Ch.16 - Aqueous Equilibria: Acids & Bases

Problem 13

Chapter 16, Problem 13

Consider the reaction: SO2 + OH- S HSO3 -. Which reaction scheme shows the correct use of the curved arrow notation representing the donation of an electron pair and the correct labeling of the Lewis acid and Lewis base? (LO 16.14) (a) (b) (c) (d)

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Identify the Lewis acid and Lewis base in the reaction. A Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair.

Examine each reaction scheme option to determine which one correctly shows the OH- ion donating an electron pair. The OH- ion should be depicted with a curved arrow originating from it, indicating the donation of an electron pair.

Check the direction of the curved arrow; it should point from the electron pair donor (Lewis base) to the electron pair acceptor (Lewis acid).

Verify that the Lewis acid in the reaction scheme is correctly labeled. In this reaction, the SO2 molecule should be labeled as the Lewis acid since it is accepting an electron pair.

Ensure that the product formed in the reaction scheme is HSO3-, which results from the addition of the OH- ion to the SO2 molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lewis Acids and Bases

Lewis acids are substances that can accept an electron pair, while Lewis bases are those that can donate an electron pair. In the given reaction, SO2 acts as a Lewis acid because it accepts an electron pair from the hydroxide ion (OH-), which serves as the Lewis base by donating its electron pair.

Curved Arrow Notation

Curved arrow notation is a visual representation used in organic chemistry to illustrate the movement of electron pairs during chemical reactions. The tail of the arrow indicates the source of the electron pair (the Lewis base), while the head points to the atom that will receive the electrons (the Lewis acid), helping to clarify the mechanism of the reaction.

Reaction Mechanism

A reaction mechanism describes the step-by-step process by which reactants are converted into products. Understanding the mechanism involves identifying the roles of different species, such as the Lewis acid and base, and how they interact through electron transfer, which is crucial for accurately depicting the reaction using curved arrows.

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Related Practice

Open Question

Determine the following concentrations for a 0.40 M H2Se solution that has the stepwise dissociation constants of Ka1 = 1.3 * 10^-4 and Ka2 = 1.0 * 10^-11. (LO 16.10) (a) [H2Se] = 0.35, [HSe^-] = 5.0 * 10^-2, [H3O^+] = 3.0 * 10^-3, [Se^2-] = 1.3 * 10^-4 (b) [H2Se] = 0.39, [HSe^-] = 7.2 * 10^-3, [H3O^+] = 7.2 * 10^-3, [Se^2-] = 1.0 * 10^-11 (c) [H2Se] = 0.31, [HSe^-] = 9.0 * 10^-2, [H3O^+] = 9.0 * 10^-2, [Se^2-] = 1.0 * 10^-11 (d) [H2Se] = 0.40, [HSe^-] = 1.3 * 10^-4, [H3O^+] = 1.3 * 10^-4, [Se^2-] = 1.0 * 10^-11

Textbook Question

Ammonia 1NH32 has base dissociation constant 1Kb2 of 1.8 * 10-5. What is the concentration of an aqueous ammonia solution that has a pH of 11.68? (LO 16.11) (a) 0.28 M (b) 3.6 M (c) 9.0 * 10-3 M (d) 1.3 M

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Open Question

Using values of Kb in Appendix C, calculate values of Ka for each of the following ions: (b) Hydroxylammonium ion, NH3OH+; (c) Anilinium ion, C6H5NH3+; (d) Pyridinium ion, C5H5NH+.

Open Question

For each of the Lewis acid–base reactions in Problem 16.138, draw electron-dot structures for the reactants and products, and use the curved arrow notation to represent the donation of a lone pair of electrons from the Lewis base to the Lewis acid.

Open Question

For a solution of two weak acids with comparable values of Ka, there is no single principal reaction. The two acid dissociation equilibrium equations must therefore be solved simultaneously. Calculate the pH in a solution that is 0.10 M in acetic acid (CH3CO2H, Ka = 1.8 * 10^-5) and 0.10 M in benzoic acid (C6H5CO2H, Ka = 6.5 * 10^-5). (Hint: Let x = [CH3CO2H] that dissociates and y = [C6H5CO2H] that dissociates; then [H3O+] = x + y.)

Textbook Question

What is the pH and the principal source of H3O+ ions in 1.0 * 10-10 M HCl? (Hint: The pH of an acid solution can’t exceed 7.) What is the pH of 1.0 * 10-7 M HCl?

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