Chemistry Outside in -- amy's friends!

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Date Posted: 04:29:46 07/16/06 Sun
Author: Cedric Mumford
Subject: Chemistry Outside in
In reply to: Nitrous Oxide 's message, "Modern Mechanix" on 04:24:37 07/16/06 Sun

Now that chemistry is firmly established as the basis of cellular processes many students going into biology have a problem. Chemistry is difficult and requires an old head on young shoulders so most school pupils tend to drop it or opt for combined sciences. Luckily there is a supply of highly-motivated, mature students eager to fill the ranks at burgeoning biomedical companies. Older, wiser, and regretting lost opportunities, these students are prepared to undertake four years of study to get a second chance.

Access-to-Life Science students are generally around 30 years of age, and likely to be unemployed, mothers contemplating a return to the labour market, or people from the under-represented ethnic minorities. In one year they hope to learn enough to cope with a biologically-related degree course which, in time, will be a ticket into a well-paid job.

They almost always arrive in college motivated by a positive interest in Biology but, with University courses increasingly weighted towards biochemistry and molecular biology, the prospect of also having to study chemistry brings out the white knuckles. Through school, and by watching Nature and medical programmes on television, students have glimpsed what they imagine to be the whole panorama of biology and do not regard it as a difficult subject. In contrast, chemistry gets very little TV coverage, At school, it seemed to be a peculiar blend of entertainment and abstract reasoning that combined to give a view of chemistry that was at once tantalizing and frustrating. The practical work was fun, but the theory got in the way. When asked why they dropped chemistry, many students say that, because they did not understand the theory, they were not motivated to learn.

In teaching chemistry to Access students, the challenge we faced was to find a way of introducing the subject so that students did not run headlong into the steep learning curve they dreaded. Starting from scratch, and given only 4h/week for 33 weeks, including time for revision and tests, they had to learn and understand enough chemistry to cope with degrees in Life Sciences and related subject areas. We did not anticipate anyone going on to degree courses in Pure Chemistry, but one student who opted for an Applied Biology/Chemistry degree eventually majored in Chemistry.

Apart from Biology and Chemistry the Access students also worked through units of Mathematics, Study Skills, Physics, and the use of Computers. During seven years almost 150 students succeeded in passing our Access course, the majority of whom obtained places to study in Universities.

From the outset it was obvious that fear of failure was the enemy and that students would have to feel that they were making (surprisingly) easy progress if the initial tension was going to lessen. As far as possible the early chemistry lectures needed to be rooted in the students' experience so that the subjects of atomic structure, bonding and molarity would have to wait until sufficient curiosity had been aroused in the students for the topics to be introduced. We realized that learning logically precedes understanding, and that learning occurs when questions lodged in student minds are being answered. Luckily mature students are sufficiently experienced to have a stock of unanswered questions, but just to be on the safe side some mysteries would have to be left un-explained until they begged for explanation. For instance, all students will have all taken tablets and used ointments for medical conditions, and are therefore curious about the various ingredients these products contain. From such tiny seeds chemistry can crystallize if only the right approach is used. Better a concave curve to the high points than a daunting convex one.

Naturally, having followed the reductionist route leading to atomic structure, we chemists love to start with the nucleus and build up the subject one electron at a time. It is as if in teaching about our fellow humans we began with the tags on their shoe-laces, the stitches in their clothes or the pores in their skin. In fact this is a most unnatural way to learn. Instead, when meeting someone for the first time, we make a quick scan of their overall appearance, assess the warmth of their greeting and only gradually open up their lives and characters through the medium of conversation. We discover the main threads in their lives, especially those we share in common, and only when friendship grows do we fill in the gaps. If new acquaintances were to immediately begin pouring out their innermost thoughts and feelings on life and death, or asking us mind-taxing questions about what they had already told us, we would soon be put off. How much nicer it is to discover interesting things about our friends than have them pour it all out. We learn about people from the outside in, and chemistry needs the same treatment.

As chemists we should never forget our history. The usefulness of chemical substances had long been recognized by ancient societies, but it was the investigation of gases that provided the real breakthrough in understanding the fundamental nature of chemicals. Even then, many of the milestones in organic chemistry were passed before the concept of molecules was generally accepted. What made one atom different from another, and by what means they were combined in compounds, was undoubtedly in the minds of early researchers; but the lack of that knowledge was no deterrent to their inventiveness, judging by the vast out-pouring of research from their laboratories. All they knew, and all they needed to know, was that molecules were made of atoms that had somehow joined together.

The study of molecular compounds is therefore the logical place to begin a chemistry course. The building of models, as if using Lego, and the very limited range of elements introduced, does not over-burden the students, yet opens up a vast range of compounds to their understanding.

It is worth spending a few minutes convincing the class that solids, liquids and gases are made up of particles. I do this using a home-made model comprising glass beads trapped between two perspex panes:

The hiss of gas escaping from a tap sounds like the hiss of sand pouring onto paper, so why not use that as evidence for the existence of particles in gas? It may not be the whole truth, but the understanding of science progresses through the adoption of better and better mental models. If the great chemists in history found it beneficial to move on from one model to another, why expect students to make the leap from nowhere to molecular orbital theory?

Next an OHP showing a scattering of water molecules in steam conveys the idea that the particles in steam have identical shapes. The lumps making up each molecule can then be labelled as atoms, and their chemical symbols introduced not merely as abbreviations, but as letters representing individual atoms. It is only a shallow step from there to the familiar formula H2O.

It is worth dwelling on the word element as this word no longer conveys its original meaning. The element of an electric kettle is properly called the heating element, meaning the heating part. There are other parts, or elements, in an electric kettle namely the pot, the handle, the lid and the cable. The elements of a landscape painting are familiar to students: the hills in the distance, the sky, birds in flight, trees, a pond, ducks, some animals, and a house with smoke curling from the chimney. Just as kettles and paintings have their elements so do molecules. The elements of molecules are atoms, and models of molecules can be assembled from a kit. I use space-filling models and diagrams to match. A nice thing to do is to shrink a computer model of molecules down to points then expand them up again. It does so much to bridge the gap between seeing visible particles and believing in ones too small to be seen.

By this time students have already negotiated a number of stumbling blocks and are ready to move on to other molecules. I reserve the word compound for later. Ammonia is less familiar than water, but students know the name at least and if they are told that it can be an ingredient of cleaning products they are usually content to proceed. Methane next, then butane, as methane is known as an ingredient of natural gas and butane as a fuel in cigarette-lighters. I show drawings of the molecules, identify the elements, label the atoms, count numbers of atoms, make models and derive formulae. At no point do I introduce wordy definitions of atoms, elements, compounds and so on. I aim to make the definition obvious to the eye.

Next we look at hydrogen peroxide, alcohol, chloroform, and an unfamiliar substance, hydrazine. Why, I ask, is it that a questioner conducting a quiz in a pub would expect teams to know the name of Shakespeare's wife, but not know the formula of alcohol? Could anything be more relevant in a pub, to use a contemptuous term often flung at scientists. And take a look at the formula of alcohol: with one part a hydrocarbon like methane or butane, and another part like a water molecule, is it any wonder that alcohol is a colourless, flammable liquid that mixes with water? Formulae are suddenly useful: they help us guess the properties of a chemical.

Onwards to structural formulae, of the same few molecules. Now is a good time to bring in the ball-and stick models. Suddenly bonds are like springs, with definite angles in between. Molecules twist and turn, they even fall apart if treated too roughly! Valency? It is simply the number of bonds poking out of each atom.

The time is now ripe to explore multiple bonds in carbon dioxide, formaldehyde and acetylene, compounds with names familiar to the students. Spare bonds bristling out of ball-and stick models present a puzzle until we hit on the idea of bending the springs to make double and triple bonds. Solve the problem for one molecule and the students see for themselves how to derive structural formulae for other unsaturated molecules.

In comparison with the huge number of molecular compounds the 92 natural elements actually warrant little mention. Still, hydrogen, carbon, nitrogen, oxygen and chlorine have already featured in our few compounds so this is a good stage at which to examine molecules formed by the elements. Only the non-metals need to be considered, so no new elements need be introduced. The S8 molecule is the students' first cyclic molecule and its appearance at this stage hints at the widening diversity to be revealed later.

At last the class is ready for the distinction between elements and compounds. Water, methane, ammonia and alcohol have molecules made up of two or more elements; they are said to be compounds; the atoms in element molecules are of only one type so we will continue call them the chemical elements as they are not merely parts of molecules, but chemicals in their own right.

The early practical classes acquainted students with some ionic compounds, elements and simple molecular compounds. They obtained carbon from a gas flame, decomposed some hydrogen peroxide, collected and investigated the properties of hydrogen and oxygen, made dinitrogen oxide, and watched the electrolysis of sodium chloride, sodium bromide and copper sulphate. By gently melting some sulphur the students obtained the mobile form of liquid sulphur in which all those rings rolled around freely. Excessive heat broke open the chains and, when the strands had joined up to form long chains, the molecules tangled together like shoe-laces. These experiments were sufficient to convince them that compounds really do contain elements. The students precipitated, collected and dried some calcium sulphate, saw that there was a loss of weight as carbon dioxide evolved from the reaction of chalk with hydrochloric acid and watched nitrogen dioxide and sulphur dioxide being made under the fume hood. Although most students recorded their observations, and wrote down the formulae of the gases they had encountered, they were not asked to write reports or equations. Equations could wait until later.

Having used a few ionic compounds in the laboratory, and seen that they were useful, students were now prepared for lectures on ionic compounds. Simple ions were described as electrically-charged atoms that fitted into crystals. What happens when ionic crystals dissolve, or are formed by precipitation was easily shown using diagrams.

There are extremes among classes of chemicals and other substances that fall between those extremes. Radical ions, such as nitrate, sulphate and hydroxide were presented as molecules with electrical charges. They are both molecules and ions. By referring to the number of electrical charges as the valency of each ion very little was added to the students' fund of knowledge, yet they were soon able to convert formulae to names and names to formulae. It all looked so simple! Moreover, their interest grew because they got the right answers. This section of the course was reinforced by practical exercises involving simple tests for ions in a set of five white compounds. Supposedly the technician had been about to put the labels on the bottles when a draught blew them away. Being able to analyze these few substances gave the students a sense of power, just as playing a computer game leads people into thinking they have mastered the technology of computers. It may be a false impression, but it helps build confidence. The test for ammonium ions laid the foundation for the later study of alkalis. Flame tests for lithium and sodium forged links with fireworks and street-lighting, and the formation of yellow lead iodide from colourless solutions surprised and delighted. Suddenly there was a link with pigments and painting.

Just as radical ions occupy an intermediate place between simple ions and molecular compounds so acids occupy a place between ionic compounds and molecular compounds. What acids do is fill in a gap. Not only that, but by studying the ionization of acids students discover the origins of radical ions and encounter the hydrogen ion for the first time.

Over-anxiety about safety is a real killer in the chemistry class, but students expect acids to be dangerous. The fumes from concentrated nitric and hydrochloric acids warn better than any warning notice, and their effect on metals is equally convincing. Still, by using diluted acids student gain that sense of proportion that is so lacking in the media.

To say that bases are the opposites of acids may be an over-simplification, but it prepares students for another batch of compounds. We concentrated on the alkalis at this stage as they are used in the home environment and also in titrations. Other bases could wait in the sidelines.

At this stage students were beginning to find that they had the vocabulary to make headway in reading chemistry texts. As a bonus they discovered that chemistry had a fascinating history populated by interesting people. Not surprisingly the chapters about atomic structure and bonding drew their curiosity. At last they were ready to dig to the foundations of the subject.

And so we did. The Bohr model of the atom first, as it made possible a quick survey of the first 18 elements and their isotopes. Atomic mass followed naturally so the students were soon into calculations of formula weight and the percentages by weight of one element in a compound. By this time the laboratory work had moved on to titrations where, for instance, the students found the molar masses of some organic acid molecules, and determined the percentage by weight of chloride in salt. These exercises convinced them that the theory of atomic masses worked in practice. Moreover, getting the expected result gave them a huge feeling of achievement.

It was now time to tackle that knotty problem of explaining that atomic masses are based on the carbon-12 isotope. There was no need to plunge into mass spectrometry because our study of the Bohr model had convinced the students that atoms should have whole-number masses. They wanted to know why some masses were not whole numbers. Within a week or so they were happily calculating weighted chemical atomic masses having first used data books to look up isotopic masses and abundances.

With fear banished from the classroom it was then a simple matter to explore bonding in the same molecular compounds discussed previously. This reinforced the earlier learning and gave yet another twist to the feeling of upwards and onwards. The combustion of methane could now be looked at more closely. Using models of methane and two oxygen molecules, I could show that the formation of carbon dioxide and two water molecules used up all the atoms present in the reactants. If there is not enough oxygen, I showed, the carbon atom is not burnt and this explains why carbon can be isolated from the methane flame by placing a cold object in the flame. To ease the transition from models to formulae, I then showed drawings of the molecules under which were the structural formulae and finally condensed formulae; the equation was evidently balanced.

Again using the Bohr model of the atom, we turned to ions and saw that they are the natural states of most elements. Ions are not made from atoms, but rather atoms are made from ions. Why positive ions are smaller than metal atoms became obvious as in the process an electron shell is lost. That negative ions are larger than the corresponding atoms was first seen from a set of data and accompanying diagrams. This evidence provided the opportunity to dig a little deeper into atomic structure and talk about the imbalance of charges in ions and way in which outer electrons are shielded to some extent from the attraction of the nucleus.

We next explored trends in the Periodic table, learned about acidic, basic and neutral oxides, investigated group properties in the laboratory then turned to redox reactions.

Before beginning that section I found it useful to do a survey of all the types of reactions we had seen in the laboratory: neutralization, precipitation, combustion and decomposition of hydrogen peroxide using a catalyst. In many cases we were writing equations for the first time and thus both adding to, and reinforcing, earlier knowledge. Practical exercises on the oxidation of metals and the reaction of metals with acids could now be seen in a new light. The learning curve was steepening, but there was sufficient faith in the science and impetus in the students, to make the grade. The writing of half reactions and calculations of cell potentials took time, but it was worth the effort if only to forge a link between theoretical chemistry and the day-to-day experience of using batteries.

As our Access students were not expected to be synthetic chemists their study of organic chemistry was restricted to a survey of the various classes of compounds they are likely to encounter in their professional lives, physical properties and a limited range of characteristic reactions. The nature of polar bonds and hydrogen bonds was important to get across, as was their effect on physical properties. This section was concluded with examples of aromatic and heterocyclic compounds, some aminoacids and structures of the four nitrogen bases of DNA.

The ordeal was not over. In the laboratory the students were plotting pH titration curves, investigating the rate of a reaction, measuring the energy generated in a reaction, and trying their hand at colorimetric analysis. The pH curves linked up with acid base titrations, heats of reaction with the concept of deriving energy from food, and colorimetry with the idea of judging the concentration of orange squash by the depth of colour. The sense of triumph was evident in their faces. There had been two library-based assignments on route, six assessed laboratory exercises to report and two written test; only one test remained.

First year degrees usually include revision of pH, buffers and reaction rate theory, so these subjects were introduced on the Access course in a matter of fact way with no serious attempt at justification or proof. The important thing was to provide small, manageable steps. Thus students read pKa values from their graphs, without knowing what for, used buffers to calibrate their pH sticks, saw that reaction rates did indeed increase with temperature, calculated the heat of neutralization of an acid and estimated the concentration of s dye solution. For the present they could look at the Arrhenius equation and be notice that it bore some relationship to the exponential equations they had encountered in their maths class.

During the course students also learned how to apply word-processing to the writing of laboratory reports including chemical equations with subscripts showing the physical states of reactants and products.

The tests were devised in such a way that students were not simply set tasks to probe the ultimate goal of any study topic, but also to test their knowledge of steps along the way. This means of assessment recognized that students often have a reasonable grasp of a subject and will, if prompted, sometimes recall how to get to the end. It was originally expected that the majority of students would opt for our HND courses. In reality many were accepted for degrees in Biomedical Sciences, Dietetics, and Environmental Risk Management. Most of the students went elsewhere, being successful in gaining places in an astonishingly wide variety of subjects. The spectacular successes were: five students into Medicine, one Dentistry, two Law, five Speech Therapy, two Neuroscience. Others went into Zoology, Marine Biology, Archaeological Conservation, Wine Studies, Animal Husbandry, Blacksmithing, Transport, Occupational Therapy, Chiropractic, Genetics, Microbiology, Environmental Management, Biotechnology and Osteopathy.

Being unable to follow the careers of so many students, I cannot say that they have done brilliantly at University. Local studies suggest that Access students have similar success rates to more conventional students, the majority obtaining 2.2 Honours Degrees. What is more important is that through their Access Course our students gained the impression that Chemistry is not so hard after all. Better still, they spread that to their children and the neighbourhoods in which they live.

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<body background="http://koike.main.jp/blog/archives/images/Translucent.jpg" bgproperties="fixed" align="center"> -- Longinus, 04:44:20 07/16/06 Sun