b. Write the formula of the precipitate formed. If there is no precipitate, write NONE.

Transcription

1 I. (41 points) A. (6 points) 1. A solution is made up of equal amounts of 0.1M lead nitrate and 0.1M potassium chromate. a. What ions are present in solution after equilibrium is established? b. Write the formula of the precipitate formed. If there is no precipitate, write NONE. 2. A solution containing 1 mole of sodium chloride and 1 mole of potassium bromide is mixed and allowed to equilibrate. Write the formula of the precipitate formed. If there is no precipitate, write NONE. B. (12 points) The solubility product constant (K sp ) of magnesium hydroxide is 8.9x If solid magnesium hydroxide remains at equilibrium, calculate 1. the concentration of magnesium ions (M) if the solvent is water. 2. the concentration of hydroxide ions (M) if the solvent is water. 3. the concentration of magnesium ions (M) if the solvent is 0.05 M NaOH. 4. the concentration of hydroxide ions (M) from the dissolved magnesium hydroxide if the solvent is 0.05 M NaOH. 1

2 C. (2 points) Calculate the solubility product constant (K sp ) of pure lead (II) sulfate in water. The solubility of lead sulfate in water at 25 Cis1.25x10 4 moles/l. D. (20 points) Ammonia can be used to dissolve AgCl by forming the complex ion Ag(NH 3 ) + 2. The formation constant for Ag(NH 3) + 2 is 1.74 x 107. The K sp for AgCl is 1.8 x Write a balanced net ionic equation for the reaction bewteen AgCl and NH Calculate the equilibrium constant for the reaction in part Calculate the concentration (M) of the chloride ion after 1.0 M NH 3 is added and equilibrium is established. 4. What is the volume of 1.0 M NH 3 required to dissolve mol of AgCl(s)? 2

3 II. (42 points) A. (4 points) Place the following reactions in order of increasing S. 1. NH 3 (g) + HCl (g) NH 4 Cl (s) 2. H 2 O (g) H 2 O(l) 3. 2 O 3 (g) 3O 2 (g) 4. H 2 (g)+f 2 (g) 2HF(g) < < < B. (8 points) Answer the following questions about a process whose standard free energy ( G is plotted vs temperature (in K) in the graph below: 1. At what temperature does the process just become spontaneous? 2. Is the process exothermic? 3. What is the sign of S? 4. What is the equilibrium constant for this process at 300 K? G (kj) Temperature (K) 3

4 C. (12 points) Red phosphorus and white phosphorus have the following thermodynamic data: Red Phosphorus: H f = kj/mol S = 22.8 J/mol K White Phosphorus: H f = 0 kj/mol S = J/mol K 1. What is S for the reaction: P red P white 2. What is H for that reaction? 3. What is G at 25 C? 4. At what temperature are the two forms of phosphorus in equilibrium if the pressure is 1 atm? 4

5 D. (18 points) Consider the following reaction carried out at 25 C. Fe(OH) 3 (s) Fe 3+ (aq) + 3 OH (aq) H f (kj/mol) S o (kj/mol K) Calculate the enthalpy change for the reaction. 2. Calculate the entropy change for the reaction. 3. Calculate the standard free energy change for the reaction. 4. Is the reaction at standard conditions spontaneous at 25 C 5. Put an X in the blanks in front of the correct statement about the reaction: The reaction is spontaneous at standard conditions at all temperatures. The reaction is nonspontaneous at standard conditions at all temperatures. The reaction is becomes spontaneous at standard conditions at a certain temperature. If you checked this statement: Write the temperature at which the reaction just becomes spontaneous. 6. What is the K sp for Fe(OH) 3? 7. At what concentration of the ions does the reaction become spontaneous at 25 C (i.e. G = -1 kj/mol)? 5

6 III. ( 42 points) A. (12 points) Circle the letter T if the statement is true and F if the statement is false. 1. A voltaic cell where E > 0 T F a) has G < 0 T F b) has K > 1. T F c)mayhavee<0. 2. A 25 C, a voltaic cell is dead (delivers zero voltage) when T F a)q=k T F b) E o = lnq n T F c)e=e B. (8 points) Given the following standard reduction potentials: Mn 2+ (aq) + 2 e Mn (s) Co 2+ (aq) + 2 e Co (s) Cu 2+ (aq) + 2 e Cu (s) MnO 2 (s) + 4H + (aq) + 2 e Mn 2+ (aq) + 2 H 2 O Cl 2 (g) + 2e 2Cl (aq) For the following questions you have more blanks than you need. Consider the following species: Mn 2+, MnO 2,Co,Cu 2+, and Cl, 1. Using the table above, which are reducing agents?,,,, 2. Using the table above, which are not reducing agents?,,,, 3. Rank the reducing agents that you listed in part (1) in order of increasing strength. < < < < 6

7 C. (22 points) A. Consider the following voltaic cell where E = Al(s) + 6H + (aq) 2Al 3+ (aq) + 3 H 2 (g) 1. Write the half reaction that takes place at the cathode. 2. What is E red for Al 3+ (aq) + 3e Al (s) 3. How many electrons are exchanged for the reaction between Al and H +? 4. Calculate Q at 25 Cif[Al 3+ ] = M, the pressure of all H 2 is atm, and [H + ] = M. 5. Calculate E for the conditions given in part 4 above. 7

8 I. (25 points) A. (6 points) Fill in the 3 blanks at the right to indicate what should be added to separate each of the groups of ions B. (10 points) Fill in the blanks at the right to indicate either the reagent to be added, the ions that separate out, or the precipitates that are formed. Do not forget to write the conditions under which the reagent should be added. (Mn and Ni are in Group III, Cu and Hg are in Group II.) C. (9 points) Write a balanced net ionic equation 1. for the formation of a precipitate for Cu after the first addition in (B) for the formation of a precipitate for Ni after the second addition in (B). 3. for the reaction between iron(iii) ions and thiocyanate 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. For a certain reaction, the equilibrium constant is 56.6 at 100 K and 3.20 x 10 3 at 200 K. Calculate H and S. H = S = 9