Quality Levels and the ~rgnstedTheory
Westminster Hieh School is located in the northern suburbs of Denver, ~oloradoo.At present the student enrollment is just un der 2000. Most students come from families who own or are hu ying their homes, which tends to lend stability to the enro! Ilment. However. due to the lame number of new homes going up in our area, the total enrollment has been increasing stt!adilv. The maioritv of our students come from homes of hhl e c&ar or subprofessional workers. Only ahout 35%subselpently enroll in college. Consequently our courses must he ge.ared to satisfy the needs of a diversity of students. We offer re1d a r hizh school chemistrv, advanced h la cement chemistrv, an (1 :i nvw senlcster course called practical chemistry. Practical chemistrv was initiated this school year with a n en rollment of 50 in t h o classes each semester. This is a lahor ieuted course in which students, usually without any prev i)us ~ chemistry course, work on individual projects making Such things as cosmetics, cleansers, fertilizers, etc., using as a Iruide such hooks as Stark's "The Formula Book." Advanced placement chemistry is open to students who earn a t least a "B" in hieh school chemistrv. and the class USually consists of 15-20itudents. High schbol chemistry is OPten to students who have earned a t least a "C"in aleehra. an d attracts students of all ability levels. We currentl; have aho u t 120 students enrolled in five classes.
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Miaintaining Standards for a Diversity of Students In spite of the diversity of students in high school chemistry, wc: believe that it is important to teach a rigorous course so t h a t those students going on in science or related fields will haIve an excellent hackeround in chemistrv. This can be done i f ]~tis made clear to allstudents that they are not all expected to learn evervthine covered in the course. For this reason we haIve divided our list of expected student accomplishments as follows:
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FC>rAll Students 1) Students shall understand the conservation laws and how they apply to chemical reactions. 2) Students shall have an amreciation of the historical developmlmt of our knowledge of atom&structure and the periodic table.
R Y Dr. Bank's piece "Quality Levels and the Br$nsted Theory" is the first of a series of personal accounts of chemistry teaching experiences, strategies andlor philosophies written by respected high sehwl teachers. VIEW will appear on even-numbered months in the future. Dr. Bank received an AB degree from Brooklyn College, and both MS and PhD degrees from the University of Denver. She has served on the staffs of Brooklyn Evelyn Bank Westminister High School College and Colorado College. She has been teaching at WestWestminster. Colorado minister Hieh School since 1958. Her message seems to he that quality chemistry need not be the stepchild of instructional innovation. 548 1 Journal of Chemical Education
3) Students shall have an appreciation of how atomic structure and the oeriadic table relate to chemical reactivitv and bondine. 4 1 Srurlenrc shnll haven hsiir underitandmg of the kinetic mw lccular theory and [he idral gas laws. 51 Srudeurs shall have an appwciatron of elementary thermodynamics, equilibrium, chemical kinetics, electrochemistry, and oxidation-reduction. 6) Students shall have some understanding of how substances behave in solution, including colligative properties, ionization, dissociation. and acids and hases. 7 1 Students shall have wme knowledge oi descriptibe inorganic. orynnlc, nnd nuclcx chemistry. h, Studcnra shnll haven hmir idea of how scientists work, and the potential of chemistry in solving today's problems. 9) Students shall be able to carry out quantitative laboratory experiments involving the use of such measuring devices as the balance, buret, pipet, graduated cylinder, thermometer, barometer, etc. 101 Students shall he able to make descriotive observations in the laboratory, and draw conclusions from these observations. 11) Students shall be able to perform tests to identlfy selected anions and cations. For Students Going on in Chemistty or Related Scientific Fields 1) Students shall have a greater depth of understanding of atomic structure, the periodic table, and chemical bonding including hybridization of bonds and shapes of molecules and ions. 2) Students shall be able to balance equations including oxidatian-reduction equations, and work problems based on stoichiometry and the ideal gas laws. 3) Students shall have a basic understandingof elementary thermodynamics, electrochemistry,equilibrium, and kinetics. 41 Students shall he able to solve orohlems based on simole therrndynamirs, the ndl~gilr~rr properties of sul,stanccv in r h t i o n , and molsrity and normahty.
Thus if students who do not plan to go on in chemistry or a related field. cannot balance enuations or do stoichiometrv problems, ~ ' m n o too t concernei, provided they know there is a law of couservation of matter which makes i t ~ o s s i b l eto balance equations and work stoichiometry problems. Unless students eo on in chemistrv. thev are not eoine to remember long they have spent trying to all the de'tails, no matter master the oroblems and intricate theoretical ideas and concepts. Too k e n , I think, chemistry teachers slow down the Dace in a n attemvt to accommodate the slower students, and as a consequence, they must leave out portions of the text. I feel this is a mistake. All students, whether thev po on in science or not, should have the opportunity t o he exposed to the entire range of topics covered in a good high school chemistry text. From evaluations which I request my students to write a t the conclusion of each year, I find many students, esperially those nor science-oriented, indicate that they enjoyed the second semeitcr more than the first semester. This . is the semester when most of the descrintive chemistrv is covered, and this is the very portion left out of many courses. All students profit from a course so paced as to cover all the material in the text during the school year. T h e fact that a student fails to grasp the concept when it is first presented does not mean it is forever lost to him. By continually relating new concepts and facts to those previously covered, students who didn't quite get i t the first time around, will often suhsequently "see the light". This type of enriching reinforcement leads to better understanding and greater retention than
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would otherwise he the case. In our judgment, i t is hetter for a student to learn 50% of 100% of the text than 70% or even 80% of 60%or less of the text. Consequently we have adjusted our grading scale accordingly, so 50% of the total points possible constitutes a "C", 70% a "B", and 85%an "A". This has been our policy for over fifteen years. I t has been shown to he valid in that, of those students who go on in chemistry in college, very few, if any, receive lower grades than they earned in high school chemistry, and some even receive higher grades. For many years i t has been our custom to give quarterly assignment sheets to students before the beginning of each quarter. This enables students to plan their work. They know when exams and labs are to he given, and when lab reports and assignments are due. Hopefully, they can schedule study time even if they have a commitment the night before a scheduled exam. We feel that teaching students to he responsible, to plan ahead, and to he self-reliant is far more important than the chemistry they learn. Teaching Hydrolysis with the Brlnsted-Lowry Theory
Manv recent texts used in high school chemistry do a fine job with the theoretical aspectsof chemistry, hut one area of chemistry which I feel is not usually presented in a manner consistent with present knowledge &&concepts, is hydrolysis. Most texts and probably most high school teachers still go along with the statement that the-salts which hydrolyze are those which contain the negative ion of a weak acid or the positive ion of a weak hase. This gives no indication of what actually is involved. I orefer to aonroach hvdrolvsis as a natural conseauence of the ~rQnsteci-'Lowrydifinitibn of an acid and a base. After oresentine the definition of an acid as a oroton donor and a base as aproton acceptor, we proceed t o make a tahle of Brdnsted acids and their coniuaate hases according to decreasing acid strength, which w; define as the tendency to give up a proton. Following is a typical tahle. Acid HCIOI HI
Base C101I-
Acid CHaCOOH
HCI . .-.
CI..
NH>+
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HNOa HzSOI H:IO+ HSOa-
NOsHSOIHzO SOP
H C ~ ~ HP042Hz0 OH-
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From this tahle we find that the stronger the Hr0nsted acid, the we~kerwill b rheconiurate Brdnsted hase, and rice versa. The leveling effect is then very easy to understand. Any acid which is stronger than hydronium ion, when put into water, will immediately donate its proton t o water and so the strongest BrQnsted acid which can exist in water solution is the hydronium ion. Acids which are weaker than hydronium ion will not give up their protons completely to water and an equilibrium will exist between the BrQnsted acid, hydronium ion and the conjugate base. HB + H20 = H30+ + BI t must he made clear that Brdnsted acids are not necessarily neutral compounds hut may he ions containing a t least one nroton which can he donated. Examokes of such Brdnsted ~ + At , acids are HS-, HzP04-, Ag(HzO)zf, F ~ ? H ~ o ) ~ o Hktc. this time i t can he noted that all metal ions except Group IA and Group IIA elements from magnesium on down, are actually aquo complexes in water solution, and thus all are BrQnsted acids. They donate protons in water solution in the manner illustrated for a typical trivalent metal ion M ( H Z O ) ~+ + HZO = M(H~O)SOHZ+ +H~O+ The M(HZO)SOHZ+may also be aBrQnsted acid and proceed to donate another proton.
M(HzO)sOH2++ Ht0 = M(HzO)dOH)z++ H30+ and again
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M(H20)1(0H)z+ Hz0 = M(H20)dOH)d + H30' When the neutral hydroxide precipitates, the whole hydrolysis process continues a t a greater rate in the forward direction since one of the products is removed from the field of action as a precipitate. I t is easy t o demonstrate this with a solution of iron(II1) chloride whichhas remained on the shelf for a period of time. Now it can he stated that hydrolysis of a salt leading t o an acid solution will take place when the positive ion is a strong enough BrQnsted acid that a t any given instant some of the BrQnsted acid will have given up its proton to water forming hydronium ion. Positive ions which contain no protons cannot hydrolyze in water solution. These are the metal ions of Groups IA and IIA except for beryllium ion which forms an aquo complex in water. Just as the strongest acid in water solution is the hydronium ion, the strongest hase is the hydroxide ion. If a Bqhsted hase which is stronger than hydroxide ion is placed in water solution it will immediately react to remove a proton from water Hz0 HB + leaving hydroxide ion in the solution: BOH-. Examples of such strong hases are 02-, H-, CzHsO-, etc., all of which accept protons hetter than the hydroxide ion. BrQnsted bases which are weaker than hydroxide ion will not accept protons completely from water and an equilibrium will exist between the BrQnsted hase and the conjugate acid and hydroxide ion.
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B-+H20=HB+OHBrQnsted bases may he neutral molecules (NHs, H20, etc.), positive ions (A1(Hz0)50H2+,Cu(Hz0)30H+, etc.) and any negative ion. Since all negative ions are Br6nsted hases, hydrolysis will take place when the negative ion of a salt is a strong enough BrQnsted hase to remove protons from water. I t can he noted that the negative ions (conjugate bases) of the molecular monoprotic acids which are stronger than hydronium ion will not he able to pull protons from water, and thus hydrolysis will not take place. From the foregoing it can he noted that a salt will hydrolyze in water solution when i t has a positive ion which can donate protons to water, in which case the solution will he acidic, or a negative ion which can accept protons from water, in which case the solution will he basic. When both the positive and negative ions hydrolyze the solution may he acidic, basic, or neutral. If the positive ion is stronger as a BrQnsted acid than the negative ion is as a BrQnsted hase, the solution will he acidic. That is the positive ions will give up protons to a greater extent than the negative ions accept them. On the other hand, if the negative ion is stronger as a BrQnsted hase than the positive ion is as a BrQnsted acid, the solution will he basic. That is the negativeions will accept protonsto a greater extent than the positive ions donate them. If the acid and hase strength of the two ions is the same, the solution will he neutral since the positive ions will give up protons to the same extent that the negative ions accept them. This interpretation of hydrolysis seems to me to be more logical, more accurate, and less confusing than the traditional interpretation. In conclusion,I would like to make a plea to chemistry teachers on both the secondary and college level, to take the time to revise their presentations on a regular basis in order to incorporate the latest ideas and information available. Even with periodic revisions, textbooks lag behind, and as in the case of hydrolysis, are often tied to a traditional approach. We are all creatures of habit. I t is only through conscious effort that we can change time-honored, traditional interpretations and approaches. Hopefully,teachers of chemistry on all levels will begin to shed their prejudices, based in many cases on the way they were taught, and use their ingenuity and creativity to devise new and hetter ways to present chemistry. Volume 54. Number 9. September 1977 1 549
