Final 3 Topics (brief)

Final 3 Topics (brief)

• Indicators

• Polyprotic Acids

• Structural Factors That Affect Acid Strength

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Indicators change color in a range around their pKa

• Indicators are acids (HA) that have a different color between HA and its conjugate base, A- (one of these may be colorless)

• Since at pKa, [HA] = [A-], this is the “breakeven” point for color—it will be “half and half”

• The solution will be the color of HA if pH is 1-2 units below pKa

• The solution will be the color of A-, if pH is 1-2 units above pKa

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Polyprotic Acid Comments

• In general, 1st ionization dominates: Ka1 >> Ka2 >>Ka3 (See Table 15.10 →)

• To determine pH, can ignore the 2nd (or 3rd) ionizations! (See Ex. 15.17 →)

• To determine [A2-], use [HA-]eq and [H3O+]eq from 1st ionization as equilibrium values for 2nd ionization (See Ex. 15.19 →)–Because such a tiny amount of 2nd ionization will “occur” (K value is so much smaller)

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Table 15.10 (cont.)

Exception! (H’s are so far away from each other)

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EXAMPLE 15.17 Finding the pH of a Polyprotic Acid Solution

Find the pH of a 0.100 M ascorbic acid (H2C6H6O6) solution.

2nd step shown on next slide (where you can ignore “extra” hydronium ion from 2nd ionization)

1st step involves 1st ionization (“weak acid ionization problem”)

(approximation valid)

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15.17 (cont.) and 15.19

HA- + H2O H3O+ + A2- 2nd ionization (Ka2)

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Factors Affecting Acid Strength

• Main one (for me): effective electronegativity of “X” in:

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Three ways to make “X” more electronegative

• Change X itself: F > Cl > Br > I Or have X be a highly charged metal cation!

• Have more and more O’s bonded to X

• If X is Carbon, have more and more Cl’s or F’s (or O’s) bonded to C (vs H’s)

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Change X itself: F > Cl > Br > I

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Change X itself: Have X be a highly charged metal cation!

Acid pKa Ka

OH2 14.00 1.0 x 10-14

Fe(H2O)62+

OR

(H2O)5Fe2+-OH2

9.49 3.2 x 10-10

Fe(H2O)63+

OR

(H2O)5Fe3+-OH2

2.20 6.3 x 10-3

(H2O)5Fe O

H

H

x+

12

Have more and more O’s bonded to X

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If X is Carbon, have more and more Cl’s or F’s (or O’s) bonded to C (vs H’s).

Acid pKa Ka

CH3COOH 4.76 1.7 x 10-5

HOCH2COOH 3.83 1.5 x 10-4

CCl3COOH 0.66 2.2 x 10-1

CF3COOH 0.23 5.9 x 10-1

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