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Chapter 16: Problem 92

Oxalic acid, HOOCCOOH, a weak diprotic acid, has \(\mathrm{p} K_{\mathrm{a}_{1}}=1.25\) and \(\mathrm{p} K_{\mathrm{a}_{2}}=3.81 .\) A related diprotic acid, suberic acid, \(\mathrm{HOOC}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{COOH}\) has \(\mathrm{p} K_{\mathrm{a}_{1}}=4.21\) and \(\mathrm{p} K_{\mathrm{a}_{2}}=5.40 .\) Offer a plausible reason as to why the difference between \(\mathrm{pK}_{\mathrm{a}_{1}}\) and \(\mathrm{pK}_{\mathrm{a}_{2}}\) is so much greater for oxalic acid than for suberic acid.

Short Answer

Expert verified
The difference between pKa1 and pKa2 in Oxalic acid is larger than in Suberic acid because in Oxalic acid, the negative charge developed after losing the first proton is more locally concentrated due to the direct connection of the carboxyl groups. This leads to stronger repulsion when trying to remove the second proton. In Suberic Acid, the long carbon chain allows better separation of the resultant negative charges, resulting in less repulsion when removing the second proton.

Step by step solution

01

Understanding pKa values

The pKa value of an acid is a measure of its acidity. The lower the pKa value, the stronger the acid. In the case of diprotic acids, we have two pKa values. The first, pKa1, indicates the ease of removal of the first proton, and the second, pKa2, indicates the ease of removal of the second proton.
02

Comparing the pKa values of Oxalic and Suberic Acids

The difference between pKa1 and pKa2 for Oxalic acid is 2.56, while for Suberic acid it's 1.19. It's noticeable that the difference for Oxalic acid is larger, meaning that the removal of the second proton is a lot more difficult (i.e. the acid becomes considerably weaker) than the removal of the first proton.
03

Analyzing the Structure of the Acids

The structural difference between Oxalic acid and Suberic acid need to be considered. Oxalic acid has two carboxyl groups directly linked together, whereas Suberic acid has a long carbon chain separating the two carboxyl groups.
04

Making Inferences based on Structure

After losing a proton, the remaining structure of the acid molecule becomes negatively charged. This negative charge is more localized in Oxalic acid because the carboxyl groups are directly connected, thus resulting in a stronger negative-negative repulsion when the second proton is being removed. In Suberic acid, the large carbon chain between the carboxyl groups allows effective separation of the negative charges, resulting in less repulsion and thus easier removal of the second proton.

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Most popular questions from this chapter

In \(0.10 \mathrm{M} \quad \mathrm{CH}_{3} \mathrm{NH}_{2}(\mathrm{aq}), \quad\) (a) \(\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]=0.10 \mathrm{M}\) (b) \(\left[\mathrm{OH}^{-}\right]=0.10 \mathrm{M} ;(\mathrm{c}) \mathrm{pH}<7 ;(\mathrm{d}) \mathrm{pH}<13\).

The three following reactions are acid-base reactions according to the Lewis theory. Draw Lewis structures, and identify the Lewis acid and Lewis base in each reaction. (a) \(\mathrm{B}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}\) (b) \(\mathrm{N}_{2} \mathrm{H}_{4}+\mathrm{H}_{3} \mathrm{O}^{+} \longrightarrow \mathrm{N}_{2} \mathrm{H}_{5}^{+}+\mathrm{H}_{2} \mathrm{O}\) (c) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{O}+\mathrm{BF}_{3} \longrightarrow\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{OBF}_{3}\)

Indicate whether each of the following is a Lewis acid or base. (a) \(\mathrm{OH}^{-} ;\) (b) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{B} ;\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

One handbook lists a value of 9.5 for \(\mathrm{p} \mathrm{K}_{\mathrm{b}}\) of quinoline, \(\mathrm{C}_{9} \mathrm{H}_{7} \mathrm{N},\) a weak base used as a preservative for anatomical specimens and to make dyes. Another handbook lists the solubility of quinoline in water at \(25^{\circ} \mathrm{C}\) as \(0.6 \mathrm{g} / 100 \mathrm{mL} .\) Use this information to calculate the \(\mathrm{pH}\) of a saturated solution of quinoline in water.

Use Lewis structures to diagram the following reaction in the manner of reaction (16.19) $$2 \mathrm{NH}_{3}+\mathrm{Ag}^{+} \longrightarrow\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}$$ Identify the Lewis acid and Lewis base.
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