What is chemistry? – Steve Lower stuff

Chemistry is such a broad subject and one so full of detail that it is easy for a newcomer to find it somewhat overwhelming, if not intimidating. The best way around this is to look at Chemistry from a variety of viewpoints:

The scope of chemical science

Chemistry is too universal and dynamically-changing a subject to be confined to a fixed definition; it might be better to think of chemistry more as a point of view that places its major focus on the structure and properties of substances particular kinds of matter and especially on the changes that they undergo.

In some ways, physics might be considered more "fundamental" to the extent that it deals with matter and energy in a more general way, without the emphasis on particular substances. But the distincion can get pretty fuzzy; it is ultimately rather futile to confine any aspect of human endeavour to little boxes.

The real importance of Chemistry is that it serves as the interface to practically all of the other sciences, as well as to many other areas of human endeavor. For this reason, Chemistry is often said (at least by chemists!) to be the "central science".

Chemistry can be "central" in a much more personal way: with a solid background in Chemistry, you will find it far easier to migrate into other fields as your interests develop.

[adapted from image found here]

Do you remember the story about the group of blind men who encountered an elephant? Each one moved his hands over a different part of the elephant's body the trunk, an ear, or a leg and came up with an entirely different description of the beast.

Chemistry can similarly be approached in different ways, each yielding a different, valid, (and yet hopelessly incomplete) view of the subject. Thus we can view chemistry from multiple standpoints ranging from the theoretical to the eminently practical:

At the most fundmental level, chemistry can be organized along the lines shown here.

This view of Chemistry is a rather astringent one that is probably more appreciated by people who already know the subject than by those who are about to learn it, so we will use a somewhat expanded scheme to organize the fundamental concepts of chemical science. But if you need a single-sentence"definition of Chemistry, this one wraps it up pretty well:

Chemistry is the study of substances; their properties, structure, and the changes they undergo.

Chemistry, like all the natural sciences, begins with the direct observation of nature in this case, of matter. But when we look at matter in bulk, we see only the "forest", not the "trees" the atoms and molecules of which matter is composed whose properties ultimately determine the nature and behavior of the matter we are looking at.

This dichotomy between what we can and cannot directly see constitutes two contrasting views which run through all of chemistry, which we call macroscopic and microscopic.

In the context of Chemistry, "microscopic" implies detail at the atomic or subatomic levels which cannot be seen directly (even with a microscope!) The macroscopic world is the one we can know by direct observations of physical properties such as mass, volume, etc.

The following table provides a conceptual overview of Chemical science according to the macroscopic/microscopic dichotomy we have been discussing. It is of course only one of many ways of looking at the subject, but you may find it a helpful means of organizing the many facts and ideas you will encounter in your study of Chemistry. We will organize the discussion in this lesson along similar lines.

2 Chemical composition

In science it is absolutely necessary to know what we are talking about, so before we can even begin to consider matter from a chemical point of view, we need to know something about its composition; is the stuff I am looking at a single substance, or is it a mixture? (We will get into the details of the definitions elsewhere, but for the moment you probably already have a fair understanding of the distinction; think of a sample of salt (sodium chloride) as opposed to a solution of salt in water a mixture of salt and water.)

The manufacturer probably claims that their peanut butter is "pure"; is it really what a chemist would call a "pure substance"?

It has been known for at least a thousand years that some substances can be broken down by heating or chemical treatment into "simpler" ones, but there is always a limit; we eventually get substances known as elements that cannot be reduced to any simpler forms by ordinary chemical or physical means. What is our criterion for "simpler"? The most observable (and therefore macroscopic) property is the weight.

The idea of a minimal unit of chemical identity that we call an element developed from experimental observations of the relative weights of substances involved in chemical reactions. For example, the compound mercuric oxide can be broken down by heating into two other substances:

2 HgO 2 Hg + O2

... but the two products, metallic mercury and dioxygen, cannot be decomposed into simpler substances, so they must be elements.

The definition of an element given above is an operational one; a certain result (or in this case, a non-result!) of a procedure that might lead to the decomposition of a substance into lighter units will tentatively place that substance in one of the categories, element or compound. Because this operation is carried out on bulk matter, the concept of the element is also a macroscopic one.

Painting by Joseph Wright of Derby (1734-97) The Alchymist in Search of the Philosopher's Stone discovers Phosphorus [link]

The atom, by contrast, is a microscopic concept which in modern chemistry relates the unique character of every chemical element to an actual physical particle.

The idea of the atom as the smallest particle of matter had its origins in Greek philosophy around 400 BCE but was controversial from the start (both Plato and Aristotle maintained that matter was infinitely divisible.) It was not until 1803 that John Dalton proposed a rational atomic theory to explain the facts of chemical combination as they were then known, thus being the first to employ macroscopic evidence to illuminate the microscopic world.

It took almost until 1900 for the atomic theory to became universally accepted. In the 1920's it became possible to measure the sizes and masses of atoms, and in the 1970's techniques were developed that produced images of individual atoms.

Cobalt atom imaged by a scanning tunneling microscope [link]

The formula of a substance expresses the relative number of atoms of each element it contains. Because the formula can be determined by experiments on bulk matter, it is a macroscopic concept even though it is expressed in terms of atoms.

What the ordinary chemical formula does not tell us is the order in which the component atoms are connected, whether they are grouped into discrete units (molecules) or are two- or three dimensional extended structures, as is the case with solids such as ordinary salt. The microscopic aspect of composition is structure, which in its greatest detail reveals the relative locations (in two or three dimensional space) of each atom within the minimum collection needed to define the structure of the substance.

The molecule of water has the structure shown here.

The S8molecule is an octagonal ring of sulfur atoms. The crystal shown at the left is composed of an ordered array of these molecules.

(No, they don't actually move around like this, although they are in a constant state of vibrational motion.)

As we indicated above, a compound is a substance containing more than one element. Since the concept of an element is macroscopic and the distinction between elements and compounds was recognized long before the existence of physical atoms was accepted, the concept of a compound must also be a macroscopic one that makes no assumptions about the nature of the ultimate .

Thus when carbon burns in the presence of oxygen, the product carbon dioxide can be shown by (macroscopic) weight measurements to contain both of the original elements:

C + O2 CO2

10.0 g + 26.7 g = 36.7 g

One of the important characteristics of a compound is that the proportions by weight of each element in a given compound are constant. For example, no matter what weight of carbon dioxide we have, the percentage of carbon it contains is (10.0 / 36.7) = 0.27, or 27%.

A molecule is an assembly of atoms having a fixed composition, structure, and distinctive, measurable properties.

"Molecule" refers to a kind of particle, and is therefore a microscopic concept. Even at the end of the 19th century, when compounds and their formulas had long been in use, some prominent chemists doubted that molecules (or atoms) were any more than a convenient model.

Computer-model of the nicotine molecule, C10H14N2, by Ronald Perry

Molecules suddenly became real in 1905, when Albert Einstein showed that Brownian motion, the irregular microscopic movements of tiny pollen grains floating in water, could be directly attributed to collisions with molecule-sized particles.

Finally, we get to see one! In 2009, IBM scientists in Switzerland succeeded in imaging a real molecule, using a technique known as atomic force microscopy in which an atoms-thin metallic probe is drawn ever-so-slightly above the surface of an immobilized pentacene molecule cooled to nearly absolute zero. In order to improve the image quality, a molecule of carbon monoxide was placed on the end of the probe.

The image produced by the AFM probe is shown at the very bottom. What is actually being imaged is the surface of the electron clouds of the molecule, which consists of six hexagonal rings of carbon atoms with hydrogens on its periphery. The tiny bumps that correspond to these hydrogen atom attest to the remarkable resolution of this experiment.

The original article was publshed in Science magazine; see here for an understandable account of this historic work.

The atomic composition of a molecule is given by its formula. Thus the formulas CO, CH4, and O2 represent the molecules carbon monoxide, methane, and dioxygen. However, the fact that we can write a formula for a compound does not imply the existence of molecules having that composition. Gases and most liquids consist of molecules, but many solids exist as extended lattices of atoms or ions (electrically charged atoms or molecules.) For example, there is no such thing as a "molecule" of ordinary salt, NaCl (see below.)

Maybe the following will help:

Composition and structure lie at the core of Chemistry, but they encompass only a very small part of it. It is largely the properties of chemical substances that interest us; it is through these that we experience and find uses for substances, and much of chemistry-as-a-science is devoted to understanding the relation between structure and properties. For some purposes it is convenient to distinguish between chemical properties and physical properties, but as with most human-constructed dichotomies, the distinction becomes more fuzzy as one looks more closely.

[image link]

This concept map offers a good overview of the ideas we have developed so far. Take some time to look it over and make sure you understand all the terms and the relations between them.

For a more in-depth treatment of much of the material covered here, please see The basics of atoms, moles, formulas equations, and nomenclature.

Chemical change

Chemical change is defined macroscopically as a process in which new substances are formed. On a microscopic basis it can be thought of as a re-arrangement of atoms. A given chemical change is commonly referred to as a chemical reaction and is described by a chemical equation that has the form

reactants products

In elementary courses it is customary to distinguish between "chemical" and "physical" change, the latter usually relating to changes in physical state such as melting and vaporization. As with most human-created dichotomies, this begins to break down when examined closely. This is largely because of some ambiguity in what we regard as a distinct "substance".

Elemental chlorine exists as the diatomic molecule Cl2 in the gas, liquid, and solid states; the major difference between them lies in the degree of organization. In the gas the molecules move about randomly, whereas in the solid they are constrained to locations in a 3-dimensional lattice. In the liquid, this tight organization is relaxed, allowing the molecules to slip and slide around each other.

Since the basic molecular units remain the same in all three states, the processes of melting, freezing, condensation and vaporization are usually regarded as physical rather than chemical changes.

Solid salt consists of an indefinitely extended 3-dimensional array of Na+ and Clions (electrically- charged atoms.)

When heated above 801C, the solid melts to form a liquid consisting of these same ions. This liquid boils at 1430 to form a vapor made up of discrete molecules having the formula Na2Cl2.

Salt dissolves in water to form a solution containing separate Na+ and Cl ions to which are loosely attached varying numbers of H2O molecules. The resulting hydrated ions are represented as Na+(aq) and Cl(aq).

Because the ions in the solid, the hydrated ions in the solution, and the molecule Na2Cl2 are really different chemical species, so the distinction between physical and chemical change becomes a bit fuzzy.

Energetics of chemical change

You have probably seen chemical reaction equations such as the "generic" one shown below:

A + B C + D

An equation of this kind does not imply that the reactants A and B will change entirely into the products C and D, although in many cases this will be what appears to happen. Most chemical reactions proceed to some inermediate point that yields a mixture of reactants and products.

For example, if the two gases phosphorus trichloride and chlorine are mixed together at room temprature, they will combine until about half of them have changed into phosphorus pentachloride:

PCl3 + Cl2 PCl5

At other temperatures the extent of reaction will be smaller or greater. The result, in any case, will be an equilibrium mixture of reactants and products.

The most important question we can ask about any reaction is "what is the equilibrium composition"?

The aspect of "change" we are looking at here is a property of a chemical reaction, rather than of any one substance. But if you stop to think of the huge number of possible reactions between the more than 15 million known substances, you can see that it would be an impossible task to measure and record the equilibrium compositions of every possible combination.

Fortunately, we don't need to do this. One or two directly measurable properties of the individual reactants and products can be combined to give a number from which the equilibrium composition at any temperature can be easily calculated. There is no need to do an experiment!

This is very much a macroscopic view because the properties we need to directly concern ourselves with are those of the reactants and products. Similarly, the equilibrium composition the measure of the extent to which a reaction takes place is expressed in terms of the quantities of these substances.

Virtually all chemical changes involve the uptake or release of energy, usually in the form of heat. It turns out that these energy changes, which are the province of chemical thermodynamics, serve as a powerful means of predicting whether or not a given reaction can proceed, and to what extent. Moreover, all we need in order to make this prediction is information about the energetic properties of the reactants and products; there is no need to study the reaction itself. Because these are bulk properties of matter, chemical thermodynamics is entirely macroscopic in its outlook.

[image link]

Dynamics of chemical change

The energetics of chemical change that we discussed immediately above relate to the end result of chemical change: the composition of the final reaction mixture, and the quantity of heat liberated or absorbed.

The dynamics of chemical change are concerned with how the reaction takes place:

These details constitute what chemists call the mechanism of the reaction. For example, the reaction between nitric oxide and hydrogen (identified as the net reaction at the bottom left), is believed to take place in the two steps shown here. Notice that the nitrous oxide, N2O, is formed in the first step and consumed in the second, so it does not appear in the net reaction equation. The N2O is said to act as an intermediate in this reaction. Some intermediates are unstable species, often distorted or incomplete molecules that have no independent existence; these are known as transition states.

The microscopic side of dynamics looks at the mechanisms of chemical reactions. This refers to a "blow-by-blow" description of what happens when the atoms in the reacting species re-arrange themselves into the configurations they have in the products.

[image link]

Mechanism represents the microscopic aspect of chemical change. Mechanisms, unlike energetics, cannot be predicted from information about the reactants and products; chemical theory has not yet advanced to the point were we can do much more than make educated guesses. To make matters even more complicated (or, to chemists, interesting!), the same reaction can often proceed via different mechanisms under different conditions.

Because we cannot directly watch the molecules as they react, the best we can usually do is to infer a reaction mechanism from experimental data, particularly that which relates to the rate of the reaction as it is influenced by the concentrations of the reactants. This entirely experimental area of chemical dynamics is known as kinetics.

Reaction rates, as they are called, vary immensly: some reactions are completed in microseconds, others may take years; many are so slow that their rates are essentially zero. To make things even more interesting, there is no relation between reaction rates and "tendency to react" as governed by the factors in the top half of the above diagram; the latter can be accurately predicted from energetic data on the substances (the properties we mentioned in the previous screen), but reaction rates must be determined by experiment.

Catalysts can make dramatic changes in rates of reactions, especially in those whose un-catalyzed rate is essentially zero. Consider, for example, this rate data on the decomposition of hydrogen peroxide. H2O2 is a by-product of respiration that is poisonous to living cells which have, as a consquence, evolved a highly efficient enzyme (a biological catalyst) that is able to destroy peroxide as quickly as it forms. Catalysts work by enabling a reaction to proceed by an alternative mechanism.

Visit link:
What is chemistry? - Steve Lower stuff