Assignment No. 6. Models of solution chemistry

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1 Chemistry 213 Summer 2010 Assignment No. 6 Name: KEY Models of solution chemistry Due Date : Wednesday, July 28, 1. Write the expression for the equilibrium constant for each of the following reactions. Write the pressure of a gaseous molecule, X, as P X. (a) Cl 2 (g) + 2OH (aq) Cl (aq) + OCl (aq) + H 2 O(l) [Cl ][OCl ] K = [ ][OH ] P O 2 2 (b) Hg(l) + I 2 (g) HgI 2 (s) 1 K = P I 2 2. rom the equations CuN 3 (s) Cu + + N 3 K = HN 3 H + + N 3 K = find the value of K for the reaction Cu + + HN 3 CuN 3 (s) + H +. All species are aqueous unless otherwise indicated. Soln: Cu + + N 3 CuN 3 (s) K 1 = 1/( ) = HN 3 H + + N 3 K 2 = Summing up : Cu + + HN 3 CuN 3 (s) + H + K = K 1 K 2 = =

2 3. Suppose that the following reaction has come to equilibrium: Br 2 (l) + I 2 (s) + 4Cl (aq) 2Br (aq) + 2ICl 2 (aq) If more I 2 (s) is added, will the concentration of ICl 2 in the aqueous phase increase, decrease, or remain unchanged? Soln: Concentrations of solids are defined in their pure states and are always unity. Therefore, they are omitted from the equilibrium equation. Iodine is added in solid form and therefore the concentrations of ICl 2 will remain unchanged. 4. ind ΔG for the reactions (a) Ca(OH) 2 (s) Ca OH K = G = RTlnK = (8.314 J K 1 mol 1 ) (298 K) ln( ) = 23.9 kj/mol (b) Mg(OH) 2 (s) Mg OH K = G = RTlnK =(8.314 J K 1 mol 1 ) (298 K) ln( ) = 63.6 kj/mol 5. or the reaction Mg 2+ + Cu(s) Mg(s) + Cu 2+, K = and ΔS = +18 J/(K.mol). (a) Under standard conditions, is ΔG positive or negative? The term "standard conditions" means that reactants and products are in their standard states. G = RTlnK = =(8.314 J K 1 mol 1 ) (298 K) ln (10 92 ) = kj/mol Therefore, G is positive. (b) Under standard conditions, is the reaction endothermic or exothermic? G = H T S H = G + T S = kj/mol + (298 K)(18 J K 1 mol 1 ) = kj/mol kj/mol = kj/mol Since H is positive the reaction is endothermic.

3 6. or the sum of two reactions, we know that K 3 = K 1 K 2. Show that this implies that ΔG 3 = Δ G 1 + Δ G 2 A + B C + D... K 1 D + E B +...K 2 A + E C +..K 3 Soln. or the first and second reactions steps, K 1 = e ( G 1 /RT) and K 2 = e ( G 2 /RT). Since the third reaction is derived by adding the first and the second reactions, the equilibrium constant for the third reaction can be expressed as the product of the individual equilibrium constants for reaction 1 and 2. Therefore, K 3 = K 1 K 2 Representing the total free energy change in the standard state for the third reaction as G 3, K 3 = e ( G 3 /RT) = e ( G 1 /RT) e ( G 2 /RT) = e (( G 1 + G ) /RT) 2 Therefore, G 3 = G 1 + G 2 7. Consider a reaction between gaseous fluorine ( 2 ) and solid graphite (C) that produces tetrafluoroethylene; 2 2 (g) + 2C (s) 2 C=C 2 (g) luorine Graphite Tetrafluoroethylene A bacteria called Teflon eats up C 2 4 and make Teflon (shown below) for their cell wall. 2 C C 2 Will the reaction shift to the left or right if these bacteria are added? Explain. Soln. The reaction will shift to the right. This will occur because, according to La Chatelier s principle, if the bacteria eats up C 2 4 (the product of the reaction), the equilibrium will be shifted in a direction such that more product is formed.

4 MULTIPLE CHOICE QUESTIONS (CHOOSE THE RIGHT ANSWER) 8. The mathematical equation which represents the solubility product when the insoluble compound Mn 2 S 3 is dissolved in water is (a) [Mn 2+ ][S 2 ] = K sp. (b) [Mn 3+ ] 3 [S 2 ] 2 = K sp. (c) [Mn 3+ ] 2 [S 2 ] 3 = K sp (d) [Mn 2+ ] 3 [S 2 ] 2 = K sp. 9. Given that the solubility product for La(IO 3 ) 3 is 1.0 x 10 11, what is the concentration of La 3+ in a saturated solution of lanthanum iodate? (a) 7.8 x 10 4 molar (b) 1.0 x 10 3 molar (c) 7.18 x 10 5 molar 10. If a M solution of NaOH is added to a solution containing M Ni 2+, M Ce 3+, and M Cu 2+, which metal hydroxide will precipitate first? K sp for Ni(OH) 2 = 6.0 x 10 16, K sp for Ce(OH) 3 = 6.0 x 10 22, and K sp for Cu(OH) 2 = 4.8 x (a) Ni(OH) 2 (b) Ce(OH) 3 (c) Cu(OH) What is the lead concentration of a saturated solution of lead(ii) sulfate containing molar Na 2 SO 4? K sp for PbSO 4 = 6.3 x (a) 7.9 x 10 4 molar (b) 5.6 x 10 3 molar (c) 3.2 x 10 5 molar 12. Consider the following equilibria: AgCl(s) Ag + + Cl K sp = AgCl(s) + Cl AgCl 2 K 2 = AgCl 2 + Cl 2 AgCl 3 K 3 = 0.49 At equilibrium, if [Ag + ] is M, then the concentrations of Cl, AgCl 2, and AgCl 2 3, respectively are (a) 0.20 M ; M; M (b) 0.2 M ; M; M (c) 2.0 M; M; M 13. The reaction of EDTA with a trivalent metal ion at a ph of 4.0 may be written as: (a) M 3+ + Y 4 MY. (b) M 3+ + H 2 Y 2 MY + 2H +. (c) M 3+ + H 4 Y MY + 4H +

5 14. Calculate the conditional formation constant K f ' for the formation of an EDTA complex with copper(ii) at a ph of 5.00, if log K f = (a) 2.3 x (b) 7.0 x 10 6 (c) 5.9 x Calculate the pco 2+ after ml of M EDTA in the titration of ml of M Co 2+ (K f = 2.04 x ) at ph = (a) 1.04 x 10 3 molar (b) (c) Calculate the pco 2+ after ml of M EDTA in the titration of ml of M Co 2+ (K f = 2.04 x ) at ph = (a) (b) (c) 1.89 x 10 7 molar 17. In the following reaction, identify the conjugate acidbase pair: NO 2 + H 2 O HNO 2 + OH. (a) NO 2, OH (b) NO 2, H 2 O (c) NO 2, HNO A solution was found to contain 8.26 x 10 4 M NaOH. Calculate the ph of the solution. (a) 3.08 (b) 10.9 (c) Calculate the hydrogen ion concentration of a M solution of methyl amine. K a for methyl amine hydrochloride = 2.3 x (a) 1.5 x 10 6 molar (b) 6.9 x 10 3 molar (c) 1.5 x molar 20. ind the concentration of phenol and phenolate ion in a M solution of phenol. K a for phenol = 1.05 x (a) 1.02 x 10 6 molar phenol and 1.02 x 10 6 molar phenolate ion (b) 1.02 x 10 6 molar phenol and 1.00 x 10 2 molar phenolate ion (c) 1.00 x 10 2 molar phenol and 1.02 x 10 6 molar phenolate ion