97

Research in Science Education, 198/4, 14, 97-103

MODEL CONFUSION IN CHEMISTRY

Malcolm Cart

INTRODUCTION

Much of the discussion (e.g. Gilbert & Watts, 1983) of what have been variously

labelled as preconceptions (Novak, 1977) misconceptions (Helm, [980), a l ternat ive

conceptions or al ternat ive frameworks (Driver & Easiey, 1978), and children's science

(Gilbert, Osborne, & Fensham, 1982) has largely focussed on concepts wi th a

discernible relat ion to everyday experience such as force, e lect r ic i ty , heat, l ight,

particle, l iv ing and animal. Understanding of scientists' concepts is considered to be

hindered by adherence to everyday understanding of these words and/or to an

alternative set of explanations for phenomena associated wi th these words (Dr iver &

Ericksen, 1983). Confusions and d i f f icu l t ies over a number of chemical concepts may

require a di f ferent perspective, since these are abstract and formal explanations of

invisible interactions between particles at a molecular level and are not l ikely to be

arrived at from confrontat ion with the world (~f experience. Champagne, K[opfer,

and Gunstone (1981) have suggested that interaction between 'prior knowledge' and

formal instruct ion is more pronounced in mechanics than in other science subjects,

and this view has been questioned (Gilbert & Watts, 1983). This paper examines the

chemist's concept of acids and bases and suggests that students' d i f f i cu l t ies in this

area may be more usefully Perceived in terms of confusion about the models used in

teaching the concept rather than as a conf l ic t between preconceptions and the

scient i f ic view. Such an analysis may be valuable for many other concepts in

chemistry suchas the mote, balanced equations, and even the nature of fundamental

particles such as atoms, molecules, and ions.

CONCRETE TO ABSTRACT MODELS OF ACIDS A N D BASES

The tradi t ional approach is to begin wi th descriptions d i rect ly relat ing to sense

data. Acids taste sharp or sour, they 'eat away' metals and change the colour of

vegetable dyes. Alkal is or bases feel soapy and change the colour of vegetable dyes

d i f ferent ly from acids. Final ly an equation (which is often accurately recalled even

by students who have had l i t t le interaction with chemistry) is introduced.

ACID + BASE -~ SALT § WATER

The next pedagogic model relates observation to invisible occurrences between

particles at the molecular level. Such a molecular model is unl ikely to have been

pre-formed by the student, however intel l igent, though chemists (including teachers)

98

who have successfully grappled with this abstract concept often behave in discussion

and teaching as i f students can 'see' chemical reactions in the same way as, e.g., they

can fol low the path of a thrown object.

ARRHENIUS AND LOWRY-BRONSTED MODELS

The modern model of acids and bases in terms of proton transfer reactions at a

molecular level (Lowry-Bronsted) was formulated after a model which was, in some

important respects, transitional towards a completely part icle view (Arrhenius). The

Arrhenius model has three main features:

(i) the description 'acid' or 'base' remains molecular in that i t is applied to a

substance which can exist in a bott le. Thus HCI, C H s c o o H and H2SO 4

are acids because they dissolve in water to increase the concentration of H +

ions, NaOH is a base because it dissolves in water to increase the

concentration of OH- ions. Acids and bases can be further classified as

weak and strong.

(ii) the fundamental equation remains as

ACID + BASE -~ SALT + WATER

(iii) The acid-base behaviour of solutions of salts requires complex argument based on

hydrolysis. In terms of the equation in ( i i )a salt is identif ied as being formed

by reaction of a weak or strong acid with a weak or strong base. Thus, a

solution of sodium ethanoate (acetate) is basic because it is a salt of a weak

acid and a strong base. Arrhenius theory here is conceptually very d i f f icu l t

since many substances from bottles ('salts' such as ammonium chloride)

dissolve in water to increase the concentration of H + ions but are not

classifed as acids although they exactly f i t the fundamental definit ion.

This d i f f icu l ty is overcome in the Lowry-Bronsted model, which was formulated

after the concept of ions in solids and in aqueous solution had been accepted in

chemistry (a recent event historically). The significant features of the

Lowry-Bronsted model are:

(i) the description 'acid' or 'base' is applied to molecules and ions. The lat ter

category cannot have a separate existence in bottles. Acids are proton donors

(HCI, C H 3 c o o H and H2SO 4 as before, as well as NH~,

H2PO ~ etc), bases are proton acceptors (OH-, not NaOH as well as

CH3COO-, HP0 2- etc). Aqueous acids are classified as weak" and

strong by reference to their re la t i ve abil i t ies to transfer protons onto water

molecules; bases are classified in a parallel fashion.

(ii) The fundamental equation changes to

ACID l + BASE 2 ~ BASE l + ACID 2

99

(i i i ) The acid-base behaviour of solutions of salts can now be described simply after a

consideration of the ions present has been completed. Sodium ethanoate

(acetate) solution is basic because the salt dissociates to hydrated sodium ions

(which are nei thr acidic nor basic) and hydrated ethonoate (acetate) ions which

are weakly basic. Ammonium chloride solution is acidic because the solution

contains ammonium ions (weak acidic) and chloride ions (neither acidic nor

basic).

The differences in the two theories are suf f ic ient ly important to be worth

tabulating.

Arrhenius Lowry-Bronsted

Substance in bott le view.

NaOH is a base

Molecules and ions view.

OH- in NaOH is the base

Acid + Base -) Salt + Water Acid + Base -~ Base + Acid

Complex theory of hydrolysis

of salts

Hydrolysis of salts clear

extension of theory

MODEL CONFUSION

Students learning about the chemist's concept of acids and bases w i l l develop

most of their understanding from experiences in laberatory~ in the classroom and in

textbooks. The task of comprehension wi l l be more d i f f i uc l t i f transit ion from model

to model is not careful ly sign-posted. Very fundamental words and ideas change their

meaning in proceeding from the Arrhenius to Lowry-Bronsted model, I f we ref lect

that the process of changing paradigms in science has caused confusion and dissent

(Kuhn, 1970) we should further ref lect that changing models is not a facile

procedure. Clear indications of when a new model is being introduced9 of how this

new model dif fers from previous models and of why the new model works better

would seem to be v i ta l ly important in the teaching process. In the case of acids and

bases there is al l - too-clear evidence of model confusion from text-books used widely

in secondary schools and in universities. Not only are the models of Arrhenius and

Lowry-Bronsted not clearly separated (so that when the new model is being used is

unclear), the fundamental difference between them (the how they di f fer) and the

greater usefulness and coherence of the new model (the why) is not explicated.

100

A TEXT-BOOK EXAMPLE

The text recommended to f i rst year students in several New Zealand

universit ies, and widely overseas (Brady & Hums[ton, 1982) provides a typical

example of the treatment of acids and bases in many texts at this level, a treatment

which is followed at a less d i f f i cu l t level in many secondary school texts. [n Chapter

6 the topic is introduced using an Arrhenius def ini t ion, hence defining NaOH as a

base. In Chapter 14, devoted ent i rely to acids and bases, the Arrhenlus def in i t ion is

repeated followed by the Lowry-Bronsted def ini t ion.

In a clear account the exemplars are mostly molecular species and the status of

NaOH is not reconsidered. Rather an extension to other solvent systems is discussed

(dealing with the why of this model since an important feature is the general[sat[on

to solvent systems other than water) and this discussion leads to yet another

acid-base model, the Lewis def in i t ion of acids and bases. (This la t ter model should,

in my view, only be introduced to pupils who have understood the Lowry-Bronsted

model, since i t introduces further confusions too extensive to outl ine in this paper.)

Chapter 15 of the text deals wi th acid-base equi l ibr ia in aqueous solution. Early

(p.464) we find the statement 'Metal hydroxides are strong bases - they are

completely dissociated' applied to NaOH solution. An Arrhenius model view. Next

'weak electrolytes include weak acids and bases' which is incorrect, confusing and

based on the Arrhenius model. This statement is contradicted (p.471) when sodium

acetate (which contains the weak base, acetate ion) is correct ly stated to be

completely dissociated (a strong electrolyte). Now follows a Lowry-Bronsted model

discussion of polyprot ic acids and buffers. In the section on hydrolysis (pp.480-86) the

model returns to the Arrhenius view and salts are considered in terms of a 'hydrolysis

constant' (p.482) indistinguishable from the Lowry-Bronsted base ion[sat[on constant'

defined earl ier (p.466). The treatment of hydrolysis is complicated and confusing. I t

is salutary to quote here from a tex t published in 1949 (Philbrick, Holmyard, &

Palmer).

One advantage of the modern view (Bronsted-Lowry) is the great

s impl i f icat ion possible in the explanation of salt hydrolysis. We can

say that sodium acetate yields an alkaline solution simply because i t

contains a base, acetate ion, and ammonium salts give acid solutions

because they contain an acid, NH 4 +.

The Chapter ends wi th a discussion of t i t ra t ions and indicators which continues an

Arrhenius view of hydrolysis of salts at the equivalence point (pp.487-z~92.

101

In the problems at the end of the chapter (15.35) students are asked about 'a

weak base BOH'. Here the authors are purely Arrhenius again, since they require the

base to react BOH r-~ B + + OH- whereas Lowry-Bronsted treatment would be

BOH + H + ~' BOH 2 +

Another textbook (Chang, 1984) used widely in universit ies displays a confusion

between Arrhenius and Lowry-Bronsted models which is even more extreme. A f te r a

treatment of the Lowry-Bronsted model the hydrolysis of salts is considered from a

muddled Arrhenius view. One sentence wi l l i l lustrate the confusion students would

have with this t r ea tmen t (p.457). 'Since NaOH is a strong base, Na + is an

extremely weak conjugate acid; therefore, i t has no tendency to react wi th H20 to

form NaOH and H + ion'. The f i rst statement is Arrhenius, the second can only be

seen as Lowry-Bronsted (as well incorrect).

A similar analysis of many texts shows the same switching from model to model.

The texts display model confusion. Teaching relying on these texts as sources wi l l

escape the same confusion only i f i t is clearly recognised and avoided.

A notably clear exception to this model confusion in textbooks is provided by an

Austral ian t ex ta l so used in secondary schools in New Zealand (Stranks et al. 1970).

The treatment is clearly and consistently Lowry-Bronsted. i t is unfortunate that

ter t iary level texts may confuse this basically sound preparation.

5TUOENT PERCEPTION AND PERFORMANCE

Students in secondary school perceive acids and bases to be a d i f f i cu l t topic

(Burns, 1982). Students fe l t that their understanding of ionic acid-base equations was

poor (ranked third poorest of the 50 topics surveyed). In interviews wi th late

secondary and f i rst year ter t iary students (Note l ) only about a quarter of the sample

of 2/4 appeared convinced that aqueous hydrogen chloride contained ions, and some of

these students did not consider the acid to be fu l ly dissociated. This reluctance to

accept ions may stem in part from reluctance to abandon Arrhenius views~ since

molecules are saved in that model. The Lowry-Bronsted model requires that ions are

recognised as important components of aqueous solutions.

In a recent national examination for students terminat ing secondary school in

New Zealand (Univers i t ies Entrance Board Bursaries Examination in Chemistry,

198]), the author formulated a quest ion on the 'hydrolysis of salts' which was

102

considered to be simple to answer from a Lowry-Bronsted viewpoint, but which would

be much more d i f f i u l t from an Arrhenius viewpoint.

The question asked that students l ist the species present in a 0.1 mol l - l

aqueous solution of sodium ethanoate (acetate) and that they choose the approximate

concentration of each species from (i) 0. l mol 1 - l ( i i) l0 -5 mol l - l and (i i i)

l0 -9 tool l - I given that IK b for ethanoate ion was l0 -9. A Lowry-Bronsted

perception of this system leads to the understanding that the solution contains

Na+(aq) and CH3coo- (a q) ions, and that a small proportion of the lat ter ions

(which are a weak base) react

CH3COO- + H20 ~=~ CH3OOH + OH-

An Arrhenius perspective wi l l argue in terms of substances in bottles rather than

separated ions. Analysis of the responses is not yet complete but a random sample of

24 students in the top octi le showed that Arrhenius views were used by one third of

these candidates, using equations such a~

CH3COON a + H20 # CH3OOH + Na + + OH-

CH3COON a # CH3COO" + Na +

to begin their answer. A quarter of these 24 students concluded that there was

approximately 0.1 mol 1 "1 of CH 3 COONa in the solution. This la t ter statement

could be made by a student who confused weak electrolytes and weak bases~ a view

reinforced by many discussions of Arrhenius bases. This question was the most poorly

answered section of the entire paper.

THE HISTORICAL APPROACH TO TEACHING

This pedagogic method often traverses several models in arr iv ing at the most

recent, and has the merits of acquainting students wi th the development of a

concept, and of revealing science as a progression through models which have been

subjected to experimental cr i t ic ism. From the perspective of model confusion the

pedagogic method has very clear dangers since concepts from one model can all too

easily be retained after they should have been discarded. I t is par t icular ly important

when teaching an historical sequence of ideas that changes in models are clearly

signposted and that danger signals are displayed.

CONCLUS[ON

This paper has been a prel iminary discussion of model confusion about acids and

bases, presenting evidence (some of i t to be elaborated) that the Arrhenius and the

Lowry-Bronsted models are confused in some textbooks, and in many students' minds.

103

A similar analysis of other concepts in chemistry (are some problems about ions a

result of carrying Daltonian and Newtonian models of atoms beyond their u t i l i ty -

since in those models atoms are unbreakable; are covalent bonding ideas served at all

well by the Bohr model of the atom?) may be a valuable area for research,

REFERENCE NOTES

Note 1. RUSSELL, S. Private communication, 1983.

REFERENCES

BRADY, i .E., & HUMISTON, G.E. General Chemistryt Principles and Structure (3rd Edition), John Wiley & Sons, New York, 1982.

BURNS, 3.R. An Evaluation of 6th and 7th Form Chemistry in Terms of the Needs of the Students and the Community. Report to the Department of Education, Wellington, New Zealand, 1982, pp.96-100.

CHAMPAGNE, A., KLOPFER, E.L., & GUNSTONE, R.F. Cognitive research and the design of science instruction. Paper presented at the International Workshop on problems concerning students' representation of physics and chemistry knowledge, Padagogische Hochschule Ludwigsburg, September 198l.

CHANG, R. Chemistry (2nd Edition) Random House, New York, 1984.

DRIVER, R. & EASLEY, 3. Pupils and paradigms: A review of l i terature related to concept development in adolescent science students. Studies in Science Education, 1978, 5, 61-8Z~.

DRIVER, R. & ERICIKSON, G. Theories-in-Action: Some theoretical and empirical issues in the study of students' conceptual frameworks in science. Studies in Science Education, 1983, I0, 37-60.

GILBERT, 3.K., OSBORNE, R.3., & FENSHAM, P.3. Children's Science and its consequences for teaching. Science Education, 1982, 6--6,(4)623-33.

GILBERT, 3.1(. & WATTS, O.tvl. Concepts, misconceptions and alternative conceptions: Changing perspectives in science education. Studies in Science Education, 1983, 10, 61-98.

HELM, H. Misconceptions in physics amongst South African students. Physics Education, 1980, 7__~, 92-105.

KUHN, T. The Structure of Scientific Revolution. University of Chicago Press, Chicago, 1970.

NOVAK, 3. A Theory of Education. Cornell University Press, Ithaca, 1977.

PHILBRICIK, E.A., HOLMYARD, E.3., & PALMER, W.G. A Textbook of Theoretical and Inorcjanic Chemistry. Dent & Sons, London, 1949, p.193.

STRANKS, O.R., HEFFERNAN, M.L., LEE BOW, K.C., McTIGUE, P.T., & WITHERS, G.R.A. Chemistr),. A structural view (2nd Edn) Melbourne University Press, Melbourne 1970.