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Chapter 4: Problem 32

Explain the following observations: (a) \(\mathrm{NH}_{3}\) contains no \(\mathrm{OH}^{-}\) ions, and yet its aqueous solutions are basic; (b) HF is called a weak acid, and yet it is very reactive; (c) although sulfuric acid is a strong electrolyte, an aqueous solution of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) contains more \(\mathrm{HSO}_{4}^{-}\) ions than \(\mathrm{SO}_{4}^{2-}\) ions.

Short Answer

Expert verified
(a) NH3 does not contain OH- ions, but when dissolved in water, it reacts with water molecules to form NH4+ and OH- ions, making the solution basic. (b) HF is a weak acid due to its partial ionization in aqueous solutions; however, its reactivity stems from fluorine's high electronegativity, which makes it a strong nucleophile and an excellent ligand. (c) In aqueous H2SO4 solutions, there are more HSO4- ions than SO4^2- ions because H2SO4 ionizes almost completely into H+ and HSO4-, while the second ionization step (HSO4- to H+ and SO4^2-) is not as complete since HSO4- is a weaker acid.

Step by step solution

01

(a) Absence of OH- ions in NH3 and basicity of its aqueous solutions

Ammonia, NH3, is a neutral compound, as it neither contains OH- ions nor H+ ions. However, when dissolved in water, NH3 molecules react with water molecules to form NH4+ and OH- ions: \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] The presence of OH- ions in the solution indicates that the solution is basic. Therefore, although NH3 itself does not contain OH- ions, it forms a basic solution when dissolved in water.
02

(b) HF as a weak acid and its reactivity

A weak acid is defined by its partial ionization in aqueous solutions. When dissolved in water, hydrofluoric acid, HF, does not completely ionize into H+ and F- ions, making it a weak acid: \[ HF 鈬� H^+ + F^- \] The equilibrium very much lies to the left, which means less H+ ions are released compared to strong acids. Nonetheless, HF is very reactive due to the high electronegativity of fluorine, making it a strong nucleophile and an excellent ligand. Fluorine can form strong bonds with other atoms, and this reactivity (not referring to basicity or acidity) is independent of HF's ionization.
03

(c) Relative concentrations of HSO4- and SO4^2- ions in aqueous H2SO4 solutions

Sulfuric acid, H2SO4, is a strong acid, meaning that it ionizes almost completely in aqueous solutions into H+ ions and its conjugate base, HSO4-: \[H_2SO_4 \rightarrow H^+ + HSO_4^-\] However, the bisulfate ion (HSO4-) can also act as an acid, donating an additional H+ ion: \[HSO_4^- 鈬� H^+ + SO_4^{2-}\] This second ionization step is not as complete as the first one, as HSO4- is a weaker acid. Consequently, while sulfuric acid is a strong electrolyte, the equilibrium for the second ionization step lies more to the left, resulting in a higher concentration of HSO4- ions compared to SO4^2- ions in an aqueous solution of H2SO4.

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Most popular questions from this chapter

The labels have fallen off two bottles, one containing \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) and the other containing \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\). You have a bottle of dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\). How could you use it to test a portion of each solution to identify which solution is which? [Section 4.2]

A solution is made by mixing \(12.0 \mathrm{~g}\) of \(\mathrm{NaOH}\) and \(75.0 \mathrm{~mL}\) of \(0.200 \mathrm{M} \mathrm{HNO}_{3}\). (a) Write a balanced equation for the reaction that occurs between the solutes. (b) Calculate the concentration of each ion remaining in solution. (c) Is the resultant solution acidic or basic?

Federal regulations set an upper limit of 50 parts per million (ppm) of \(\mathrm{NH}_{3}\) in the air in a work environment [that is, 50 molecules of \(\mathrm{NH}_{3}(g)\) for every million molecules in the air]. Air from a manufacturing operation was drawn through a solution containing \(1.00 \times 10^{2} \mathrm{~mL}\) of \(0.0105 \mathrm{M}\) HCl. The \(\mathrm{NH}_{3}\) reacts with HCl as follows: $$ \mathrm{NH}_{3}(a q)+\mathrm{HCl}(a q) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(a q) $$ After drawing air through the acid solution for \(10.0 \mathrm{~min}\) at a rate of \(10.0 \mathrm{~L} / \mathrm{min}\), the acid was titrated. The remaining acid needed \(13.1 \mathrm{~mL}\) of \(0.0588 \mathrm{M} \mathrm{NaOH}\) to reach the equivalence point. (a) How many grams of \(\mathrm{NH}_{3}\) were drawn into the acid solution? (b) How many ppm of \(\mathrm{NH}_{3}\) were in the air? (Air has a density of \(1.20 \mathrm{~g} / \mathrm{L}\) and an average molar mass of \(29.0 \mathrm{~g} / \mathrm{mol}\) under the conditions of the experiment.) (c) ls this manufacturer in compliance with regulations?

We have learned in this chapter that many ionic solids dissolve in water as strong electrolytes, that is, as separated ions in solution. What properties of water facilitate this process?

Write a balanced molecular equation and a net ionic equation for the reaction that occurs when (a) solid \(\mathrm{CaCO}_{3}\) reacts with an aqueous solution of nitric acid; (b) solid iron(II) sulfide reacts with an aqueous solution of hydrobromic acid.
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