AqChemPS1SolutionSet

- 254 Aquatic Chemistry Problem Set#1 Solution Set 2.3 3.4 3.6 3.7 3.8 Problem 2.3 In concentrated ferric iron solutions numerous species may

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Unformatted text preview: 254 Aquatic Chemistry Problem Set #1 Solution Set 2.3, 3.4, 3.6, 3.7, 3.8 Problem 2.3 ' In concentrated ferric iron solutions, numerous species may appear as intermediates before iron precipitates as Fc(OH)3(s). What is the total ferric iron concentration in mgfL as Fe if the concentrations of Fe“, Fe(OH)2*, Fe(0H);, and Fez(OH)2‘*are, respectively, 1.0x] 0‘3 M. 8.9x 10"1 M, 4.9x10’T M, and 1.2x10'3 M. Purpose of problem Units conversion and stoichiometry Relevant section(s) of text Sections 2.4.2 and 2.6 and Table 2.2 Solution The total concentration as Fe is: {Fe} (mol/L) = (1 eq Fe/mol)[Fe3‘} + (1 eq Fe/mol)[Fe(OI-I)2+] + (1 eq Fclmol)[Fc(OH)f] - + (2 eq Fe/mol)[Ee3(OH)2“’] = (1 eq Felrnol)(1.0><10"3 mol/L) + (1 eq Fe/mol)(8.9><10"‘ moi/L) + (1 eq Fe/mol)(4.9><10‘7 moi/L) + (2 eq Fe/tnoi)(l.2)<10‘3 molfL) =4.3><10'3 Incl/L / ' [Fe] ( rug/L) = [Fe in tnoI/LKMW of Fe in g/moi)( 1000 mg/g} = (-’-1.3><10‘3 moi/L)(55.85 g/mol)(1000 mg/g) = 240 mg/L The total iron concentra tion is 240 nag/L as Fe Problem 3.4 _ Consider the reaction: HSO4“ = 5042' + H". Is the reaction endothermic or exothermic if all concentrations are 1 M? Does the reaction proceed spontaneously as written if all concentrations are 1 M? For H8041 H"; = — 887.3 kamolfif = 132 J/molJ’K, and G; = —756.0 lei/mot. For 804213; 2 —909.2 kJ/mol,§; = 20.1 J/mol—“K, and G} = —744.6 kI/rnol. For H“: H} = kJ/molfi, : 0 J/mol»°K, and G“! I O kJ/mol. Purpose of problem Use of thermodynamic equations Relevant section(s) of text Section 3.7 Solution Since alt concentrations are l M, you can use thermodynamic properties evaluated at standard state. To determine if the reaction is endothermic or exothermic, calculate AH"m assuming all concentrations are 1M: Aanm=§a 1- +§FH+“£t—£a flu-3'04 49,336: = _909-2 + 0 _ = ‘21.9 Heat is released (AQp= AH?” < 0) and therefore the reaction is eitothermic if all concentrations are l M. To determine if the reaction proceeds spontaneously, calculate A011,, assuming all concentrations are 1M: AG; =35“. + iii”. — aims —744.6 + o m (— 756.0) = +1 1.4'ldfmol A62” > 0 and therefore the reaction does not proceed spontaneously as written (assuming that all concentrations are l Problem 3.6 Calculate K for the reaction in Problem 3.4. (At equilibrium. you cannot assume all species concentrations are 1 M.) At what pH are the equilibrium activities of H804“ and 8042' equal? Purpose of problem I Test understanding of equilibrium and use of thennodynamic equations Relevant section{s) of text Section 3.9 ' Solution From eq. 3.18: K : exp(—AG"M/RT) From Problem 3.4: AG?“ a +1 1.4 kJ/mol Thus, at 25“C ("4 298°K): K : exp[—(11.4 klinlol)/(8.3l4><10"3 kJ/mole—“K)(298°K)] m 1.9X10'2 Also: K = {3042' } {If}! {HSOA’} Thus, {8042'} = {F130;} when {IF} 2K1“ 1.(l><10‘2 or pH = —log{H"} = ~log(1.0><10‘2) : 2.0 Problem 3.7 Calculate K for the reaction in Problem 3.5. (At equilibrium, you cannot assume aii species concenn‘ations are 0.01 M.) At What range of pH is {NH‘C} > {NFL} at equilibrium? Purpose of problem Test understanding of equilibrium and use of thermodynamic equations Relevant seetiou{s) of text Section 3.9 Solution From eq. 3.18: K = eprAG‘Q/RT) From Problem 3.5: A61," = +52% kJ/mol Thus, at 25“C (= 298°K): K = cum—(52.8 lernol)f(8.3 14X 10‘3‘ kJ/mol.e-°K)(298“K)} = 5.6X10‘ "' Also: .K = {N143} {11"}! {NI-14*} Thus, {NRC} > {NHB} when {H} > K= 5.6><10“” This occurs when or pH = ~iog{H*} < —log(5.6x 10' '0) or pH < 9.3 (Note: {I—I‘} > K =» log {H*} > iogK a ~log {F} < —iogK, since the inequality sign is reversed when you multiply both sides of an inequality by a negative number) Problem 3.8 What is the criterion for equilibrium in terms of G? AGm? A0,“? Purpose of problem Test understanding of equilibrium Relevant section(s) of text Sections 3.8 and 3.9 Solution From the text: 6' is minimized at equilibrium AGm = 0 at equilibrium (since G is minimized) A61“ = -RTanat equilibrium ...
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This note was uploaded on 02/02/2012 for the course CEE 254 taught by Professor Jenniferjay during the Fall '11 term at UCLA.

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- 254 Aquatic Chemistry Problem Set#1 Solution Set 2.3 3.4 3.6 3.7 3.8 Problem 2.3 In concentrated ferric iron solutions numerous species may

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