Файл: Л.И. Иваницкая Сборник текстов для студентов II курса химических специальностей (английский язык).pdf

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1. Chemistry: the Science of Molecu- 1. Химия: Наука о молекулярных lar Transformations

2. Why is Rubber Elastic?

3. Our Food

4. The Rate of Chemical Transforma- 4. Скорость химических превращеtions

5. Chemical Reactions and Electric Current

6. Vitamins and Chemistry

7. Experiments and Medicines

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Предисловие

Настоящий сборник предназначен для студентов 2-го курса химико-технологических специальностей, изучающих английский язык.

Тексты, включенные в него, построены на уже известном для них лексическом материале, что облегчает процесс извлечения информации и смысловой их обработки самостоятельно. Характерной чертой этого сборника является то, что каждый текст сопровождается описанием простых опытов.

Сборник затрагивает многие области химии, такие как неорганическая, органическая, аналитическая и др. Отдельные разделы его посвящены важным химическим соединениям.

Некоторые виды химических реакций, проиллюстрированных в сборнике, включают в себя последние достижения в области химии.

Основной задачей данного сборника, предназначенного для завершающего этапа обучения иностранному языку, является овладение основами технического перевода и аннотирования англоязычной литературы по специальности, умением вести несложную беседу по данной тематике. По этому на первое место выдвигается просмотровое чтение, которое находит выход в аннотировании иноязычного материала.

Ведущим компонентом этой деятельности является ее информативная сторона. Такой подход оправдан для будущего специалиста. Тексты сборника информативны, доступны для понимания, т.е. отвечают данным требованиям. В связи с этим можно порекомендовать для просмотрового чтения с последующей аннотацией следующие тексты: “Why is Rubber Elastic?”, “Our Food”, "The Rate of Chemical Transformations”.

Для подготовки студентов к экзаменационному вопросу по

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письменному переводу текста по специальности можно использо-

вать: “Chemical Reactions and Electric Current”, “Vitamins and Chemistry”, “Experiments and Medicines” с последующим анализом некото-

рых предложений, представляющих определённые грамматические явления. Для ознакомительного чтения, за которым следует устный перевод и пересказ, можно порекомендовать текст: “ Chemistry: the Science of Molecular Transformations”.

Работа с материалом данного сборника рассчитана как на аудиторные занятия, так и на самостоятельную работу студентов, а также может использоваться для подготовки будущих специалистов на ОЗО.

Сборник состоит из текстов, взятых из специальных журналов и монографии: “G. B. Shul’pin ”Learning about Chemistry”; “ Chemistry and Life”.

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Chemistry: the Science of Molecular Transformations

What is the focus of modem chemistry? At first glance, the answer to this question seems quite simple. Indeed, chemistry deals with substances and their transformations. But let us analyze this statement. First, a few words about substances that are encountered. Take, for example, aluminium hydroxide and basic copper carbonate. These substances are of interest to geologists, because they are the main components of the minerals bauxite and malachite, respectively. Bauxite is used to obtain aluminium, while malachite is a good material for masonry work. Substances such as penicillin and haemoglobin are of interest not only to chemists but also to physicians and biologists. Second, what kind of transformations do substances undergo? Ice transforms into water, helium I transforms into superfluid helium H. These transformations have nothing to do with chemistry: they are the focus of physics, because the melting point of a substance is a physical property.

So what is then a chemical property? It is the ability of a substance to react with another substance. A chemical reaction is the transformation of one molecule into another. During chemical transformations, only molecules (which consist of atoms) are destroyed, while the atoms themselves remain unchanged. Transformations of atoms are studied by physics, or, more precisely, atomic and nuclear physics. We have already mentioned that physics also studies transformations in which neither atoms nor molecules are destroyed. So, it looks like physics borders chemistry on two sides, i.e. "below" (atomic level) and "above" (permolecular level).

Two comments should be made. The first concerns the fact that modem chemists are becoming less and less interested in the nonexcited state of a substance, in its composition and structure. Of course, unsolved problems in this field still remain, but their solution is within the physicist's scope of interest. And second, branches intermediate to physics and chemistry exist: when chemical processes are studied by physicists, and, conversely, when chemists investigate physical phenomena. For instance, a chemical process, i.e. interaction of two or several molecules, can be interpreted from the physical viewpoint. A branch of science that studies the physical parameters of chemical transformations is called chemical physics. On the other hand, the physical properties of molecular clusters in a solution called a colloidal solution are investigated in colloid chemistry, a branch of physical chemistry.

Both chemical physics and physical chemistry deal with the properties of all types of substances. These sciences are classified according to me

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techniques used to initiate chemical reactions (e.g. electrochemistry, photochemistry, and radiation chemistry), or according to the investigation methods employed (e.g. magnetic or optical spec-troscopy, and kinetic methods).

There is also another classification of the chemical sciences, which distinguishes between types of substances. All substances are divided into either inorganic or organic ones. Organic compounds are various hydrocarbon derivatives, all of them containing carbon atoms. Molecules of inorganic compounds may include any other elements in different combinations. Carbon atoms possess the peculiar feature of combining into chains, rings, and other configurations, so that one molecule may contain a hundred carbon atoms. It is therefore not surprising that the number of compounds comprising carbon atoms is much greater than the number of inorganic compounds. Organic compounds form the basis of living organisms, and a science that deals with the substances and processes occurring in organisms is called biochemistry. Recent years have seen the appearance of one more new branchof chemistry, i.e. bioorganic chemistry, which deals with the organic reactions pro- ' ceeding in a cell. |Ions of various metals can bind to orgatrtc molecules in a living organism to form enzymes (biological catalysts), haemoglobin (the carrier of oxygen), and other important substances. These are the compounds studied in bioinorganic chemistry, a science which appeared a few years ago. There are also other branches of chemistry related to biology, medicine, and agriculture, e.g. pharmaceutical, tox-icological, and agricultural chemistries. It is also necessary to mention here a chemistry of high-molecular compounds (polymers). Molecules of these compounds, both organic and inorganic, consist of a large and indefinite number of identical units.

We have briefly informed the reader about the focus of chemistry and the relationship between chemistry and physics. It should be noted, however, that scientists have not yet arrived at a general conclusion on what should be considered the focus of chemistry. Only a few (far from all) branches of modem chemistry were mentioned here; other chemical branches, and the most important chemical concepts, substances and techniques employed in chemistry will be discussed former on.

Why Is Rubber Elastic?

Try to bend or stretch out an iron nail with your hands and you will see that your attempts will fail. But when you take a rubber band and repeat your test, the results will be quite different. Indeed, in order to stretch a rubber

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band by only a one-hundredth fraction of its length, a force must be applied that is 100 000 times less than that required to stretch an iron nail by the same amount. However, the force that must be applied is not the only thing that makes these materials different. The rubber band can be stretched to ten times its original length, and the band will not break. The ability of rubber to experience tension is 1000 times greater than the extensibility of solids under normal conditions. What is responsible for this remarkable property? Surely, the answer must lie in the molecular structure of the substance. People began to use rubber articles a long time ago, but the theory that could explain its elasticity was only developed in 1932 by the Swiss scientist Mayer. Rubber is formed from caoutchouc (crude rubber), which had formerly been cured over a fire into a solid, dark mass. This substance consists of long polymer molecules, in which certain carbon atoms are connected by double bonds. Each molecule of caoutchouc includes several thousands of units, and therefore the molecular mass. of the substance reaches hundreds of thousands. What is the length of the polymer molecule? If the polymer molecule is stretched out to form a thread, its length will equal about a micron. A silk thread that is one-half metre long can be a "model" of such a molecule because it reflects the relationship between the width of the molecule and its length. We used the expression "stretch out" most appropriately because in reality the molecules of solid or liquid polymers are shaped like a zigzag. When placed on the surface of water, the thread, i.e. the model of the molecule, will acquire the form of an unusual curve. The form of the molecule can be predicted theoretically. Run an experiment that resembles blind man's buff. Cover your friend's eyes with a kerchief, turn him around several times and then ask him to take a step. Plot the direction of his motion on paper. Then, repeat everything once more and you will obtain a broken curve - a mathematical model of a polymer molecule. The more times the experiment is run, the more the form of the sketch will approach an average statistical molecule. One can determine the dimensions of such a ball. It should be mentioned, however, that real molecules occupy a threedimensionalspace, while the experiment was run in a plane. Nevertheless, the principle of analysis is the same. The distance between the ends of the

molecule r, as well as the distance between the start and uhe end of your friend's path, can be expressed by the formula

r = l n ,

where l is the length of a step and n is me number of steps. Of course,

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this doesn't mean that if you measure the distance r with a ruler (in metres), the result will coincide with the value calculated from the formula. But the more times you repeat the game, me closer r will approach the calculated value. Therefore, r is called an average statistical quantity. The length of a molecule, i.e. the path covered by your friend, is expressed by h = ln. The ratio h/r shows the degree of "convolution" of the molecule or the degree of "futility" of your friend's wanderings. It is equal to

h / r = n ?

i.e. the longer the molecule, the more it is coiled up.

One can assume that the reason for the elasticity of rubber is that when it is stretched out, the coiled molecules become straightened. The assumption is correct, except for one "but". Let us model the following process: place several threads on a water surface so that they do not contact. Take the ends of two threads and move the threads apart. The threads will not return to the initial positions, and reversible elasticity will not be observed. We have thus created a model soft plastic paraffin, rather than rubber. Now, tie nine threads together into a regular network and place mis chain on the water surface. Carefully pull on one end of the network, while holding the other end. When you release the ends of the network, it will return to its initial position. This is a model of rubber. The "ties" that bind together separate caoutchouc molecules in the rubber are the bridges of sulphur atoms. These bridges are introduced during the vulcanization of caoutchouc when it is treated with sulphur at elevated temperatures.

Several questions may have arisen during this explanation.

What makes the stretched network return to the initial state after the tensile force is removed? The fact is that molecules possess kinetic energy and therefore they are always in motion. In rubber, cross-links do not permit large molecules to move relative to one another under the action of heat. However, thermal motion "pushes apart" separate parts of the molecules and coils up long molecules. It is precisely this thermal motion that returns the network to its initial position. If a piece of rubber is stretched and then freezed at a very low temperature, its molecules lose a considerable part of their kinetic energy, and me stretched rubber cannot return to its initial state. But when heated to room temperature, it shrinks again. Rubber has another remarkable property. It is known that all "normal" bodies and liquids expand when they are heated. Rubber, on the contrary, shrinks when heated. You can check this by running the following experiment. Hammer two nails into a stick and connect them with a metallic spring and a rubber band. If the

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rubber band is placed in hot water, it shrinks, which can be seen from the extension of the spring. When the temperature is raised, the kinetic energy of the molecules is increased, and the network tends to shrink.

Caoutchouc does not contain sulphur cross-links and therefore separate molecules are not connected to each other. Nevertheless, caoutchouc can be reversibly extended (though it resembles paraffin). This occurs because there are "ties" in the caoutchouc that connect long molecules together. The "ties" are formed when these molecules are entangled or interwoven like separate fibres in a piece of cotton wool. Since there are no strong "ties" that can bind separate caoutchouc molecules, the latter is less elastic than rubber, and can dissolve in benzene, while rubber only swells (you can check this experimentally). Why can many polymers, such as polyethylene, be extended irreversibly, i.e. why don't they shrink when the tensile force is removed? The point is that unlike caoutchouc molecules, the molecules of polyethylene are arranged as in a crystal. Such molecules can only slide relative to one another.

We mentioned that the molecular masses of polymers are very large and have been determined for all polymers; we also noted that the real length of the polymer molecule is smaller than the length of the stretched molecule. How do scientists determine these quantities? - Since even large, long polymer molecules cannot be seen under a microscope, indirect methods, are used. The simplest way to determine the molecular mass of a substance is to measure the increase in the boiling point of a solution. Dissolve some table salt in water and heat the solution to boiling. Heat the same volume of pure water under the same conditions, and you will see that the solution boils at a higher temperature than the pure water. In the same way, a salt solution freezes at a lower temperature than pure water. This can be verified by placing two liquids in a refrigerator. The difference between the boiling and freezing points of solution and pure solvent depends on the concentration of solute and its molecular mass. How can the shape of molecules be determined? The shape of molecules in solution can be established by measuring the viscosity of the solution. The longer the molecules and the more they are stretched into a chain, the easier they become entangled and the greater the resistance they exert against the "flowing" water molecules. The following rough analogy can be drawn: the sand from the bottom of a river, which consists of fine particles, can easily be passed through a conical funnel. A piece of cotton wool, however, is drawn through the funnel with difficulty because the wool consists of long entangled fibres.

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In concluding this discussion, let us express an idea from the realm of science fiction. The principle of rubber elasticity can be expressed as follows

And now imagine that a substance is synthesized, whose molecules consist of a variety of rings coupled together to form a chain. Scientists already know how to connect two or three rings into a chain. But if a molecule consists of hundreds or even thousands of such rings, the stretched

Our Food molecule will be represented as

Thus, it becomes obvious that the extensibility of such a polymer should be thousands of times greater than the extensibility of common rubber.

Our Food

A human organism can, in a sense, be compared to an internalcombustion engine, because it converts the chemical energy of substances taken in with the food into motion and heat. The components of food - proteins, carbohydrates, fats, vitamins, salts, and water - are vitally important and necessary for an organism, and each of them has a specific function. Carbohydrates are a "fuel" for humans and animals. These substances have tlie general formula Cx (H2O)y and this is why they are called carbohydrates. For example, the formula of the most well-known and important carbohydrate, glucose, is C6H12O6 or C6(H2O)6. The structure of a glucose molecule reveals that this substance can be simultaneously regarded as an alcohol with five hydroxyl groups OH and an aldehyde that contains a CHO group. Glucose molecules can exist in three forms that are in a dynamic equilibrium, i.e. two cyclic and one opened forms. Hydroxyl and aldehyde groups in the glucose can be identified by the following test. Add 2 or 3 drops of sodium hydroxide to a few drops of an aqueous glucose solution in a glass, and then add a copper sulphate solution dropwise until a persistent turbidity forms.