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Chemistry 123/125
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Name _____________________________
Name _____________________________
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Ch. 17 In-Class Exercise
In this exercise you will explore the relationship between the molecular structure
of an acid and its strength (degree of dissociation).
As you know, the strength of an acid is a measure of how easily the acid will lose
a proton (an H+ ion). The hydrogen atom in the acid molecule which becomes a
proton by dissociating and leaving an electron pair behind is called the acidic
hydrogen. An acid molecule can have one acidic hydrogen (a monoprotic acid),
two acidic hydrogens (a diprotic acid) or three acidic hydrogen (a triprotic acid).
There are two factors that determine the strength of an acid:
• the strength of the bond between the acidic hydrogen and the atom it’s
attached to,
• the partial charge of the acidic hydrogen.
Acid Strength and Bond Strength
H --- O --- H
O-H bond strength = 436 kJ/mol
H --- S --- H
S-H bond strength = 367 kJ/mol
1. Which bond is easier to break, O-H or S-H?
2. Which is the stronger acid, H2O or H2S? Explain.
3. Recall that, everything else being equal, the larger the atom, the weaker the
bond (the bond length is longer to accommodate the larger atomic size).
Which is the stronger acid, NH4+ or PH4+? Explain.
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Table 1. Characteristics of some acids with related structure.
Acid
Bond
Bond Enthalpy
(kJ/mol)
Ka
H2 O
H2 S
NH4+
PH4+
HF
HCl
HBr
HI
H-O
H-S
H-N
H-P
H-F
H-Cl
H-Br
H-I
463
367
390
325
568
432
366
298
1.8 x 10-16
1.0 x 10-7
5.6 x 10-10
≈ 1014
7.2 x 10-4
1 x 106
1 x 109
3 x 109
4. Based on the information in Table 1, explain why the strength of the
hydrohalide acids (H-halogen) increases down the halogen group.
Acid Strength and Partial Charge
O-H bonds have about the same bond energy (463 kJ/mol)
0.14
0.01
H
Cl
-0.24
H
0.40
C
C
-0.01
-0.35
O
0.14
H
0.30
H
0.14
O
-0.53
Cl
0.38
C
C
-0.30
O
0.01
H
0.32
Cl
0.01
O
-0.42
5. Identify the acidic hydrogen in both molecules above (acetic acid and
trichloroacetic acid).
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6. Which acid is the stronger acid? Explain.
Table 2. Characteristics of some acids with related structure.
Acid
CH3COOH
CH2ClCOOH
CHCl2COOH
CCl3COOH
HOCl
HOBr
HOI
Partial Charge on Acidic
Hydrogen
Ka
0.298
0.308
0.317
0.325
0.280
0.275
0.270
1.8 x 10-5
1.4 x 10-3
5.1 x 10-2
0.22
2.9 x 10-8
2.4 x 10-9
2.3 x 10-11
7. Consider the data in Table 2 and explain why the partial charge on the acidic
hydrogen increases in the series of organic acids (first four molecules in the
table).
8. Consider the data in Table 2 and explain why the partial charge on the acidic
hydrogen increases in the series of hydrohalide acids (second set of three
molecules in the table).
9. Predict the value of Ka for CF3COOH. Explain your reasoning.
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Practice Exercises
10. For each of the following pairs of acids, predict which will have the larger
value of Ka and explain your reasoning.
a) H2S and H2Se
b) HONO and HOPO
c) NH4+ and Cl3NH+
d) HOSeO2 and HOSeO3
e) H2S and H2Te
f) HONO2 and HONO
g) HOSO2 and HOSO3
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11. Rank the following 1M solutions in order of increasing pH.
a) HBr
b) CH3COOH
c) CF3COOH
d) KBr
e) NH3
12. Consider the relative acid strengths of H2O and HF. Which of the two factors
described at the beginning of this exercise dominates the determination of
relative acidity for these acids? Explain.
Adapted from CHEMISTRY, A Guided Inquiry by Richard S. Moog and John J. Farrell,
preliminary edition.