Chemistry 65.100 A and V

Final Exam

April 14, 1997

Calculators only permitted

Part A. Answer each question with a few sentences or equations where required (5 marks each)

1. Define all terms in the equation DG^o = -nFE^o

2. The rate of evaporation of liquid hexane (C₆H₁₄) is much greater than that of liquid water at the same temperature. Why?

3. What is the fundamental reason for colligative properties such as freezing point depression, boiling point elevation and osmotic pressure?

4. Given that the value of K_a for acetic acid is 1.8 x 10^-5 and that for hypochlorous acid is 3 x 10^-8, which is the stronger base: OCl^-_(aq) or CH₃COO^-_(aq)? Justify your answer.

5. What is a unit cell?

6. Why is the rate of a reaction affected by the temperature (two reasons)?

7. In a nuclear reaction involving a decrease in mass, large amounts of energy are released. Why can we not measure a decrease in mass in an exothermic chemical reaction?

8. Why do alkanes not exist as cis-trans isomers?

Part B. Answer each question (15 marks each)

1. One of the few electrochemical processes that produces an organic compound is used by the Monsanto Company to produce 1,4-dicyanobutane according to the reduction reaction:

2 CH₂CHCN + 2 H⁺ + 2 e^- à NC-(CH₂)₄-CN

What current (in Amperes) must be used to produce 150 kg of the product per hour?

2. The enthalpy of vaporization of acetone is 32.0 kJ/mol. The normal boiling point of acetone is 56.5^oC. What is the vapor pressure of acetone (in mm Hg) at 25.0^oC?

3. Calculate the freezing point depression and the osmotic pressure at 25^oC of an aqueous solution of 1.0 g/L of a protein (molar mass = 9.0 x 10⁴ g/mol) if the density of the solution is 1.00 g/cm³. K_f for water = 1.86^oC kg mol^-1. Which of these two quantities could be practically used to measure the molar mass of a protein? Why?

4. (a) Calculate the pH of a 1 L aqueous solution of 0.50 M NH₄Cl + 0.50 M NH₃. K_a for NH₃ = 1.8 x 10^-5.

(b) Calculate the pH of the solution after addition of 0.020 mol of HCl.

5. Lead crystallizes in a face centred cubic unit cell with an edge length of 495 pm.

(a) What is the radius of a Pb atom in pm?

(b) What is the density of Pb in g/cm³?

6. The reaction 2 NO_(g) + Cl_2(g) à 2 NOCl_(g) was studied at -10^oC and the following results were obtained ("Rate" = -d[Cl₂]/dt):

(a) Write the differential rate law.

(b) What is the value of the rate constant? (Don’t forget the units).

(c) Does the reaction occur as written, or is the mechanism more complex? How do you know?

7. Radioactive ³²P is commonly used in biochemical research. Its half life is 14.3 days.

(a) ³²P decays via b^- decay. What nuclide is produced?

(b) Calculate the decay constant of ³²P.

(c) What mass of ³²P is left from an original sample of 175.0 mg after 35.0 days?

(d) What is the activity of 175.0 mg ³²P in mCi? (1 mCi = 3.7 x 10⁷ disintegrations/s)

8.	(a) Give correct names for the following compounds:

(i)  		(ii)  	(iii)  	(iv) CH3CH2CH2CCl3

(b) Predict the products (draw or give correct names) of the following reactions:

(i) CH₂CH₂ + Br₂ à

(ii) benzene + iodine à

(iii) phenylamine + HCl_(aq) à

(iv) acetic acid + 1-propanol à

(v) acetic acid + diethylamine à

 

Some useful constants

R = 0.082 L atm K^-1 mol^-1

R = 8.314 J K^-1 mol^-1

F=96500 C mol^-1

N_AV = 6.022 x 10²³ mol^-1