Chemistry Module 2 homework

CHEM 205 Worksheet (WS2)

52 Points Total

Key Topics: Acid-Base Equilibria, Keq Approximation, Henderson-Hasselbalch Applications

1. (a) For each of the Bronsted-Lowery acid base reactions below, draw the conjugate acid and conjugate

base pairs most likely to form. (b) Circle the set of arrows that best describe the equilibrium for each

reaction. (3pts for each correct conjugate pair, 1pt for each correct set of arrows)

2. (a) Using the structures provided, draw an arrow-pushing mechanism for the acid base reaction below.

(3pts). (b) Estimate the equilibrium constant for this reaction (Keq). Show work which led to this value.

(3pts). (c) Draw a different acid (assuming the same base) that would reverse the equilibrium to favor the

other side. (2pts).

3. One form of Henderson-Hasselbalch Equation shown below (See also Chapter 19.4 and 26.3) can be

derived from the generic equation for an acid-dissociation reaction. It is useful for approximating the pH

of an aqueous buffer. The expression can also be arranged to solve for pKa or the ratio of [conjugate

base]:[acid] or [base]:[conjugate acid] depending on the functional group being ionized.

Consider the two different cases below. Decide if the compound shown is most likely acting as a base or

acid then draw the most likely ionized form of the compound as it would exist at pH 7. For each case

calculate the % ionization. For Case 2 only, using the appropriate form of the Henderson-Hasselbalch

expression, show your work which led to each % ionized answer. (2pts for each correct structure, 1pt

for each correct % ionization, 4pts for relevant work shown). Note, the % ionized value

approximated for each case should be some value < 100% and > 0%.

Note: You will need to use Table 1.8 in your text to find the relevant pKa. Round the pKa to the nearest

whole number.

Show work for how you solved for the % ionization for Case 2 below:

4. Suppose Compound B is the conjugate base of Compound A. Calculate the approximate concentration

of Compound A that must be present in a 0.042 M solution of Compound B to produce a pH of 4.96. The

Ka for Compound A is 1.8 x 10-5. Show your work. Round your final answer to the nearest hundredth

decimal place. (5pts)

5. Consider the structure below. Then using the empty boxes, answer parts (a)-(c).

(a) (1pts) Draw a constitutional isomer expected to have approximately the same pKa as the compound

above.

(b) (2pts) Draw a constitutional isomer that you expect to be less acidic than the compound above.

(c) (2pts) Draw a constitutional isomer that is roughly 1012 times more acidic than the compound above.

6. The relative acidity (or basicity) of compounds is influenced by factors including: periodic trends,

resonance, induction, and orbitals. For each case below there are two protons circled. Indicate which one

is most acidic and briefly explain the factor that is most responsible. If resonance is the determining factor,

draw relevant resonance structures to clearly illustrate. (1pt each correct answer, 2pt for each valid

explanation).

CHEM 205 Worksheet (WS2)

52 Points Total

Key Topics: Acid-Base Equilibria, Keq Approximation, Henderson-Hasselbalch Applications

1. (a) For each of the Bronsted-Lowery acid base reactions below, draw the conjugate acid and conjugate

base pairs most likely to form. (b) Circle the set of arrows that best describe the equilibrium for each

reaction. (3pts for each correct conjugate pair, 1pt for each correct set of arrows)

2. (a) Using the structures provided, draw an arrow-pushing mechanism for the acid base reaction below.

(3pts). (b) Estimate the equilibrium constant for this reaction (Keq). Show work which led to this value.

(3pts). (c) Draw a different acid (assuming the same base) that would reverse the equilibrium to favor the

other side. (2pts).

3. One form of Henderson-Hasselbalch Equation shown below (See also Chapter 19.4 and 26.3) can be

derived from the generic equation for an acid-dissociation reaction. It is useful for approximating the pH

of an aqueous buffer. The expression can also be arranged to solve for pKa or the ratio of [conjugate

base]:[acid] or [base]:[conjugate acid] depending on the functional group being ionized.

Consider the two different cases below. Decide if the compound shown is most likely acting as a base or

acid then draw the most likely ionized form of the compound as it would exist at pH 7. For each case

calculate the % ionization. For Case 2 only, using the appropriate form of the Henderson-Hasselbalch

expression, show your work which led to each % ionized answer. (2pts for each correct structure, 1pt

for each correct % ionization, 4pts for relevant work shown). Note, the % ionized value

approximated for each case should be some value < 100% and > 0%.

Note: You will need to use Table 1.8 in your text to find the relevant pKa. Round the pKa to the nearest

whole number.

Show work for how you solved for the % ionization for Case 2 below:

4. Suppose Compound B is the conjugate base of Compound A. Calculate the approximate concentration

of Compound A that must be present in a 0.042 M solution of Compound B to produce a pH of 4.96. The

Ka for Compound A is 1.8 x 10-5. Show your work. Round your final answer to the nearest hundredth

decimal place. (5pts)

5. Consider the structure below. Then using the empty boxes, answer parts (a)-(c).

(a) (1pts) Draw a constitutional isomer expected to have approximately the same pKa as the compound

above.

(b) (2pts) Draw a constitutional isomer that you expect to be less acidic than the compound above.

(c) (2pts) Draw a constitutional isomer that is roughly 1012 times more acidic than the compound above.

6. The relative acidity (or basicity) of compounds is influenced by factors including: periodic trends,

resonance, induction, and orbitals. For each case below there are two protons circled. Indicate which one

is most acidic and briefly explain the factor that is most responsible. If resonance is the determining factor,

draw relevant resonance structures to clearly illustrate. (1pt each correct answer, 2pt for each valid

explanation).