COLLEGE OF SAINT MARY        FALL 2008     FOUNDATIONS OF CHEMISTRY, CHM -100     

INSTRUCTOR: DR. PETER ILICH         YOUR NAME _________KEY_____________________

EXAM # 3  -- ACID-BASE EQUILIBRIUM and ALKANES, ALKENES, ALKYNES & BENZENES

NOTE:  Show the work or your answer will NOT be graded.

1. (15 pts)  Two acids, AH and EH, dissociate when added to water.  80 % of AH dissociates into A^- and H⁺ while only 0.05 % of EH dissociates into E- and H+. Write down the acid-base dissociation reactions for these acids, write the expressions for the equilibrium constants and, from the dissociation data given above, calculate the constants.  

                                   80 %    dissocicated                                                                  0.05 %  dissocicated                                                          

SOLUTION:

AH = A^- + H⁺ 

Keq = [A^- ] [H⁺] / [AH]

Keq = (0.8) (0.8) / (0.2) = 3.2

EH = E^- + H⁺ 

Keq = [E^- ] [H⁺] / [EH]

Keq = (0.0005) (0.0005) / (0.9995) = 2.6*10^-7

2. (15 pts) Formic acid (the stingy substance injected subcutaneously by ants), HCOOH, is the simplest carboxylic acid, with equilibrium constant Ka =  2.95*10^-4.  Use the Hendersohn-Hasselbalch's equation to calculate the ratio of dissociated, HCOO^-, to undissociated, HCOOH, formic acid in the acid aqueous solution at pH = 4.0.  (Hint: HH equation:  pKa = pH - lg{[A^-] / [HA]}.)

SOLUTION:

pKa = pH - lg{[HCOO^-] / [HCOOH]}

pKa = -lg Ka = -lg 2.95*10^-4  = 3.53

3.53 = 4 - lg{[HCOO^-] / [HCOOH]}

- lg{[HCOO^-] / [HCOOH]}= 3.53 - 4 = -0.47

or  lg{[HCOO^-] / [HCOOH]}= 0.47  and  [HCOO^-] / [HCOOH] = 10^0.47 = 2.95 ≈ 3

3. (10 pts) Name the following alkanes:

4. (10 pts) In the presence of light, alkanes react with dibromine, Br₂, to give a bromoalkane.  First to be replaced by a bromine is the hydrogen on tertiary, 3°, carbon atom.   Identify the tertiary carbon atoms in the molecule 4 (a), (b), (c), (d), (e) and write the formulas of the corresponding bromoalkanes.

6. (15 pts) Name the following alkenes and alkynes, specify the (E)- (or trans-) and (Z)- (or cis-) stereochemistry where appropriate:

7. (15 pts) Indicated below are the addition reactions of alkenes and alkynes.  Complete each reaction by writing down the starting hydrocarbon, or the reagent, or both:

8. (20 pts) Complete the following benzene substitution reactions by writing down and naming the products in each reaction:

BONUS # 1: (10 pts)    According to the Brønsted-Lowry theory an acid is a molecule that can give away a proton and a conjugate base, according to the reaction scheme, AH = A^- + H⁺.  Inspect the molecules given below and determine which one is a Brønsted acid.  Write an acid-base reaction for each one.

BONUS # 2: (10 pts)   Write down the formula (line, or structural or connectivity) of (a) a branhed ALKANE with 5 carbon atoms; (b) and ALKENE with 5 carbon atoms, (c) a CYCLOALKANE with 4 carbon atoms, and (d) a CYCLOALKADIENE of your choice of size.

(a)                                               (b)                                                             (c)                                                          (d)

EXTRA BONUS (5 pts): Name each of these compounds:

(a) ___________________      (b)   ____________________     (c) _____________________       (d) _______________________