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JChemEd.chem.wisc.edu Vol. 78 No. 11 November 2001 Journal of Chemical Education 1495

As a teacher of general chemistry for more than 20 years,I have sought to teach each subject efficiently and effectively.One subject that provokes many questions and takes its tollon student grades is the myriad relationships existing betweenstrong and weak acids, bases, and their conjugates. Adding to students confusion is the inexactness of the termsstrong ,weak , and very weak in regard to these species and how thesequalifiers affect the interaction of acids and bases with eachother and with water, itself both acid and base. Adding to thisthe concept of the leveling effect that occurs whenstrong acids/bases are placed in water and the non-interaction whenvery weak acids/bases are involved can create chaos in the mind of a student trying to make predictions and draw insight andsolve

quantitative problems . The problem is that each relationship,although simple, is linked to every other and we teach themindividually in separate sections and expect students to bediligent and thoughtful enough to make the connections thatpracticing chemists/scientists take for granted.

Spurred by the persistence of a particularly challenging class during the summer of 2000, which demanded succinctanswers, I devised a figure (Fig. 1) that depicts or implies

all the relationships simultaneously. The students found ithelpful, and compared with students in all previous classesthey improved their grades on the examination that washeavily weighted with acid/base equilibria and buffer questions.

The Figure

The assumptions implicit in Figure 1 are (i) the use of logarithmic pK values; (ii) the relationship that pK a + pK b =pK w ; and (iii) ignoring the leveling effect of water in orderto suggest it! Logarithmic pK a values for a series of inorganicand organic monobasic acids including the hydrohalic acids andthe weak base ammonia are included. In each example the pK a value for the conjugate acid and the pK b value for its comple-mentary conjugate base are shown aligned with the numericscale above and with the corresponding symbol/formula atthe left. Most acids and their conjugates are considered dilute;the one exception is water, which as both conjugate acid andbase is by nature 55.33 mol/L at 25 C. This skews the valuesfor the hydroxide and oxonium ions to 1.74 pH units greaterthan 14 and less than zero. The pK a and pK b values tabulatedare derived fromK a and K b values commonly quoted in generalchemistry texts (1, 2 ) and a general inorganic text ( 3). Thefigure is descriptive, not quantitative, although an effort wasmade to not misrepresent values for any chemical species. Foreach species, the sum of the pK a and pK b values is 14.00. In

fact, this relationship was used to calculate approximate valuesfor the conjugate base strengths for chloride, bromide, iodide,and nitrate ions.

Application of the Figure

The concepts of strong, weak and very weak can now be quantified for both acids and bases, although few strong nonionic, non-hydroxide bases are commonly known. Quan-tification of these qualifiers allows important and definitiveinferences to be made about the reactions possible when acidsand bases in each category are dissolved in water. For example,values for the hydrohalic acids HX (X = I, Br, Cl) are estimatedfrom thermodynamic considerations, since they immediately react with water to form the hydrated oxonium ion. Allacids stronger than the oxonium ion are thus leveled to theacid H3O +(aq) when actually dissolved in (reacted with)

water. The corresponding conjugate bases of these acids, theanions I , Br , and Cl are very weak and will not hydrolyze.

A strong acid is one whose pK a is smaller than 1.74;only a negligible amount of the original acid will remain aftermixing, and the reaction with water is effectively 100%. Thisreaction is the leveling effect of water on acids stronger thanthe aqueous hydrogen ion. A very weak acid is one whosepK a is greater than 15.74 and it will not react with water toeffectively alter pH. This point allows one to logically explain

why the very weak conjugate bases of strong acids (iodide,

Teaching BrnstedLowry AcidBase Theory in a Direct Comprehensive Way

Jamie L. Adcock Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600; Jadcock@utk.edu

Figure 1. The numerical values refer to pK a values for the conjugateacids (CA, ) and p K b values for the conjugate bases (CB, ). Themodifiers strong, weak, and very weak apply to either acid or base.The divisions implied by the shading are somewhat arbitrary, butencompass the 014 pH scale where p K a /p K b values apply mostimportantly to buffers.

-5

HII

HBrBr

HClCl

HFF

H3O+

H2O

HNO 3NO3

HClO 2ClO 2

H2CO 2HCO 2

HOAcAc

HCNCN

NH4+

NH3

H2OOH

Strong Weak Very weak

0 5 10 15 20

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1496 Journal of Chemical Education Vol. 78 No. 11 November 2001 JChemEd.chem.wisc.edu

bromide, chloride, etc.) do not hydrolyze; their pK b valuesare larger than 15.74.

A weak acid is one whose pK a is between 15.74 andabout 2.6 (if one assumes 5% dissociation); it will produce a non-negligible change in pH due to partial reaction (equili-bration) with water. Conjugates of weak acids are also weak bases and as a result undergo hydrolysis. Conjugate base pK bvalues are displayed on the same scale with the conjugate acidspK a value and utilize the same qualifiers. A strong base is one

whose pK b is smaller than 1.74; only a negligible amount of the base will remain because the reaction with water is effec-tively 100%. This reaction is the leveling effect of water onbases stronger than the aqueous hydroxide ion. A very weak base is one whose pK b is larger than 15.74. Very weak bases

will not react with water to effectively alter pH; that is, they do not hydrolyze if they are ions. The pK b of a weak base isbetween 15.74 and about 2.6 (if one assumes 5% dissociation)and a weak base will produce a non-negligible change in pH.

This layout illustrates the complementary nature of theconjugates of acids and bases and makes the point that thereis essentially no difference in nature between the acids HCN

and NH 4+ except in the way we, as chemists, have previously classified them. The same relationship is seen with the con-

jugate bases CN and NH 3. We thus explicitly describe thehydrolysis of salts of weak acids and bases as simply a conse-quence of having an ionic or charged acid or base rather thana neutral, molecular one.

ConclusionThis figure enables a lecturer to unify all the Brnsted

Lowry acidbase concepts in a quantitative way. It is frustrating when students who are introduced to acidbase behaviormemorize numerous specific equations as a problem-solving approach. These students fail to see the exquisite unity presentedin the general concept. A few of my better students will realize

that two simple equilibria and two simple equations can be usedto solve all acid, base, buffer, and hydrolysis problems. Using this diagram in my general chemistry classes as a handouthas resulted in more students realizing this unity of concept,and that makes the effort worthwhile.

Acknowledgment I would like to acknowledge the helpful suggestions of

George K. Schweitzer and the persistence and determinationof a gifted summer 2000 class in general chemistry.

Literature Cited

1. Hill, J. W.; Petrucci, R. H. General Chemistry , 2nd ed.; PrenticeHall: Upper Saddle River, NJ, 1999.2. Ebbing, D. D.; Gammon, S. D. General Chemistry , 6th ed.;

Houghton Mifflin: Boston, NY, 1999.3. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements ,

2nd ed.; Butterworth Heinemann: Boston, 1997.

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