Chem 128, Exam III April 23, 2004

Transcription

1 I. (35 points) A. (10 points) Consider an aqueous solution of PbI 2 with solid lead(ii) iodide present. K sp =8.4x Write a balanced net ionic equation for the equilibrium established between the solid and its corresponding ions in solution. 2. What is the K sp expression? 3. What is the concentration of I at equilibrium in a saturated solution of PbI 2? B. (10 points) Aluminum hydroxide solid (K sp =2x10 31 ) reacts with an excess of hydroxide ions to form the complex ion Al(OH) 4 (K f =1x10 33 ). 1. Write a balanced net ionic equation for the reaction. (Do NOT forget to write the states of the species.) 2. What is the numerical value of K for the reaction? 3. Determine the molar solubility (mol/l) of aluminum hydroxide when [OH ] is 1.0 x10 3 M. 1

2 C. (15 points) You need to show your work for parts 1, 2, and 3. You do not need to do so for parts 4 and 5. A solution is made up by mixing 50.0 ml of M AgNO 3 with 50.0 ml of M HCl. A precipitate of AgCl (K sp =1.8x10 10 ) is obtained. 1. How many moles of precipitated AgCl are obtained after equilibrium is established? 2. What is the concentration (in moles/l) of Ag + when equilibrium is established? 3. What is the concentration (in moles/l) of Cl when equilibrium is established? 4. To the solution at equilibrium described above, NaCl is added. [Ag + ] increases, decreases remains the same Circle the correct answer. 5. In a separate experiment, AuNO 3 is added to the solution at equilibrium described above. K sp of AuCl is 2.0 x [Ag + ] increases, decreases remains the same Circle the correct answer. 2

3 II. (45 points) A. (8 points) A solid and liquid are mixed at 25 C and 1 atm. Some observations are: The solid disappears completely. The beaker feels warmer to the touch after the compounds are mixed. A gas bubbles out. One can state with a high degree of accuracy that (Fill in the blanks with <, >,=,ormi.) 1. H 0 2. S 0 3. G 0 4. K 1 B. (10 points) Sodium carbonate can be obtained from sodium hydrogen carbonate: 2 NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O(l) H = kj S = 339 J/K 1. Calculate G at 85 C. 2. What is the molar entropy for NaHCO 3 given the following molar entropies? Na 2 CO 3 = J/mol-K CO 2 = J/mol-K H 2 O(l) = 69.9 J/mol-K 3. At what temperature is the reaction at equilibrium? 4. Does the reaction become spontaneous above or below that temperature? 3

4 C. (14 points) Answer the following questions by writing Y if the statement about the diagram is true and N if the statement is false. 1. Spontaneous reactions are always fast reactions. 2. S is positive when dry ice sublimes. 3. G is not temperature dependent. 4.IfQ=1, G = G 5. One can figure out the K sp of AgCl using only tables of H f and S o. 6. At the normal melting point, G =0. 7. In all reactions where S > 0, G < 0 D. (5 points) Given the following data for bromine at 25 C: Br 2 (l): S o = J/mol-K Br 2 (g): S o = J/mol-K H f = kj/mol Estimate the normal boiling point for bromine. (Bromine is a liquid in its standard state.) E. (8 points) Acetic acid, HC 2 H 3 O 2,hasaK a of 1.8 x What is G for acetic acid at 25 C? 2. Show by calculation whether the ionization of acetic acid at 25 C where the ph is 4 and [HC 2 H 3 O 2 ] = M is spontaneous. (Hint: [H + ]=[C 2 H 3 O 2 ]) 4

5 III. ( 45 points) A. (16 points) Use the following data to answer the questions below: Cu 2+ (aq) + 2 e Cu (s) E red = V Cu 2+ (aq) + e Cu + (aq) E red = V Cu + (aq) + e Cu (s) E red = V Fe 2+ (aq) + 2 e Fe (s) E red = V Zn 2+ (aq) + 2 e Zn (s) E red = V Ni 2+ (aq) + 2 e Ni (s) E red = V Fe 3+ (aq) + e Fe 2+ (aq) E red = V For questions 1 6, consider the following species: Zn, Ni 2+,Fe 2+,Cu + 1. Using the table above, which are reducing agents? (You have more blanks than needed.) 2. Which of the reducing agents is the strongest? 3. Which of the reducing agents is the weakest? 4. Using the table above, which are oxidizing agents? (You have more blanks than needed.) 5. Which of the oxidizing agents is the strongest? 6. Which of the oxidizing agents is the weakest? 7. What is E for the cell Fe Fe 2+ Cu 2+ Cu 8. Is the cell described in (7) a voltaic cell? 5

6 B. (12 points) Consider a voltaic cell in which the following reaction takes place 2NO 3 (aq) + 3 H 2 (g) 2NO (g) + 2 OH (aq) + 2 H 2 O E = V 1. What is n? 2. What is G? 3. What is Q (reaction quotient) under the following conditions: [NO 3 ] = M, P NO = atm, P H2 = atm, ph = What is E under the conditions of (4)? 6

7 C. (8 points) Consider a cell with the following reaction and standard reduction electrode potentials: Pb (s) + 2 H + (aq) Pb 2+ (aq) + H 2 (g) Pb 2+ (aq) + 2 e Pb (s) E red = V 2H + (aq) + 2e H 2 (g) E red = V 1. Calculate E for this cell. 2. Chloride ions are added to the Pb Pb 2+ half cell to precipitate PbCl 2. The voltage is measured to be V. If [H + ] = 1.0 M and P H2 = 1.0 atm, what is [Pb 2+ ]? 3. Taking [Cl ] in (2) to be 0.10 M, calculate K sp for PbCl 2 D. (9 points) A solution containing a 3+ metal ion (M 3+ (aq)) is electrolyzed by a current of 5.0 A for 10.0 min. A metal (M (s)) is obtained. 1. How many coulombs are supplied by the battery? 2. How many moles of electrons pass through the cell in 10 minutes? 3. What is the metal if 1.19 g of metal was plated out in this electrolysis. 7

8 IV. (25 points) A. (4 points) Answer the following questions on the blanks provided. 1. What is the precipitating agent for group I ions? 2. At what ph do Group II ions precipitate as sulfides? B. (11 points) Write a balanced net ionic equation using smallest whole number coefficients for the following reactions. Do not forget to write the physical states of all the species. A point will be subtracted for each extra specie. 1. The reaction of the silver ammonia complex with a chloride ion and a strong acid. 2. The reaction between iron(iii) ions and thiocyanate ions. 3. The precipitation of antimony (III) ions as a sulfide. C. (10 points) When A + ions react with B 2 ions a precipitate forms. 2A + (aq) + B 2 (aq) A 2 B (s) In an experiment, 10.0 ml of M ACl are mixed with 10.0 ml of M Na 2 B. A green precipitate forms. When equilibrium is established, it is determined (using a spectrophotometer) that [B 2 ] is M. 1. Fill in the table below. Note that all answers must be expressed in MOLARITY. Do not write x s on the table. Only numerical values will be graded. A + (aq) B 2 (aq) [ ] o [ ] eq Using the values from the table, calculate K sp for A 2 B 8

9 BONUS (15 points) All or nothing. The bonus should be done only after you have completed the main part of this exam and checked your work for errors. The time alloted for this exam does not include time for the bonus. Trial and error solutions will not be accepted. When CaF 2 is dissolved in an HCHO 2 /CHO 2 buffer, the following reaction takes place: CaF 2 (s)+2h + (aq) Ca 2+ (aq) + 2 HF (aq) What is the molar solubility of CaF 2 in a buffer solution containing 0.30 M HCHO 2 and 0.20 M NaCHO 2? The following information may be useful in solving the problem: K sp for CaF 2 =1.5x10 10 K a forhf=6.9x10 4 K a for HCHO 2 =1.9x10 4 9