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Chapter 4: Problem 45

Offer an explanation for the following observations. (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\)is a stronger acid than \(\mathrm{NH}_{4}^{+}\). (b) Nitric acid, \(\mathrm{HNO}_{3}\), is a stronger acid than nitrous acid, \(\mathrm{HNO}_{2}\). (c) Ethanol and water have approximately the same acidity. (d) Trifluoroacetic acid, \(\mathrm{CF}_{3} \mathrm{COOH}\), is a stronger acid than trichloroacetic acid, \(\mathrm{CCl}_{g} \mathrm{COOH}\).

Short Answer

Expert verified
Question: Explain why in the given observations (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\) is a stronger acid than \(\mathrm{NH}_{4}^{+}\), (b) why nitric acid is a stronger acid than nitrous acid, (c) why ethanol and water have approximately the same acidity, and (d) why trifluoroacetic acid is a stronger acid than trichloroacetic acid. Answer: (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\) is a stronger acid than \(\mathrm{NH}_{4}^{+}\) because the oxygen in water is more electronegative, providing a more stable conjugate base after losing a proton. (b) Nitric acid is a stronger acid than nitrous acid due to the increased electron-withdrawing nature of the additional oxygen atom. (c) Ethanol and water have approximately the same acidity because the difference in electronegativity between carbon and hydrogen does not significantly affect the charge stabilization in the conjugate base. (d) Trifluoroacetic acid is a stronger acid than trichloroacetic acid because fluorine is more electronegative, resulting in better charge stabilization in the conjugate base after losing a proton.

Step by step solution

01

Observation (a): Comparing \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{NH}_{4}^{+}\)

To understand why \(\mathrm{H}_{3} \mathrm{O}^{+}\) is a stronger acid than \(\mathrm{NH}_{4}^{+}\), first, consider the conjugate bases formed after losing a proton: \(\mathrm{H}_{2} \mathrm{O}\) (water) and \(\mathrm{NH}_{3}\) (ammonia). The oxygen in water is more electronegative than the nitrogen in ammonia, and it can stabilize the negative charge more effectively because of its strong attraction to electrons, making water a more stable conjugate base. Therefore, \(\mathrm{H}_{3} \mathrm{O}^{+}\) is a stronger acid than \(\mathrm{NH}_{4}^{+}\).
02

Observation (b): Comparing \(\mathrm{HNO}_{3}\) and \(\mathrm{HNO}_{2}\)

Nitric acid (\(\mathrm{HNO}_{3}\)) and nitrous acid (\(\mathrm{HNO}_{2}\)) are oxyacids. In general, the strength of an oxyacid is determined by comparing the number of oxygen atoms bonded to the central atom. The more oxygen atoms present, the stronger the acid due to the increased electron-withdrawing nature of oxygen. Since \(\mathrm{HNO}_{3}\) has three oxygen atoms and \(\mathrm{HNO}_{2}\) has only two, nitric acid is a stronger acid than nitrous acid.
03

Observation (c): Comparing Ethanol and Water

Both ethanol and water can donate a proton to form a conjugate base. In ethanol, the acidic proton is bonded to oxygen, just like in water. However, the rest of the ethanol molecule is made mainly of carbon and hydrogen, which are not very electronegative atoms. This means that the conjugate base formed after losing a proton from ethanol will have less effective charge stabilization compared to the conjugate base formed from water. However, the difference in electronegativity between carbon and hydrogen is not significant enough to make a noticeable difference in their acidity. Therefore, ethanol and water have approximately the same acidity.
04

Observation (d): Comparing \(\mathrm{CF}_{3} \mathrm{COOH}\) and \(\mathrm{CCl}_{3} \mathrm{COOH}\)

Trifluoroacetic acid (\(\mathrm{CF}_{3} \mathrm{COOH}\)) and trichloroacetic acid (\(\mathrm{CCl}_{3} \mathrm{COOH}\)) are both carboxylic acids. To assess the difference in acidity, we need to look at the inductive effect of the attached atoms. Fluorine is a more electronegative atom than chlorine, which means it has a stronger ability to pull electrons towards itself. As a result, trifluoroacetic acid will have a larger electron withdrawal ability through the inductive effect, making its conjugate base more stable after losing a proton. Therefore, trifluoroacetic acid is a stronger acid than trichloroacetic acid.

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Most popular questions from this chapter

Write these reactions as proton-transfer reactions. Label which reactant is the acid and which is the base, which product is the conjugate base of the original acid, and which is the conjugate acid of the original base. In addition, write Lewis structures for each reactant and product, and use curved arrows to show the flow of electrons in each reaction. (a) \(\mathrm{CH}_{3} \mathrm{SH}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{CH}_{3} \mathrm{~S}^{-}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{CH}_{2}=\mathrm{O}+\mathrm{HCl} \rightleftharpoons \mathrm{CH}_{2}=\mathrm{OH}^{+}+\mathrm{Cl}^{-}\)

As we shall see in Chapter 19 , hydrogens on a carbon adjacent to a carbonyl group are far more acidic than those not adjacent to a carbonyl group. The anion derived from acetone, for example, is more stable than is the anion derived from ethane. Account for the greater stability of the anion from acetone. $$ \begin{array}{cc} \stackrel{O}{\|} & \\ \mathrm{CH}_{3} \mathrm{CCH}_{2}-\mathrm{H} & \mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{H} \\ \text { Acetone } & \text { Ethane } \\ \mathrm{p} K_{\mathrm{a}} 22 & \mathrm{p} K_{\mathrm{a}} 51 \end{array} $$

Sodium hydride, NaH, is available commercially as a gray-white powder. It melts at \(800^{\circ} \mathrm{C}\) with decomposition. It reacts explosively with water and ignites spontaneously on standing in moist air. (a) Write a Lewis structure for the hydride ion and for sodium hydride. Is your Lewis structure consistent with the fact that this compound is a high- melting solid? Explain. (b) When sodium hydride is added very slowly to water, it dissolves with the evolution of a gas. The resulting solution is basic to litmus. What is the gas evolved? Why has the solution become basic? (c) Write an equation for the reaction between sodium hydride and 1-butyne, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\). Use curved arrows to show the flow of electrons in this reaction.

Which has the larger numerical value? (a) The \(\mathrm{p} K_{\mathrm{a}}\) of a strong acid or the \(\mathrm{p} K_{\mathrm{a}}\) of a weak acid (b) The \(K_{\mathrm{a}}\) of a strong acid or the \(K_{\mathrm{a}}\) of a weak acid

If the \(\Delta G^{\circ}\) for a reaction is \(-4.5 \mathrm{kcal} / \mathrm{mol}\) at \(298 \mathrm{~K}\), what is the \(K_{\text {eq }}\) for this reaction? What is the change in entropy of this reaction if \(\Delta H^{\circ}=-3.2 \mathrm{kcal} / \mathrm{mol}\) ?
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