0.19 Acid-base equilibrium  (Page 7/7)

In the acids in the other two categories, the hydrogen atom which ionizes is attached directly to an oxygen atom.Thus, to understand acidity in these molecules, we must examine what the oxygen atom is in turn bonded to. It is very interestingto note that, in examining compounds like R-O-H, where R is an atom or group of atoms, we can get either acidic or basic properties.For examples, NaOHis a strong base, whereas HOClis a weak acid. This means that, when NaOHionizes in solution, the Na-O linkage ionizes, whereas when HOClionizes in solution, the H-O bond ionizes.

To understand this behavior, we compare the strength of the simple oxyacidsHOI, HOBr,and HOCl.The $p{K}_a$ 's for these acids are found experimentally to be, respectively, 10.6,8.6, and 7.5. The acid strength for HOXincreases as we move up the periodic table in the halogen group. This means that the H-O bond ionizes more readily when the oxygenatom is bonded to a more electronegative atom.

We can add to this observation by comparing the strengths of the acidsHOCl, HOClO,HOClO ₂ , andHOClO ₃ . (Note that the molecular formulae are more commonly written asHClO, HClO ₂ , HClO ₃ , andHClO ₄ . We have written them instead to emphasize the molecular structure.)The $p{K}_a$ 's of these acids are, respectively, 7.5, 2.0, -2.7, and -8.0.In each case, the molecule with more oxygen atoms on the central Cl atom is the stronger acid:HOClO is more acidic thanHOCL, etc . A similar result is found in comparing the oxyacids of nitrogen.HONO ₂ , nitric acid, is one of the strong acids, whereasHONO, nitrous acid, is a weak acid. Since oxygen atoms are very stronglyelectronegative, these trends add to our observation that increasing electronegativity of the attached atoms increases theionization of the O-H bond.

Why would electronegativity play a role in acid strength? There are two conclusions we might draw. First, agreater electronegativity of the atom or atoms attached to the H-O in the oxyacid apparently results in a weaker H-O bond, which isthus more readily ionized. We know that an electronegative atom polarizes bonds by drawing the electrons in the molecule towardsit. In this case, the Cl in HOCland the Br in HOBrmust polarize the H-O bond, weakening it and facilitating the ionization of the hydrogen. In comparingHOCl toHOClO, the added oxygen atom must increase the polarization of the H-Obond, thus weakening the bond further and increasing the extent of ionization.

A second conclusion has to do with the ion created by the acid ionization. The negative ion produced has asurplus electron, and the relative energy of this ion will depend on how readily that extra electron is attracted to the atoms ofion. The more electronegative those atoms are, the stronger is the attraction. Therefore, theOCl ^– ion can more readily accommodate the negative charge than can the OBr ^– ion. And the OClO ^– ion can more readily accommodate the negative charge than can the OCl ^– ion.

We conclude that the presence of strongly electronegative atoms in an acid increases the polarization of theH-O bond, thus facilitating ionization of the acid, and increases the attraction of the extra electron to the negative ion, thusstabilizing the negative ion. Both of these factors increase the acid strength. Chemists commonly use both of these conclusions inunderstanding and predicting relative acid strength.

The relative acidity of carbon compounds is a major subject of organic chemistry, which we can only visit brieflyhere. In each of the carboxylic acids, the H-O group is attached to a carbonyl C=O group, which is in turn bonded to other atoms. Thecomparison we observe here is between carboxylic acid molecules, denoted asRCOOH, and other organic molecules containing the H-O group, such asalcohols denoted as ROH.(R is simply an atom or group of atoms attached to the functional group.) The former are obviously acids whereas the latter groupcontains molecules which are generally extremely weak acids. One interesting comparison is for the acid and alcohol when R is thebenzene ring, C ₆ H ₅ . Benzoic acid,C ₆ H ₅ COOH, has $p{K}_a=4.2$ , whereas phenol,C ₆ H ₅ OH, has $p{K}_a=9.9$ . Thus, the presence of the doubly bonded oxygen atom on the carbonatom adjacent to the O-H clearly increases the acidity of the molecule, and thus increases ionization of the O-H bond.

This observation is quite reasonable in the context of our previous conclusion. Adding an electronegativeoxygen atom in near proximity to the O-H bond both increases the polarization of the O-H bond and stabilizes the negative ionproduced by the acid ionization. In addition to the electronegativity effect, carboxylate anions,RCOO ^– , exhibit resonance stabilization, as seen in [link] .

The resonance results in a sharing of the negative charge over several atoms, thus stabilizing the negativeion. This is a major contributing factor in the acidity of carboxylic acids versus alcohols.

Review and discussion questions

1. Strong acids have a higher percent ionization than do weak acids. Why don't we use percent ionization as ameasure of acid strength, rather than $K_a$ ?
2. Using the data in [link] for nitrous acid, plot [H ₃ O ⁺ ] versusc ₀ , the initial concentration of the acid, and versus[HNO ₂ ] the equilibrium concentration of the acid. On a second graph, plot[H ₃ O ⁺ ] ² versus c ₀ , the initial concentration of the acid, and versus[HNO ₂ ] the equilibrium concentration of the acid. Which of these resultsgives a straight line? Using the equilibrium constant expression, explain your answer.
3. Using Le Châtelier's principle, explain why the concentration of[OH ^– ] is much lower in acidic solution than it is in neutralsolution.
4. We considered mixing a strong base with a weak acid, but we did not consider mixing a strong acid with a weakacid. Consider mixing 0.1M HNO ₃ and 0.1M HNO ₂ . Predict the pH of the solution and the percent ionization of thenitrous acid. Rationalize your prediction using Le Châtelier's principle.
5. Imagine taking a 0.5M solution of nitrous acid and slowing adding water to it. Looking at [link] , we see that, as the concentration of nitrous acid decreases, the percent ionization increases. Bycontrast, [H ₃ O ⁺ ] decreases. Rationalize these results using LeChâtelier's principle.
6. We observed that mixing a strong acid and a strong base, in equal amounts and concentrations, produces aneutral solution, and that mixing a strong base with a weak acid, in equal amounts and concentrations, produces a basic solution.Imagine mixing a weak acid and a weak base, in equal amounts and concentrations. Predict whether the resulting solution will beacidic, basic, or neutral, and explain your prediction.
7. Using the electronegativity arguments presented above , explain why, in general, compounds like M-O-H are bases rather thanacids, when M is a metal atom. Predict the relationship between the properties of the metal atom M and the strength of the baseMOH.
8. Ionization of sulfuric acid H ₂ SO ₄ produces HSO ₄ ^– , which is also an acid. However,HSO ₄ ^– is a much weaker acid than H ₂ SO ₄ . Using the conclusions from above , explain why HSO ₄ ^– is a much weaker acid.
9. Predict and explain the relative acid strengths ofH ₂ S andHCl. Predict and explain the relative acid strengths ofH ₃ PO ₄ and H ₃ AsO ₄ .
10. Using arguments from above , predict and explain the relative acidity of phenol and methanol .

    Phenol

    

    Methanol