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Chapter 16
Acids and Bases
Concept Check 16.
Chemists in the seventeenth century discovered that the substance that gives red ants their
irritating bite is an acid with the formula HCHO 2. They called this substance formic acid after
the ant, whose Latin name is Formica rufus. Formic acid has the following structural formula and molecular model.
H C O
O H
Write the acid-base equilibria connecting all components in the aqueous solution. Now list all of the species present.
Solution
In any aqueous solution, you should consider the autoionization of water. And because we have a solution of a weak acid in water, you should also consider the equilibrium between this acid and water. Here are the two equilibria: H 2 O( l ) + H 2 O( l ) H 3 O+( aq ) + OH-( aq ) HCHO 2 ( aq ) + H 2 O( l ) CHO 2 - ( aq ) + H 3 O+( aq ) The species present in these equilibria are: H 2 O( l ), H 3 O+( aq ), OH-( aq ), HCHO 2 ( aq ), and CHO 2 - ( aq ).
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Concept Check 16.
Formic acid, HCHO 2 , is a stronger acid than acetic acid, HC 2 H 3 O 2. Which is the stronger base, formate ion, CHO 2 - , or acetate ion, C 2 H 3 O 2 -?
Solution
The stronger acid gives up its proton more readily, and therefore its conjugate base ion holds onto a proton less strongly. In other words, the stronger acid has the weaker conjugate base. Because formic acid is the stronger acid, the formate ion is the weaker base. Acetate ion is the stronger base.
Concept Check 16.
Rank the following solutions from most acidic to most basic (water molecules have been omitted for clarity).
Solution
Look at each solution, and determine whether it is acidic, basic, or neutral. In solution A, the numbers of H 3 O+^ and OH-^ ions are equal, so the solution is neutral. For solution B, the number of H 3 O+^ ions is greater than the number of OH-^ ions, so the solution is acidic. In solution C, the number of H 3 O+^ ions is less than the number of OH-^ ions, so the solution is basic. Therefore, ranking from most acidic to least acidic (most basic) is B > A > C.
Concept Check 16.
You have solutions of NH 3 , HCl, NaOH, and HC 2 H 3 O 2 (acetic acid), all with the same solute concentrations. Rank these solutions in order of pH, from the highest to lowest.
Solution
126 Chapter 16
Solution
The hydroxide ion acts as a base and donates a pair of electrons to the O atom, forming a bond with CO 2 to give HCO 3 –.
Conceptual Problem 16.
Compare the structures of HNO 2 and H 2 CO 3. Which would you expect to be the stronger acid? Explain your choice.
Solution
Nitrogen has greater electronegativity than carbon. You would expect the H—O bond in the H—O—N group to be more polar (with the H atom having a positive partial charge) than the H—O bond in the H—O—C group. Thus, based on their structure, you would expect HNO 2 to be the stronger acid.
Conceptual Problem 16.
The value of the ion-product constant for water, Kw , increases with temperature. What will be the effect of lowering the temperature on the pH of pure water?
Solution
When you lower the temperature of pure water, the value of Kw decreases. In pure water, the hydronium ion concentration equals the hydroxide ion concentration, so Kw = [H 3 O+]^2. When Kw decreases, the hydronium ion decreases, and the corresponding pH increases.
Conceptual Problem 16.
You make solutions of ammonia and sodium hydroxide by adding the same moles of each solute to equal volumes of water. Which solution would you expect to have the higher pH?
Acids and Bases 127
Solution
Sodium hydroxide is a strong base, whereas ammonia is weak. As a strong base, NaOH exists in solution completely as ions, whereas NH 3 exists in solution as an equilibrium in which only part of the NH 3 has reacted to produce ions. Thus, a sodium hydroxide solution has a greater OH–^ concentration than the same concentration solution of NH 3. At the same concentrations, the pH of the NaOH solution is greater (more basic) than the NH 3 solution.
