A Lewis acid is a species that can accept an electron pair, whereas a Lewis base has an electron pair available for donation to a Lewis acid. Complex ions are examples of Lewis acid-base adducts and comprise central metal atoms or ions acting as Lewis acids bonded to molecules or ions called ligands that act as Lewis bases. The equilibrium constant for the reaction between a metal ion and ligands produces a complex ion called a formation constant; for the reverse reaction, it is called a dissociation constant.
Practice Exercises
Even though Ca(OH)2 is an inexpensive base, its limited solubility restricts its use. What is the pH of a saturated solution of Ca(OH)2?
Under what circumstances, if any, does a sample of solid AgCl completely dissolve in pure water?
Solution
when the amount of solid is so small that a saturated solution is not produced
Explain why the addition of NH3 or HNO3 to a saturated solution of Ag2CO3 in contact with solid Ag2CO3 increases the solubility of the solid.
Calculate the cadmium ion concentration, [Cd2+], in a solution prepared by mixing 0.100 L of 0.0100 M Cd(NO3)2 with 1.150 L of 0.100 NH3(aq).
Solution
$1.8×10^{-5}\;M$
Explain why addition of NH3 or HNO3 to a saturated solution of Cu(OH)2 in contact with solid Cu(OH)2 increases the solubility of the solid.
Sometimes equilibria for complex ions are described in terms of dissociation constants, Kd. For the complex ion $AlF_6^{3-}$ the dissociation reaction is:
$AlF_6^{3-}⇌Al^{3+}+6F^-$ and $K_d=\frac{[Al^{3+}][F^-]^6}{[AlF_6^{3-}]}=2×10^{-24}$
Calculate the value of the formation constant, Kf, for $AlF_6^{3-}$.
Solution
$5×10^{23}$
Using the value of the formation constant for the complex ion $Co(NH_3)_6^{2+}$, calculate the dissociation constant.
Using the dissociation constant, Kd = $7.8×10^{-18}$, calculate the equilibrium concentrations of Cd2+ and CN– in a 0.250-M solution of $Cd(CN)_4^{2-}$.
Solution
[Cd2+] = $9.5×10^{-5}\;M$;$[CN^-]=3.8×10^{-4}\;M$
Using the dissociation constant, Kd = $3.4×10^{-15}$, calculate the equilibrium concentrations of Zn2+ and OH– in a 0.0465-M solution of $Zn(OH)_4^{2-}$.
Using the dissociation constant, Kd = $2.2×10^{-34}$, calculate the equilibrium concentrations of Co3+ and NH3 in a 0.500-M solution of $Co(NH_3)_6^{3+}$.
Using the dissociation constant, Kd = $1×10^{-44}$, calculate the equilibrium concentrations of Fe3+ and CN– in a 0.333 M solution of $Fe(CN)_6^{3-}$.
Calculate the mass of potassium cyanide ion that must be added to 100 mL of solution to dissolve $2.0×10^{-2}$ mol of silver cyanide, AgCN.
Solution
1.3 g
Calculate the minimum concentration of ammonia needed in 1.0 L of solution to dissolve $3.0×10^{-3}$ mol of silver bromide.
A roll of 35-mm black and white photographic film contains about 0.27 g of unexposed AgBr before developing. What mass of Na2S2O3·5H2O (sodium thiosulfate pentahydrate or hypo) in 1.0 L of developer is required to dissolve the AgBr as $Ag(S_2O_3)_2^{3-}$ $(K_f=4.7×10^{13})$?
Solution
0.79 g
We have seen an introductory definition of an acid: An acid is a compound that reacts with water and increases the amount of hydronium ion present. In the chapter on acids and bases, we saw two more definitions of acids: a compound that donates a proton (a hydrogen ion, H+) to another compound is called a Brønsted-Lowry acid, and a Lewis acid is any species that can accept a pair of electrons. Explain why the introductory definition is a macroscopic definition, while the Brønsted-Lowry definition and the Lewis definition are microscopic definitions.
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:
Solution:
(b)
(c)
(d)
(e)
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:
Using Lewis structures, write balanced equations for the following reactions:
Solution:
Calculate $[HgCl_4^{2-}]$ in a solution prepared by adding 0.0200 mol of NaCl to 0.250 L of a 0.100-M HgCl2 solution.
In a titration of cyanide ion, 28.72 mL of 0.0100 M AgNO3 is added before precipitation begins. [The reaction of Ag+ with CN– goes to completion, producing the $Ag(CN)_2^-$ complex.] Precipitation of solid AgCN takes place when excess Ag+ is added to the solution, above the amount needed to complete the formation of $Ag(CN)_2^-$. How many grams of NaCN were in the original sample?
Solution
0.0281 g
What are the concentrations of Ag+, CN–, and $Ag(CN)_2^-$ in a saturated solution of AgCN?
In dilute aqueous solution HF acts as a weak acid. However, pure liquid HF (boiling point = 19.5 °C) is a strong acid. In liquid HF, HNO3 acts like a base and accepts protons. The acidity of liquid HF can be increased by adding one of several inorganic fluorides that are Lewis acids and accept F– ion (for example, BF3 or SbF5). Write balanced chemical equations for the reaction of pure HNO3 with pure HF and of pure HF with BF3.
The simplest amino acid is glycine, H2NCH2CO2H. The common feature of amino acids is that they contain the functional groups: an amine group, –NH2, and a carboxylic acid group, –CO2H. An amino acid can function as either an acid or a base. For glycine, the acid strength of the carboxyl group is about the same as that of acetic acid, CH3CO2H, and the base strength of the amino group is slightly greater than that of ammonia, NH3.
(a) Write the Lewis structures of the ions that form when glycine is dissolved in 1 M HCl and in 1 M KOH.
(b) Write the Lewis structure of glycine when this amino acid is dissolved in water. (Hint: Consider the relative base strengths of the –NH2 and –$CO_2^-$ groups.)
Boric acid, H3BO3, is not a Brønsted-Lowry acid but a Lewis acid.
(a) Write an equation for its reaction with water.
(b) Predict the shape of the anion thus formed.
(c) What is the hybridization on the boron consistent with the shape you have predicted?
Solution
(a) $H_3BO_3+H_2O⟶H_4BO_4^−+H^+$;
(b) The electronic and molecular shapes are the same—both tetrahedral. (c) The tetrahedral structure is consistent with sp3 hybridization.
