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Chapter 16: Problem 24

Label each of the following as being a strong acid, a weak acid, or a species with negligible acidity. In each case write the formula of its conjugate base, and indicate whether the conjugate base is a strong base, a weak base, or a species with negligible basicity: (a) \(\mathrm{HNO}_{2}\), (b) \(\mathrm{H}_{2} \mathrm{SO}_{4},(\mathrm{c}) \mathrm{HPO}_{4}{ }^{2-}\) (d) \(\mathrm{CH}_{4}\), (e) \(\mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+}\) (an ion related to \(\mathrm{NH}_{4}{ }^{+}\) ).

Short Answer

Expert verified
(a) HNO2 is a weak acid, and its conjugate base NO2- is a weak base. (b) H2SO4 is a strong acid, and its conjugate base HSO4- is a weak base. (c) HPO4^2- has negligible acidity, and its conjugate base PO4^3- is a weak base. (d) CH4 has negligible acidity, and its conjugate base CH3- has negligible basicity. (e) CH3NH3+ is a weak acid, and its conjugate base CH3NH2 is a weak base.

Step by step solution

01

(a) HNO2

HNO2 is a weak acid because it is not one of the 7 common strong acids (HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4). To find its conjugate base, remove one proton (H+) from the acid: \(HNO_2 \rightarrow NO_2^- + H^+\) The conjugate base is NO2-. Since the conjugate of a weak acid is a weak base, NO2- is a weak base.
02

(b) H2SO4

H2SO4 is a strong acid because it's one of the 7 common strong acids. To find its conjugate base, remove one proton (H+) from the acid: \(H_2SO_4 \rightarrow HSO_4^- + H^+\) The conjugate base of H2SO4 is HSO4-. HSO4- is a weak base because it's the conjugate base of a strong acid but not a part of strong bases (hydroxides of Group 1 and Group 2 elements).
03

(c) HPO4^{2-}

HPO4^2- has already lost a proton, so it's a species with negligible acidity. To find its conjugate base, remove one proton (H+) from the species: \(HPO_4^{2-} \rightarrow PO_4^{3-} + H^+\) The conjugate base of HPO4^2- is PO4^3-. Since HPO4^2- has negligible acidity, its conjugate base PO4^3- will be a weak base.
04

(d) CH4

CH4 has negligible acidity because it's a nonpolar covalent compound with no hydrogen atoms bonded to highly electronegative atoms. To find its conjugate base, remove one proton (H+) from the molecule: \(CH_4 \rightarrow CH_3^- + H^+\) The conjugate base of CH4 is CH3-. Since CH4 has negligible acidity, its conjugate base CH3- will also have negligible basicity.
05

(e) CH3NH3+ (related to NH4+)

CH3NH3+ is related to NH4+, so it is a weak acid, as NH4+ is the conjugate acid of a weak base (NH3). To find its conjugate base, remove one proton (H+) from the ion: \(CH_3NH_3^+ \rightarrow CH_3NH_2 + H^+\) The conjugate base of CH3NH3+ is CH3NH2. Since CH3NH3+ is a weak acid, its conjugate base CH3NH2 is a weak base.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Strong Acids
Strong acids are acids that completely dissociate in water, meaning they release all their protons into the solution, leaving no undissociated molecules. They are characterized by their capability to easily donate a proton (H鈦�). Common examples include hydrochloric acid (HCl), sulfuric acid (H鈧係O鈧�), and nitric acid (HNO鈧�).

These acids are often memorized as the 7 most common strong acids, which include: HCl, HBr, HI, HNO鈧�, H鈧係O鈧�, HClO鈧�, and HClO鈧�. When a strong acid dissociates, it forms a conjugate base, which typically has negligible basicity because the equilibrium heavily favors the formation of the acid and the presence of protons in solution.
Weak Acids
Unlike strong acids, weak acids do not completely dissociate in water. Instead, they partially dissociate, establishing an equilibrium between the undissociated acid and the ions that form. This means not every molecule of a weak acid contributes a proton to the solution.

Examples include nitrous acid (HNO鈧�) and acetic acid (CH鈧僀OOH). Because weak acids partially release their protons, their ability to donate protons is not as pronounced as that of strong acids. The conjugate base that forms from a weak acid is a weak base, and it participates in reversible reactions with water.
Conjugate Bases
The concept of a conjugate base involves the idea that when an acid donates a proton (H鈦�), the remaining part of the molecule becomes the conjugate base. For instance, if HNO鈧� loses a proton, it turns into NO鈧傗伝.

Conjugate bases derived from weak acids are weak bases themselves, as they can accept protons but do so weakly. In contrast, conjugate bases of strong acids are considered to have negligible basicity because they have a minimal tendency to re-associate with protons to form the original acid. This relationship between acids and their conjugate bases is a key concept in understanding acidity and basicity.
Negligible Acidity
A species with negligible acidity hardly acts as an acid at all. This means it has little to no tendency to donate protons in a solution. Methane ( CH鈧�), for example, is a molecule with negligible acidity due to the strong covalent bonds holding onto its hydrogen atoms, which are not bonded to highly electronegative atoms.

The concept of negligible acidity often involves molecules where the hydrogen atoms are not easily removed, hence they do not readily donate protons. When such a species loses a proton (hypothetically), the conjugate base formed will also have negligible basicity, as there is a lack of tendency to accept protons back into the system.
Basicity
Basicity refers to the tendency of a substance to accept protons. In an aqueous solution, bases receive protons from water, becoming more positively charged. Strong bases, like the hydroxide ions from sodium hydroxide (NaOH), accept protons readily and completely neutralize acids by converting them to water and corresponding salts.

Weak bases, by contrast, partially accept protons. Ammonia (NH鈧�) is a classic example. Since it accepts protons less eagerly, it establishes an equilibrium with its conjugate acid (NH鈧勨伜) in solution. Basicity can vary greatly depending on the structure and composition of the molecule in question, as well as its corresponding acid.

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Most popular questions from this chapter

Identify the Lewis acid and Lewis base in each of the following reactions: (a) \(\mathrm{HNO}_{2}(a q)+\mathrm{OH}^{-}(a q) \rightleftharpoons \mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\) (b) \(\mathrm{FeBr}_{3}(s)+\mathrm{Br}^{-}(a q) \rightleftharpoons \mathrm{FeBr}_{4}^{-}(a q)\) (c) \(\mathrm{Zn}^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}(a q)\) (d) \(\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)\)

Indicate whether each of the following statements is correct or incorrect. For those that are incorrect, explain why they are wrong. (a) Every Br酶nsted-Lowry acid is also a Lewis acid. (b) Every Lewis acid is also a Br酶nsted-Lowry acid. (c) Conjugate acids of weak bases produce more acidic solutions than conjugate acids of strong bases. (d) \(\mathrm{K}^{+}\) ion is acidic in water because it causes hydrating water molecules to become more acidic. (e) The percent ionization of a weak acid in water increases as the concentration of acid decreases.

Many moderately large organic molecules containing basic nitrogen atoms are not very soluble in water as neutral molecules, but they are frequently much more soluble as their acid salts. Assuming that \(\mathrm{pH}\) in the stomach is \(2.5\), indicate whether each of the following compounds would be present in the stomach as the neutral base or in the protonated form: nicotine, \(K_{b}=7 \times 10^{-7} ;\) caffeine, \(K_{b}=4 \times 10^{-14} ;\) strychnine, \(K_{b}=1 \times 10^{-6} ;\) quinine, \(K_{b}=1.1 \times 10^{-6}\)

Lactic acid (CH \(\left._{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}\right)\) has one acidic hydrogen. A \(0.10 \mathrm{M}\) solution of lactic acid has a pH of \(2.44\). Calculate \(K_{a}\)

The acid-dissociation constant for hypochlorous acid \((\mathrm{HClO})\) is \(3.0 \times 10^{-8} .\) Calculate the concentrations of \(\mathrm{H}_{3} \mathrm{O}^{+}, \mathrm{ClO}^{-}\), and \(\mathrm{HClO}\) at equilibrium if the initial concentration of \(\mathrm{HClO}\) is \(0.0090 \mathrm{M}\).
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