Explain the Lewis model of acid-base chemistryWrite equations for the development of adducts and complex ionsPerform equilibrium calculations involving formation constants

In 1923, G. N. Lewis proposed a generalized definition of acid-base behavior in i m sorry acids and also bases are figured out by their capacity to expropriate or come donate a pair of electron and type a coordinate covalent bond.

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A coordinate covalent bond (or dative bond) occurs once one the the atom in the bond offers both bonding electrons. For example, a coordinate covalent link occurs when a water molecule combines v a hydrogen ion to form a hydronium ion. A coordinate covalent bond additionally results when an ammonia molecule combines v a hydrogen ion to kind an ammonium ion. Both of this equations are displayed here.

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A Lewis acid is any types (molecule or ion) that have the right to accept a pair that electrons, and also a Lewis base is any varieties (molecule or ion) that have the right to donate a pair of electrons.

A Lewis acid-base reaction occurs as soon as a base donates a pair of electron to an acid. A Lewis acid-base adduct, a link that includes a coordinate covalent bond in between the Lewis acid and also the Lewis base, is formed. The adhering to equations show the general application that the Lewis concept.

The boron atom in boron trifluoride, BF3, has actually only 6 electrons in its valence shell. Being short of the preferred octet, BF3 is a very an excellent Lewis acid and reacts with plenty of Lewis bases; a fluoride ion is the Lewis basic in this reaction, donating one of its lone pairs:

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In the following reaction, each of two ammonia molecules, Lewis bases, donates a pair of electron to a silver- ion, the Lewis acid:

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Nonmetal oxides act together Lewis acids and react v oxide ions, Lewis bases, to type oxyanions:

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Many Lewis acid-base reactions room displacement reaction in i beg your pardon one Lewis basic displaces another Lewis basic from one acid-base adduct, or in i m sorry one Lewis mountain displaces an additional Lewis acid:

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The last displacement reaction shows how the reaction the a Brønsted-Lowry acid v a base fits into the Lewis concept. A Brønsted-Lowry acid such together HCl is one acid-base adduct according to the Lewis concept, and also proton transfer occurs due to the fact that a much more stable acid-base adduct is formed. Thus, return the meanings of acids and also bases in the two theories are quite different, the theory overlap considerably.

Many slightly soluble ionic solids dissolve once the concentration the the metal ion in systems is lessened through the formation of complicated (polyatomic) ions in a Lewis acid-base reaction. For example, silver- chloride disappear in a systems of ammonia since the silver ion reacts with ammonia to kind the complex ion \textAg(NH_3)_2^\;\;+. The Lewis framework of the \textAg(NH_3)_2^\;\;+ ion is:

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The equations for the resolution of AgCl in a solution of NH3 are:


\textAg^+(aq)\;+\;2\textNH_3(aq)\;\longrightarrow\;\textAg(NH_3)_2^\;\;+(aq)
\textNet:\;AgCl(s)\;+\;2\textNH_3(aq)\;\longrightarrow\;\textAg(NH_3)_2^\;\;+(aq)\;+\;\textCl^-(aq)

Aluminum hydroxide disappear in a systems of salt hydroxide or another solid base due to the fact that of the development of the complicated ion \textAl(OH)_4^\;\;-. The Lewis structure of the \textAl(OH)_4^\;\;- ion is:

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The equations because that the dissolved are:


\textAl^3+(aq)\;+\;4\textOH^-(aq)\;\longrightarrow\;\textAl(OH)_4^\;\;-(aq)
\textNet:\;Al(OH)_3(s)\;+\;\textOH^-(aq)\;\longrightarrow\;\textAl(OH)_4^\;\;-(aq)
\textHg^2+(aq)\;+\;2\textS^2-(aq)\;\longrightarrow\;\textHgS_2^\;\;2-(aq)
\textNet:\;HgS(s)\;+\;\textS^2-(aq)\;\longrightarrow\;\textHgS_2^\;\;2-(aq)

A facility ion is composed of a central atom, frequently a transition metal cation, surrounded by ions, or molecules dubbed ligands. These ligands can be neutral molecules favor H2O or NH3, or ions such as CN– or OH–. Often, the ligands act together Lewis bases, donating a pair of electrons to the central atom. The ligands accumulation themselves about the main atom, creating a new ion through a charge equal come the sum of the charges and, many often, a transitional metal ion. This more complicated arrangement is why the result ion is called a complex ion. The complex ion formed in this reactions can not be predicted; it must be determined experimentally. The species of bonds formed in complicated ions are called coordinate covalent bonds, together electrons indigenous the ligands space being mutual with the central atom. Due to the fact that of this, complex ions are occasionally referred to as coordination complexes. This will be studied more in upcoming chapters.

The equilibrium continuous for the reaction of the contents of a complex ion to kind the complex ion in solution is dubbed a formation continuous (Kf) (sometimes dubbed a stability constant). For example, the complicated ion \textCu(CN)_2^\;\;- is shown here:

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It forms by the reaction:


\textCu^+(aq)\;+\;2\textCN^-(aq)\;\rightleftharpoons\;\textCu(CN)_2^\;\;-(aq)
K_\textf = Q = \frac<\textCu(CN)_2^\;\;-><\textCu^+><\textCN^->^2

The inverse of the formation continuous is the dissociation consistent (Kd), the equilibrium consistent for the decomposition that a complicated ion right into its materials in solution. We will work-related with dissociation constants further in the exercises because that this section. Attachment K and also Table 2 space tables of development constants. In general, the larger the development constant, the much more stable the complex; however, together in the situation of Ksp values, the stoichiometry of the compound must be considered.

SubstanceKf in ~ 25 °C
<\textCd(CN)_4>^2-3 × 1018
\textAg(NH_3)_2^\;\;+1.7 × 107
<\textAlF_6>^3-7 × 1019
Table 2. Common facility Ions by decreasing Formulation Constants

As an instance of dissolved by complicated ion formation, allow us take into consideration what happens when we include aqueous ammonia come a mixture of silver chloride and also water. Silver- chloride dissolves slightly in water, offering a small concentration the Ag+ ( = 1.3 × 10–5M):


However, if NH3 is present in the water, the complicated ion, \textAg(NH_3)_2^\;\;+, can type according to the equation:


\textAg^+(aq)\;+\;2\textNH_3(aq)\;\rightleftharpoons\;\textAg(NH_3)_2^\;\;+(aq)
K_\textf = \frac<\textAg(NH_3)_2^\;\;+><\textAg^+><\textNH_3>^2 = 1.7\;\times\;10^7

The huge size that this formation continuous indicates that most of the cost-free silver ions produced by the resolution of AgCl incorporate with NH3 to kind \textAg(NH_3)_2^\;\;+. Together a consequence, the concentration of silver- ions, , is reduced, and the reaction quotient because that the dissolution of silver- chloride, , falls below the solubility product the AgCl:


More silver- chloride then dissolves. If the concentration of ammonia is good enough, every one of the silver chloride dissolves.


Example 1

Dissociation of a complicated IonCalculate the concentration that the silver- ion in a systems that originally is 0.10 M with respect to \textAg(NH_3)_2^\;\;+.

SolutionWe use the familiar path to fix this problem:

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Determine x and equilibrium concentrations. we let the adjust in concentration of Ag+ it is in x. Dissociation of 1 mol the \textAg(NH_3)_2^\;\;+ provides 1 mol of Ag+ and 2 mol of NH3, for this reason the change in is 2x and that that \textAg(NH_3)_2^\;\;+ is –x. In summary:
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Solve for x and also the equilibrium concentrations. in ~ equilibrium:

Both Q and Kf space much bigger than 1, therefore let us assume that the transforms in concentrations required to reach equilibrium room small. Therefore 0.10 – x is approximated as 0.10:


Because just 1.1% the the \textAg(NH_3)_2^\;\;+ dissociates right into Ag+ and NH3, the presumption that x is small is justified.

Now we recognize the equilibrium concentrations:


The concentration of complimentary silver ion in the systems is 0.0011 M.

Check the work. The value of Q calculated using the equilibrium concentrations is same to Kf within the error linked with the far-ranging figures in the calculation.

Check your LearningCalculate the silver ion concentration, , the a solution prepared by dissolve 1.00 g that AgNO3 and also 10.0 g that KCN in enough water to do 1.00 together of solution. (Hint: due to the fact that Q f, assume the reaction goes to perfect then calculate the created by dissociation of the complex.)


Key Concepts and Summary

G.N. Lewis suggest a definition for acids and bases that depends on an atom’s or molecule’s capacity to accept or donate electron pairs. A Lewis mountain is a species that can accept one electron pair, conversely, a Lewis base has an electron pair easily accessible for donation to a Lewis acid. Complex ions are instances of Lewis acid-base adducts. In a complicated ion, we have actually a main atom, frequently consisting of a transition metal cation, i beg your pardon acts together a Lewis acid, and also several neutral molecule or ions bordering them referred to as ligands that act as Lewis bases. Complex ions type by sharing electron bag to type coordinate covalent bonds. The equilibrium reaction that occurs when forming a complicated ion has actually an equilibrium constant associated through it referred to as a development constant, Kf. This is frequently referred to together a security constant, together it represents the stability of the complicated ion. Development of facility ions in solution can have a profound effect on the solubility the a change metal compound.


Chemistry finish of thing Exercises

Under what circumstances, if any, does a sample of solid AgCl completely dissolve in pure water?Explain why the enhancement of NH3 or HNO3 come a saturated solution of Ag2CO3 in contact with solid Ag2CO3 increases the solubility the the solid.Calculate the cadmium ion concentration, , in a solution ready by mixing 0.100 together of 0.0100 M Cd(NO3)2 through 1.150 l of 0.100 NH3(aq).Explain why addition of NH3 or HNO3 come a saturated solution of Cu(OH)2 in contact with hard Cu(OH)2 boosts the solubility of the solid.Sometimes equilibria for facility ions are described in terms of dissociation constants, Kd. For the facility ion \textAlF_6^\;\;3- the dissociation reaction is:

\textAlF_6^\;\;3-\;\rightleftharpoons\;\textAl^3+\;+\;6\textF^- and also K_\textd = \frac<\textAl^3+><\textF^->^6<\textAlF_6^\;\;3-> = 2\;\times\;10^-24

Calculate the value of the formation constant, Kf, for \textAlF_6^\;\;3-.

Using the value of the formation consistent for the complicated ion \textCo(NH_3)_6^\;\;2+, calculate the dissociation constant.Using the dissociation constant, Kd = 7.8 × 10–18, calculate the equilibrium concentrations of Cd2+ and also CN– in a 0.250-M systems of \textCd(CN)_4^\;\;2-.Using the dissociation constant, Kd = 3.4 × 10–15, calculate the equilibrium concentrations of Zn2+ and OH– in a 0.0465-M equipment of \textZn(OH)_4^\;\;2-.Using the dissociation constant, Kd = 2.2 × 10–34, calculate the equilibrium concentrations of Co3+ and also NH3 in a 0.500-M equipment of \textCo(NH_3)_6^\;\;3+.Using the dissociation constant, Kd = 1 × 10–44, calculate the equilibrium concentration of Fe3+ and CN– in a 0.333 M equipment of \textFe(CN)_6^\;\;3-.Calculate the mass of potassium cyanide ion that must be added to 100 mL of equipment to dissolve 2.0 × 10–2 mol of silver cyanide, AgCN.Calculate the minimum concentration the ammonia needed in 1.0 l of systems to dissolve 3.0 × 10–3 mol of silver- bromide.A roll of 35-mm black and also white photographic film contains around 0.27 g of unexposed AgBr before developing. What massive of Na2S2O3·5H2O (sodium thiosulfate pentahydrate or hypo) in 1.0 l of developer is forced to dissolve the AgBr together \textAg(S_2\textO_3)_2^\;\;3- (Kf = 4.7 × 1013)?We have seen an introductory an interpretation of an acid: An acid is a compound that reacts with water and also increases the amount of hydronium ion present. In the chapter on acids and also bases, we experienced two an ext definitions of acids: a compound the donates a proton (a hydrogen ion, H+) to another compound is called a Brønsted-Lowry acid, and a Lewis acid is any varieties that can accept a pair the electrons. Define why the introductory meaning is a macroscopic definition, while the Brønsted-Lowry meaning and the Lewis definition are microscopic definitions.Write the Lewis frameworks of the reactants and also product of every of the complying with equations, and also identify the Lewis acid and the Lewis basic in each:

(a) \textCO_2\;+\;\textOH^-\;\longrightarrow\;\textHCO_3^\;\;-

(b) \textB(OH)_3\;+\;\textOH^-\;\longrightarrow\;\textB(OH)_4^\;\;-

(c) \textI^-\;+\;\textI_2\;\longrightarrow\;\textI_3^\;\;-

(d) \textAlCl_3\;+\;\textCl^-\;\longrightarrow\;\textAlCl_4^\;\;- (use Al-Cl solitary bonds)

(e) \textO^2-\;+\;\textSO_3\;\longrightarrow\;\textSO_4^\;\;2-

Write the Lewis frameworks of the reactants and also product of each of the complying with equations, and identify the Lewis acid and the Lewis basic in each:

(a) \textCS_2\;+\;\textSH^-\;\longrightarrow\;\textHCS_3^\;\;-

(b) \textBF_3\;+\;\textF^-\;\longrightarrow\;\textBF_4^\;\;-

(c) \textI^-\;+\;\textSnI_2\;\longrightarrow\;\textSnI_3^\;\;-

(d) \textAl(OH)_3\;+\;\textOH^-\;\longrightarrow\;\textAl(OH)_4^\;\;-

(e) \textF^-\;+\;\textSO_3\;\longrightarrow\;\textSFO_3^\;\;-

Using Lewis structures, write balanced equations because that the complying with reactions:

(a) \textHCl(g)\;+\;\textPH_3(g)\;\longrightarrow

(b) \textH_3\textO^+\;+\;\textCH_3^\;\;-\;\longrightarrow

(c) \textCaO\;+\;\textSO_3\;\longrightarrow

(d) \textNH_4^\;\;+\;+\;\textC_2\textH_5\textO^-\;\longrightarrow

Calculate <\textHgCl_4^\;\;2-> in a solution prepared by including 0.0200 mol the NaCl come 0.250 together of a 0.100-M HgCl2 solution.In a titration that cyanide ion, 28.72 mL of 0.0100 M AgNO3 is included before precipitation begins. \textAg(CN)_2^\;\;- complex.> Precipitation of heavy AgCN takes ar when overfill Ag+ is added to the solution, above the amount essential to finish the development of \textAg(CN)_2^\;\;-. How many grams of NaCN to be in the original sample?What are the concentrations of Ag+, CN–, and \textAg(CN)_2^\;\;- in a saturated systems of AgCN?In dilute aqueous equipment HF acts together a weak acid. However, pure liquid HF (boiling point = 19.5 °C) is a strong acid. In fluid HF, HNO3 acts choose a base and accepts protons. The mountain of fluid HF deserve to be increased by including one that several not natural fluorides that space Lewis acids and accept F– ion (for example, BF3 or SbF5). Write well balanced chemical equations for the reaction the pure HNO3 with pure HF and of pure HF through BF3.The simplest amino acid is glycine, H2NCH2CO2H. The typical feature of amino acids is the they save on computer the functional groups: an amine group, –NH2, and also a carboxylic mountain group, –CO2H. One amino mountain can duty as either an acid or a base. For glycine, the acid stamin of the carboxyl team is about the very same as the of acetic acid, CH3CO2H, and also the base stamin of the amino team is slightly better than the of ammonia, NH3.

(a) create the Lewis structures of the ion that kind when glycine is dissolved in 1 M HCl and also in 1 M KOH.

(b) compose the Lewis structure of glycine once this amino mountain is liquified in water. (Hint: consider the relative base staminas of the –NH2 and -\textCO_2^\;\;- groups.)

Boric acid, H3BO3, is no a Brønsted-Lowry acid yet a Lewis acid.

(a) create an equation for its reaction v water.

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(b) guess the form of the anion thus formed.

(c) What is the hybridization top top the boron regular with the form you have predicted?