Subject: further discussion on acid/base chemistry These are answers sent to students concerning some homework problems dealing with acids and bases. 16.21 (16.23 in 11th ed, 16.22 in 10th ed.) - Amphoterism and Acid-Base Strength: OH- would be considered amphoteric (can act as an acid by donating an H+ and as a base by accepting). Although OH- is a base in water it could act as an acid by losing an H+. When it does it's conjugate base is O^2-, OH- -> H^+ + O^2- The O^2- ion is a strong base and will accept an H+ to form its conjugate acid, OH-, O^2- + H+ -> OH- For NH3 (a weak base) this will act as base and will accept an H+ to form its conj. acid, NH4+ For NH4+ this will act as an acid and lose an H+ to form its conj. base, NH3 NH4+ -> NH3 + H+ NH3 + H+ -> NH4+ Since NH3 is amphoteric it could act as an acid under the right conditions: NH3 -> NH2^- + H+ The conj. base of NH3 acting as an acid is NH2^-. This makes NH2^- a strong base. OH-/O^2- and NH4+/NH3 are conj. acid/base pairs. The conj. base of an acid is found by removing an H+ from the acid. The conj. acid of a base is found by adding and H+ to the base. 16.24 (16.26 in 11th ed., 16.24 in 10th ed.) - Acid-Base Strength: When you think about the relative strengths of acids and bases and acid-base reactions you should think of them as a competition for the H+ ion. A substance that readily donates a proton to H2O is considered and acid in an aqueous solution. A substance that accepts and H+ from water is considered a base. Both NH3 and H2O are amphoteric (they have H atoms that can be donated as H+ ions and thus act as acids and lone-pair electrons that can accept an H+ and thus act as bases). Thus, either NH3 or H2O can act as an acid or a base. When NH3 is mixed in H2O there is a competition for the proton. Since the NH3 acts as the base in H2O it is accepting a proton from the H2O (and the H2O donates the H+). Thus NH3 has the stronger tendency to accept the H+ than H2O (otherwise the H2O would accept the proton and act as the base and the NH3 would act as an acid, but we know it is a base in H2O). 16.28 (16.30 in 11th ed., 16.28 in 10th ed.) - Autoionization of H2O: There are 2 ways to write the autoionization of H2O. 2 H2O <=> H3O+ + OH- or H2O <=> H+ + OH- The 2nd eqn is just a shorthand way of writing the first eqn. The H+ and H3O+ are used interchangeably to represent a hydrated proton. Kw = [H+] [OH-] H2O does not appear in Kw since the H2O is a pure liquid (only minute amounts of H2O ionize) and its conc. is considered constant. 16.42 (16.44 in 11th ed., 16.42 in 10th ed.) - Strong Bases: A strong base completely dissociates. For all ionic compounds when they dissolve the amount that dissovles completely dissociates. Group 1A and 2A hydroxides are very soluble (except Mg(OH)2) and completely dissociate. Mg(OH)2 is not very soluble compared to other group 2A hydroxides so you can't get very conc. solutions of OH-. However, what ever amount does dissolve completely dissociates so it is still considered a strong base (although you can't get a concentrated soln of OH-). Most other hydroxides are pretty insoluble (such as Fe(OH)3, Al(OH)3, etc.). Thus you could only get very dilute solutions of OH- and some don't completely dissociate so they aren't considered strong bases. Group 1A hydroxides are fairly easy to deal with since there's 1 OH- per formula unit (as noted above). Thus a 0.1 M NaOH solution is 0.1M in OH-. Group 2A is a little trickier. A 0.1 M Ca(OH)2 is 0.2 M in OH- (since there are 2 OH- per formula unit). There are a few other less common strong bases, such as H-, O^2-, S^2-, N^3-, NH^2-, NH2^-, CH3- (conj. bases of hydrocarbons), CH3O- (conj. bases of alcohols) that you might encounter. All other bases are considered weak bases, not because they don't dissolve (they do), but because they do not completely ionize. One example is NH3. When you make a solution of 1 M NH3 all the NH3 does not react with H2O to produce 1 M OH-. You get a conc. of OH- that is less than that of the NH3 (how much OH- is formed is determined by setting up an equilibrium, ICE, table for NH3). Most of the NH3 remains unreacted. NH3 + H2O <=> NH4+ + OH- For 1 M NaOH you get 1 M OH- because all the NaOH dissociates. NaOH(s) + H2O(l) ==> Na+(aq) + OH-(aq) When the NaOH dissolves you don't have NaOH "molecules" in solution. You get the ions and no NaOH "molecules". So 1 mole of NaOH produces 1 mole of Na+ and 1 mole of OH-. As described above 1 mole of NH3 does NOT produce 1 mole of NH4+ and 1 mole of OH- because the NH3 is a weak base and does not react completely with H2O.