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

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liding. This number is determined via the exponent:

n = Ne− Ea / RT ,

where N is the total number of molecules, and R is the universal gas constant. It is not far from here to the famous Arrhenius equation, which relates the reaction rate constant to the temperature:

k = Pze− Ea / RT

Pz=A and Ea/R = b. If we determine the reaction rate constant for several temperatures, we can calculate from the Arrhenius equation the value of Ea called the reaction activation energy. The value Ea is found graphically in the following way;

We plot 1/T along the c-axis and the logarithm of the reaction rate constant along the y-axis. The slope of the straight line obtained is equal to Ea / R . Note that the activation energy is an important characteristic of a

reaction; the less the activation energy, the more readily the reaction occurs. Up to this point, we have been discussing reactions that proceed to the

end, i.e. reactions that during a certain period of time completely transform initial substances A and B into substances D and E (although, theoretically, for the reaction to be completely accomplished, a large amount of time is required). Reactions are known, however, where substances D and E can also react to produce substances A and B. These reactions are called reversible

and can be expressed by the general equation

aA + bB ↔ dD + eE .

Imagine that we have poured together solutions of substances A and B; a reaction has started whose rate is W-1=k-1cAcB. In the course of the reaction, substances A and B are being used up, and their concentrations decrease; hence, the reaction slows down. On the other hand, substances D and E begin to form, and their concentrations increase. As a result, the reaction between D and E, whose rate is W-1=k-1cDcE, accelerates. Finally, the reaction arrives at equilibrium, which is characterized by a constant K (in this case W1 = W-1). The value of K is equal to the ratio of the reaction constants of theforward and reverse reactions:

Mix dilute solutions of iron(III) chloride and ammonium thiocyanate in a glass. Pour the resulting blood-stained solution into each of three empty glasses (keep a certain amount of the solution as a reference). Add a satu-

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rated solution of iron(III) chloride to the first glass, a saturated solution of ammonium thiocyanate to the second, and a saturated solution of ammonium chloride to the third. You will notice that the colour of the solutions in the first two glasses becomes more intense (compare it with the colour of the reference solution), while that of. the third solution becomes pale. How can the observed changes be explained? The reaction equation is

FeCl3+3NH4SCN↔ Fe(SCN)3+3NH4Cl

The equilibrium constant for the reaction is written as

K = [Fe( SCN )3][ NH4Cl] 3 [FeCl3][ NH4 SCN] 3

The brackets denote the concentrations of these substances c. The quantity K is constant, therefore when the concentration of FeCI3 (in the first glass) is increased, the equilibrium is shifted to the right, and the concentration of Fe(SCN)3 increases as About Catalysis well. The colour intensity is determined by the concentration of iron thiocyanate Fe(SCN)3. When ammonium chloride is added, the equilibrium shifts to the left, and the concentration of the coloured substance Fe(SCN)3 decreases. We will later return to this reaction which makes it possible to identify iron ions and determine their concentration in a solution. Now, let us make some conclusions. We have briefly considered a field of chemistry that is concerned with the rates of chemical reactions and is called chemical kinetics. You can read about chemical kinetics in books on physical chemistry and popular-science books.

Chemical Reactions and Electric Current

If you need a small amount of hydrogen, the best way to obtain it is to treat zinc with an acid. Pour a small quantity of dilute sulphuric or hydrochloric acid into a glass and drop a few pieces of zinc into it: the metal will slowly dissolve, and hydrogen bubbles will appear on its surface. Now, drop several pieces of zinc into a solution of copper sulphate (blue vitriol) for a short time and then transfer them to an acid solution. You will notice that this time the metal reacts with the acid more vigorously. How can this be explained? When zinc is dissolved in an acid, the positively charged ions of hydrogen, 112 from the acid, receive electrons from zinc atoms and are transformed into neutral molecules of I la. Having lost two electrons, zinc atoms transform into Zn2+ ions, which pass into the copper sulphate solution.

But here we have added zinc to a copper salt. As we know, copper is less active than zinc, and therefore electrons migrate from zinc to the copper

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ions. The positively charged zinc ions that result pass into the solution, while metal copper precipitates ' on the zinc surface. As a result, zinc with inclusions of copper is formed. But zinc has more free electrons, and they are more easily detached from its atoms. Therefore, zinc willingly gives its electrons to copper, and then electrons from copper migrate to the hydrogen ions, which transform into molecules. The functions of zinc and copper atoms are different: zinc atoms must only pass into the solution in the form of ions, while copper atoms must only reduce FT to Hs. In pure zinc, both functions are performed by the atoms of this metal. The surface of zinc is covered by molecules of hydrogen and the dissolution of the metal slows down.

Thus, we have discovered that electrons willingly flow from one metal to another. But if each of these two metals is immersed into a solution of its salt and a metal wire is used to connect them externally, electrons will flow through this wire, i.e. an electric current will be generated. Thus, we have explained the main principle of the chemical source of electricity. Its structure will not be dealt with in greater detail; instead we will use a flashlight battery as a chemical source of electric current for further experiments.

Attach two isolated copper wires, 30 cm long, to the terminals of the battery. Wind the other end of each wire 2 or 3 times around two separate small coins. Connect a bulb from a flashlight in series with the battery. Then, pour water into a glass to one-third of its capacity and immerse the coin electrodes into it such that they do not contact with each other. The bulb willnot light up. If you replace the water with a sugar solution, the result will be the same: in other words, no effect will be observed. Now, pour water into the glass again, immerse the coins into it and add several drops of sulphuric acid to the water: the bulb will light up and gas bubbles will rise to the surface from both coins.

I In the next experiment, the electrodes are immersed into a table salt solution containing 2 or 3 drops of alcohol phenol-phthalein solution. The bulb lights up, and a colourful sight is observed: both coins become immediately covered with gas bubbles, while the coin that is connected to the negative terminal (the cathode) is surrounded by a crimson cloud, which gradually spreads throughout the solution.

How can these phenomena be explained? Electrons from the negative terminal of the battery move to the coin cathode, and thus give it a negative charge. The positive terminal of the battery "absorbs" electrons from the anode, which becomes positively charged. When the two coins are separated by a layer of pure water, electrons car/t How from the cathode to the anode, be-

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sulphuric acid solution, move towards me negatively charged cathode. At the cathode, Cl- ions receive electrons and transform into hydrogen atoms. Two hydrogen atoms combine to form a molecule of hydrogen gas, which bubbles from the solution. When electrons leave the cathode, the battery has to supply new portions of negative charges to the cathode. This means that an electric current is flowing through the circuit and the bulb lights up.

The electrolysis of a H2SО4 solution actually proceeds in a more complex way, and different substances, for example, sulphur derivatives and hydrogen peroxide, can be among the products.

The electroconductivity of water can be increased by adding a solution of any electrolyte (a substance that dissociates in water into ions) to it. In industry, for example, water is decomposed by an electric current into hydrogen and oxygen by adding sodium hydroxide to it.

Now, let us consider the electrolysis of a table salt solution. Sodium chloride in solution dissociates into Na+ and Cl- ions. The former move to the cathode; the latter, to the anode. It would seem that sodium ions should receive electrons from the cathode and transform into metallic sodium. But this does not occur, because sodium is a very active metal that cannot exist in water in a free state. Therefore, electrons from the cathode are transferred not to Na+ ions, but to H+ ions, which are always present in an aqueous solution. As a result, hydrogen evolves at the cathode (Fig. 3b). The other constituent part of water is OН- ions, which accumulate near the cathode. It is these ions that are responsible for the crimson colour of the solution around the coin cathode when phenol-phthalein is added. The latter is known to be a reliable indicator for alkalies. When chloride ions arrive at the anode, they give their electrons to the anode, and transform into molecules of chlorine gas. Na+ and OH- ions, i.e. the components of sodium hydroxide, remain in the solution. Thus, it becomes clear why the electrolysis of an aqueous NaCI solution is used as an industrial method for me production of three important substances: hydrogen, sodium hydroxide, and chlorine.

We have briefly considered the fundamentals of electrochemistry - one of the branches of physical chemistry - that studies chemical reactions induced by an electric current, as well as reactions that produce an electric current. The reader may think that electrochemistry only deals with inorganic substances. But this is not so: many organic substances are of interest to electrochemists. During recent years, the electrosynthesis of many organic compounds has been intensively developed.

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Vitamins and Chemistry

Today we know that the normal functioning of the body requires vitamins and that the lack of vitamins in food results in severe diseases. So, what are these substances, how many vitamins are known, and what role do they play in the organism?

The history of vitamins goes back to the end of the last century. At the time, it was known that proteins, fats, carbohydrates, water, and mineral salts were indispensable components of food, and that without them the organism develop normally and will finally die. In 1881, the Russian scientist Lunin carried out an experiment in which he prepared "artificial" milk by mixing purified proteins, fats, carbohydrates and mineral salts in the same proportions they are in natural milk. This "milk" was fed to the experimental animals (mice), and after a certain time, all the animals died. Thus, it was clear that natural food contains something else essential for an animal organism. In 1911, a Polish biochemist C. Funk called these indispensable substances vitamins (from the Latin vita meaning life + amine). Although it was later shown that many vitamins are not amines, the name remained.

Dozens of vitamins and their structures are now known. Most vitamins can be synthesized industrially. In an organism, vitamins provide constituents to enzyme molecules. A living organism cannot synthesize vitamins by itself and therefore they must be eaten in food. We shall consider several vitamins. Vitamins are mainly named by the letter of the alphabet, and we shall consider them in the alphabetical order.

A deficiency of vitamin A causes metabolic disturbances, weight loss and one's hair falls out. The main danger of vitamin A deficiency is softening of the cornea and dry eyes, resulting in night blindness. In a molecule of vitamin A, which is also called retinol, double bonds alternate with single bonds in a long chain of carbon atoms that terminates with a hydroxyl group. Retinol is a pale-yellow viscous substance that is soluble in fat and insoluble in water. A derivative of vitamin A called retinene is a pigment (in which - CH2OH group is replaced by an aldehyde group - CHO) and is contained in the retina of the eye. In the rods of the retina, it combines with the protein opsin to form the visual pigment rhodopsin. When light falls on a rod, the rhodopsin decomposes into opsin and cis-retinene. Then, cis-retinene immediately transforms into trans-retinene, which is accompanied by the transmission of a nervous impulse to the brain (Fig. 3). After the impulse has reached the brain, which means that a man has perceived a quantum of light, the trans-retinene is again transformed into the cis-form. This process can be

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compared to winding of a film in a camera to take a next picture, bringing unexposed film in front of the lens. This "winding on" in the eye proceeds in the dark according to the following scheme. An enzyme and a biological reducing agent reduce the trans-retinene to trans-retinol (i.e. to vitamin A), which is then transformed into cis-retinol by the enzyme isomerase. Then, the cis-retinol is oxidized to cis-retinene, and the eye is ready to "photograph" the next picture.

Vitamin A is contained in fish-liver oil, which can be confirmed experimentally. Add five drops of chloroform and a drop of

Fig. 3 A. simplified scheme of conversions of vitamin A. derivatives during light perception by an eye. Black dots are the methyl groups

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concentrated sulphuric acid to a drop of a fresh fish-liver oil in a glass or on a glass slide: the mixture turns red. Carotene is often found in natural food. A molecule of carotene consists of two parts, i.e. two molecules of vitamin A. Carotene is converted in the body into retinol. Triturate a rose hip with sand in a glass and add several drops of chloroform. The carotene in the rose hip is extracted by the chloroform. Pour the solution into another glass and add several drops of concentrated sulphuric acid. The upper chloroform layer turns green and then blue.

Rose hips also contain another vitamin, i.e. vitamin C (ascorbic acid). This is a colourless, crystalline substance that is readily soluble in water. In aqueous solution it is unstable and it is easily decomposed by heating. Vitamin C is a good reducing agent, and it is this property that is utilized in biological processes. Ascorbic acid is involved in the synthesis of hormones, and it prevents adrenaline, which is an important biological compound, from oxidizing. Ascorbic acid is also a constituent of some enzymes. It is a cure for scurvy, and it should be taken when exercising heavily either physically or mentally. Many reactions are based on the reducing properties of ascorbic acid. Obtain an extract from a rose hip by triturating it with sand and water. Filter the resultant mass through cotton wool or filter paper and use the filtrate in the following experiments. Add a few drops of the filtrate to a few drops of iodine solution in a glass. The iodine solution decolourizes as a result of reduction, while the ascorbic acid is oxidized to dehydroascorbic acid. In a second test, add 1 or 2 drops of sodium carbonate solution and a few drops of the nitrate to a strongly diluted solution of methylene blue dye in a test tube. The dye becomes colourless when the test tube is heated. Finally, if a drop of the nitrate is added to a drop of potassium ferricyanide in a test tube, a precipitate of Prussian blue is formed. These experiments can also be carried out with pure ascorbic acid.

Fish oil, which we have already mentioned, is a useful product. In addition to vitamin A, it contains vitamin D (also known as calciferol). Vitamin D is essential for normal calcium and phosphorus deposition in bones and teeth. Deficiency of vitamin D in children results in rickets. Mix several drops of fish oil, chloroform, aniline and a drop of concentrated hydrochloric acid in a test tube. Heat the test tube over a gas burner and observe how the mixture turns red, which is characteristic of vitamin D. When a mixture of an oil solution of vitamin D, chloroform, and concentrated sulphuric acid is vigorously stirred, it turns red.

In 1936, the Hungarian scientist A. Szent-Gyorgyi concluded that per-

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meability of small blood vessels is affected by a substance called vitamin P complex. Today it is known that this effect is produced by rutin, hesperidine, and the catechins. Rutin contains phenol hydroxyl groups, and hence produces a characteristic colour with ferric chloride. Mix a saturated solution of rutin (you can buy it in a chemist or pharmacy store) with a few drops of ferric chloride in a test tube or on a glass slide: the liquid turns green. The rutin is reduced when 2 or 3 drops of concentrated hydrochloric acid and a piece of zinc are added to a few drops of vitamin P solution. As a result, the liquid turns red. The catechins, which are the components of vitamin P, are contained in tea. Pour some ethyl alcohol into a glass containing some tea leaves. The solution above the tea leaves turns yellow after some time. Then, pour the solution into another glass and add an alcohol solution of ferric chloride. Observe the green colouration.

Vitamin PP has nothing to do with vitamin P complex. Antipellagric vitamin PP is an amide of nicotinic acid, but one should not conclude from the name that it is as poisonous as nicotine. However, nicotine can be used to produce nicotinic acid, from which vitamin PP can easily be obtained

issolve a small amount of nicotinic acid in a few drops of dilute acetic acid in a test tube with heating. Boil the solution and add a few drops of copper acetate. The liquid becomes turbid and turns light blue. After a time, a dark blue precipitate separates from the liquid. Nicotinamide plays an important role in biology. It combines with sugar, ribose, phosphoric and adenylic acids into a complex, which is a component of the hydrogenase enzymes. One of these enzymes takes part in light perception by the rods of the eye retina. The reducing agents nicotinamide adenine dinucleotide (NAD) and HKXrtinamide adenine dinucleotide phosphate (NADP) convert trans-retinene to transretinol and contain vitamin PP.