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16,9 Acid-Base Properties of Salt Solutions 643 (a) (b) (c) ± Figure 16.11 Depending on the ions involved, salt solutions can be neutral, acidic, or basic. These three solutions contain the acid-base indicator bromthymol blue, (a) The NaCI solution is neutral (pH = 7.0); (b) the NH4CI solution is acidic (pH = 3.5); (c) the NaCIO solution is basic (pH = 9.5). SAMPLE EXERCISE 16.17 Predict whether the salt Na2HPO4 will form an acidic or basic solution on dissolving in water. Solution Analyze and Plan: We are asked to predict whether a solution of Na2HPO4 will be acidic or basic. Because Na2HPO4 is an ionic compound, we divide it into its component ions, Na+ arid HPO42~, and consider whether each is acidic or basic. Because Na+ is the cation of a strong base, NaOH, we know that Na+ has no influence on pH. It is merely a spectator ion in acid^base chemistry. Thus, our analysis of whether the solution is acidic or basic must focus on the behavior of the HPO42~ ion. We need to consider the fact that HPO42~ can act as either an acid or a base. '(an) ^=± H+(oq) + P043~(flfl) , .[16.45] HO H2PO4~(fl
644 Chapter 16 Acid-Base Equilibria SAMPLE EXERCISE 16.18 List the following solutions in order of increasing acidity (decreasing pH): (i) 0 1 Al Ba(C2H3O2)2; (ii) 0.1 M NH4C1; (iii) 0.1 M NH3CH3Br; (iv) 0.1 M KNO3. Solution Analyze: We are asked to arrange a series of salt solutions from least acidic to most - acidic. Plan: We can determine whether the pH of a solution is acidic, basic, or neutral by identifying the ions in solution, and by assessing how each ion will affect the pH. Solve: Solution (i) contains barium ions and acetate ions. Ba2+, is the cation of the - strong base Ba(OH)2 and will therefore not affect the pH. The anion, C2H3O2", is the conjugate base of the weak acid HC2H3O2 and will hydrolyze to produce OH~ ions - thereby making the solution basic. Solutions (ii) and (iii) both contain cations that are conjugate acids of weak bases and anions that are conjugate bases of strong acids. Both solutions will therefore be acidic. Solution (i) contains NH4+, which is the conjugate acid of NH3 (Kb = 1.8 X 10~5). Solution (iii) contains NH3CH3+, which is the conjugate acid of NH2CH3 (Kb = 4.4 X 10~4). Because NH3 has the smaller Kb and is the weaker of the two bases, NH4+ will be the stronger of the two conjugate acids. Sofa-" tion (ii) will therefore be the more acidic of the two. Solution (iv) contains the K+ ion, '- which is the cari6n of the strong base KOH, and the NO3~ ion, which is the conjugate base of the stror/g acid HNO3. Neither of the ions in solution (iv) will react with water to any appreciable extent, making the solution neutral. Thus, the order of increasing acidity is 0.1 Mfea(C2H3O2)2 < 0.1 M KNO3 < 0.1 M NH3CH3Br < 0.1 M NH4C1. PRACTICE EXERCISE In each of the following, indicate which salt will form the more acidic (or less basic) 0.010 M solution: (a) NaNO3,Fe(NO3)3; (b) KBr, KBrO; (c) CH3NH3C1, BaCl2 (d)NH4NO2,NH4NO3. Answers: (a) Fe(NO3)3; (b) KBr; (c) CH3NH3C1; (d) NH4NO3 16.10 Acid-Base Behavior and Chemical Structure Bassam Z. Shakhashiri, "Effect of Molecular Structure on the Strength of Organic Acids and Bases in Aqueous Solutions," Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989) pp. 158-161. When a substance is dissolved in water, it may behave as an acid, behave as a base, or exhibit no acid-base properties. How does the chemical structure of a substance determine which of these behaviors is exhibited by the substance? For example, why do some substances that contain OH groups behave as bases, releasing OH^ ions into solution, whereas others behave as acids, ionizing to release H+ ions? Why are some acids stronger than others? In this section we will discuss briefly the effects of chemical structure on acid-base behavior. Factors That Affect Acid Strength A molecule containing H will transfer a proton only if the H—X bond is polarized in the following way: H—X In ionic hydrides such as NaH, the reverse is true; the H atom possesses a negative charge and behaves as a proton acceptor (Equation 16.23). Nonpolar H—X bonds, such as the H—C bond in CH4, produce neither acidic nor basic aqueous solutions. A second factor that helps determine whether a molecule containing an H—X bond will donate a proton is the strength of the bond. Very strong bonds are less easily dissociated than weaker ones. This factor is important, for example, in the case of the hydrogen halides. The H—F bond is the most polar H~~" ^ bond. You therefore might expect that HF would be a very strong acid if the first factor were all that mattered. However, the energy required to dissociate HF into
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