL

2 1.[25] Aqueous Equilibria. Enter your final answer in the space. Show work below. A) [3] Obtain the ph of a M solution of hypochlorous acid (HOCl). Ka = 3.0 x X 2 / = Ka B) [2] How many ml of 0.10 M NaOH would be required to titrate 20 ml of a 0.03 M hypochlorous acid, HOCl, solution. Ka = 3.0 x ml _ 20 X 0.03 = 0.10 X C) [5] What is the ph at the equivalence point in the above titration? ANS ph = 9.94 OCl - + H 2 O = HOCl + OH - Kb = Kw / ka = 3.3 x 10-7 Ocl - ] = 20 x.03 / 26 = M X 2 / = Kb OH- = 8.7 x 10-5 D) [5] The solubility of PbI 2 (MW = 461) in water is given as grams per 100 ml of water. Obtain the Ksp for lead iodide. Ksp = 1 X 10-8 sol = X 10 / 461 = 1.36 x 10-3 PbI 2 = Pb I - Ksp = 4x 3 = 1.0 X 10-8

3 E) [5] Obtain the equilibrium constant for AgBr (s) + 2 NH 3 Ag(NH 3 ) Br - from the Ksp of AgBr = and the formation constant for Ag(NH 3 ) + 2 = 1.6 X Use this result to obtain the solubility of AgBr in 2 M aqueous NH 3. Knet = Ksp X Kf X 2 /4 = Knet K = 1.6 x 10-6 molar solubility = 2.5 x 10-3 F) [5] Obtain the concentration of [Ag + ] in the solution in E. Then determine if AgCl would precipitate from the solution in E if 0.01 moles of NaCl is added to a litre of the solution in E. (Note that E is 2 M in NH 3 and saturated in AgBr. ).Show a calculation to support your claim. Ksp for AgCl is 1.8 x [Ag + ] = 3.9 x [Ag + ] = 2.5 x 10-3 / 4 x Kf = 3.9 x NO PPT as Q < Ksp Q = 0.01 X Ag+ = 3.9 x ) [10] a) Circle the weakest acid HClO 4 HClO 3 HClO 2 HClO HCl b) Circle the salt which gives the most basic solution. NaCl KNO 3 K 2 CO 3 NH 4 ClO 4 c) Identify the ph at the equivalence point as acidic A, basic B, or neutral N in the following titrations HClO 4 with KOH N HClO 4 with NH 3 A NaOH with CH 3 COOH B d) [5] Give balanced ionic chemical reactions which explain why: i) Aluminum hydroxide, Al(OH) 3 (s) dissolves in dilute aqueous HCl. Al(OH) H+ = Al H 2 O ii) When CaO (s) is added to water a strongly basic solution results. CaO + H 2 O = Ca OH -

4 3.) [15] Putrescine (abbrev Put) is a foul smelling substance formed in rotting flesh. The distribution of the three forms PutH 2 +2, PutH +, and Put species is plotted below. Only the neutral form stinks. [2] Write out the equilibria corresponding to Ka 1 and Ka 2 for PutH 2 2+ PutH 2 2+ = PutH + + H + PutH + = Put + H + [6] Label the 3 curves [3] in the graph below and estimate the values of pka and pka from the graph. [3] [2] Obtain the equilibrium constant for Put + H 2 O PutH + + OH - Kb = 1.58 X [2] kb = Kw/ Ka2 [5] If 10 ml of 0.1 M PutH 2+ 2 is reacted with with 10 ml of 0.1 M Put. Obtain the following for this solution ph 3.5, [ Put] M, [ PutH + ] _0.074 M _, and [ H 2 Put 2+ ] _0.013 M at equilibrium. These may be obtained using the fractions estimated from the graph. No complex calculations are needed. Do not enter fractions above, enter the molar concentrations! 0.13 X 0.1 M and 0.74 X 0.1 M

5 4. [15] Complete the rate laws (a) and give the integrated form (b) for each of the following in the space provided. In each case an initial concentration [A o ] decays with constant k. i) rate is zero order in [A] a) RATE LAW b) INTEGRATED FORM ( [A] = etc ) rate = -d[a]/dt = k [A] = [Ao] - kt ii) rate is first order in [A] rate = -d[a]/dt = k [A] A = Ao e -kt iii) rate is second order in [A] rate = -d[a]/dt = k [A] 2 1/A 1/Ao = k t iv) Identify the kinetic order in [A] when : a plot of ln A vs. time is linear a plot of [A] vs. time is linear first zero a plot of 1/[A] vs. time is linear 2nd v) Write the Arrhenius equation for the temperature dependence of the rate constant. k = A e -Eact/RT vi) SHORT ANSWER The rate law for the elementary reaction 2A + B C is rate = _k [A] 2 [B] but the reaction becomes pseudo-first order with rate law : rate = k [B] when (Circle ONE) A= B A >> B B >>A The state of maximum energy along the minimum energy path from reactant to products is called the: transition state. The difference in Ea for the forward and reverse reactions is equal to H o The ratio of k forward / k reverse is equal to Keq At equilibrium the ratio of the forward rate / reverse rate would be equal to 1.0.

6 5) {12} A [6] Kinetic data for the reaction OCl - + I - OI - + Cl - at alkaline ph s is given. Expt OCl - M I - M OH - M initial rate M/s x x x x x 10-4 i) Assuming a rate law of the form Rate = k [OCl - ] X [I - ] Y [OH - ] Z obtain the numerical values of X 1 Y 1 Z -1 and the rate constant k = 60 M -1 s -1

7 5B) [6] The following mechanism is proposed for the reaction in question 5. OCl - + H + HOCl fast preequilibrium 1/Ka in this direction HOCl + I - HOI + Cl - k 2 slow HOI OI - + H + fast i) Show that this mechanism is consistent with the stoichiometry of the overall reaction in question 3. Adding the rxns gives OCl - + H + +HOCl + I - + HOI = HOCl + Cl - + H + +OI - + HOI etc ii) Derive the rate law for this mechanism. rate = k 2 [HOCl] [ I] = k 2 [OCl] [I] [H + ] / Ka iii) Using the fact that in water, Kw = [H + ] [OH - ], show that the mechanism above is consistent with the rate law you obtained in part A). rate = k 2 Kw/Ka [OCl - ] [I] /[OH]

8 6. [15] Use the standard reduction potential data in acid to answer the questions below. The reduction potential is written above each redox couple in the Latimer diagram V 0.00 V -0.76V +0.77V -0.44V O 2 H 2 O H + H 2 (g) Zn 2+ Zn (s) Fe 3+ Fe 2+ Fe (s) V +1.33V -0.44V -0.90V V V +1.53V V Ag + Ag Cr 2 O 7 2- Cr 3+ Cr 2+ Cr (s) Cu 2+ Cu + Cu(s) Co 3+ Co 2+ Co (s) a) Which metal ion will disproportionate CIRCLE ONE Ag + Fe 2+ Cu + Cr 2+ Co 2+ b) Which species shown is the strongest oxidizing agent _Co 3+ c) Will solutions of Fe 2+ be stable in air (O 2 )? YES or NO d) One of the +2 charged ions slowly evolves H 2 (g) in 1M H +. Which one? Cr 2+ d) Of the solid metals shown which is the strongest reducing agent. _ Cr e) Is there any reagent shown above that could oxidize Co 2+ to Co 3+? YES or NO If so Which. f) Obtain the reduction potential for Cu e - Cu (s) 0.34 V g) Write a complete balanced reaction in acid for the result of adding dichromate ion to a solution of Fe H + + Cr 2 O Fe 2+ = 2 Cr Fe H 2 O

9 8.[10] QUICKIES 1 mk each ANSWER TRUE OR FALSE (no penalty for guessing) a) TRUE The standard enthalpy of formation of Cl 2 (gas) at 298 K is b) FALSE Adding a catalyst speeds up a reaction by lowering the free energy change, G, for the reaction. c) FALSE For a general reaction 2A + 3B P, the overall kinetic order is 5. d) TRUE Diamonds have a higher free energy than graphite and are expected to spontaneously convert to graphite on the basis of thermodynamics alone. e) TRUE During the use (discharge) of a Ni/Cd battery <Cd\Cd(OH) 2 \\Ni(OH) 2 \NiO 2 > electrons flow from the cadmium through your calculator to the NiO 2. f) TRUE when ice melts at 0 o C, G = 0. g) TRUE when recharging a dead lead acid battery <Pb/PbSO 4, H 2 SO 4 // PbSO 4 / PbO 2 >, the PbSO 4 is both oxidized and reduced. h) TRUE In the evaporation of liquid CCl 4 both H o and S o are positive. i) FALSE as the reactants are consumed in a reaction, the rate constant decreases over time. j) FALSE The third law requires that O 2 gas at 298 K have zero entropy.