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

10

When the glass is heated in a pan with hot water, the solution turns yellow and a precipitate is formed. This reaction is the oxidation of the glucose aldehyde group to an acid group —COOH. The oxi-dant - copper(H) hydroxide - is thus reduced to copper oxide Cu2О, which settles as a yellow precipitate.

In another test, a small amount of silver nitrate is dissolved in a few drops of water in a glass and ammonium hydroxide solution is added until the precipitate thus obtained is dissolved. Then, a small amount of glucose is added. When the glass is heated in a hot water bath, silver is reduced and separates in the form of a black precipitate. These two tests can be used to identify the aldehyde group. Now conduct the reactions for the hydroxyl group. Place a few drops of copper sulphate in a glass or on a glass slide and add sodium hydroxide solution. A precipitate of copper hydroxide will result, to which glucose solution is added by drops. A blue compound is formed in which copper is connected to the oxygen of the hydroxyl groups of the glucose.

Glucose is widely encountered in nature, but most frequently it is found in a bound state. Let us discuss the best-known derivatives of glucose. A molecule of ordinary cane sugar (succharose) consists of two parts: glucose and fructose. However, they are arranged in such a way that the glucose aldehyde group cannot reduce copper hydroxide (you can check this with the reaction for an aldehyde group). On the contrary, the hydroxyl groups can easily be identified by the corresponding reaction. Sugar decomposes into

.its main components (glucose and fructose), when it is boiled with an acid. Place a small amount of glucose solution and a few drops of dilute sulphuric acid into a test tube and boil for 2 or 3 minutes. After neutralization with an alkali solution, the glucose can be identified in the resulting solution.

Cane sugar is a disaccharide (dimer) whose molecule consists of two units of the simplest carbohydrates. Glucose can form long polymer chains and, depending on the way the units are connected, starch or cellulose is obtained. When iodine tincture is added to a starch solution (or starch paste), a blue colour appears, because a starch molecule represents a long hollow cylinder, which absorbs iodine molecules to form insertion compounds. Add an excess amount of ammonium hydroxide solution to a small amount of copper sulphate solution in a glass and drop a piece of cotton wool into the obtained solution: in a few minutes the wool dissolves. If dilute hydrochloric acid is added to the contents of the glass, cellulose will again separate from the solution. When starch and cellulose are heated with acid solutions, they de-

11

compose to form glucose molecules.

Thus, food is a source of energy. But how does this "fuel" bum and how is the energy contained in it utilized? Suppose you have boiled rice, boiled potato, sweet tea, and bread for your breakfast. Bread contains about 50 % starch; potato contains 20 %; and rice, 80 %. Therefore, after such a breakfast, you have been supplied with a portion of carbohydrates. The digestion of food begins already in the mouth. Under the action of organic catalysts (enzymes), starch, cellulose, and sugar are broken down in the organism to glucose units. Enzymes accelerate this reaction so that it takes only a few seconds, while under normal conditions prolonged heating with an acid is required for this reaction to occur. Each chain of starch or cellulose decomposes into many thousands of glucose molecules. The decomposition of polysaccharides terminates in the stomach under the action of gastric juice. Glucose is then absorbed through the walls of the intestine and is carried with the blood throughout the entire organism. Each cell therefore obtains glucose with the blood. But the cells do not consume glucose uniformly, because the energy expenditures during hard work are greater than during a state of rest. If the concentration of the glucose in the blood is greater than the amount required for the cells at a given moment, the glucose excess is carried with the blood to the liver where it again polymerizes to form the reserve, nonreducing polysaccharide glyco-gen. But how does blood "know" that excess glucose should be stored for future use? This is controlled by a special hormone called insulin, which is produced by the pancreas. If the cells require additional energy, glycogen is decomposed, and the glucose thus formed is delivered by the blood to the cells.

What happens to a glucose molecule when it enters a cell? These transformations are complex and occur in many stages, but scientists today have unravelled them in general form. Here, special enzymes and an energy accumulator - adenosine tri-phosphate (ATP) - are involved. The latter is a complex compound, which, after the separation of a phosphate group, transforms into adenosine diphosphate (ADP) liberating energy that is required by the cell. At the first stage, the molecule of glucose reacts is» with the enzyme glucosidase and the ATP molecule, which loses its energy and transforms into ADP. Further, a chain of transformations occurs in which the glucose molecule splits into smaller fragments, and all these processes yield ATP, which is utilized by a cell. An intermediate substance in the transformations of glucose is pyroracemic (pymvic) acid CH3COCOOH, which is involved in new reactions and while reacting with oxygen (delivered to the cell by the

12

haemoglobin of blood),' transforms into citric, oxalic-succinic, suc-cinic, and malic acids and, at last, into carbon dioxide and water that are carried from the cell with the blood. The entire process of glucose oxidation by oxygen obtained by breathing consists of 22 consecutive chemical reactions and requires the participation of two dozen enzymes. The cell obtains 38 ATP molecules (formed from ADP molecules) for every glucose molecule that is "burned". The efficiency of cell is 45 %.

Where is this energy spent? First of all, it is used to maintain the normal temperature of the body, because many processes in the cell cannot occur at low temperatures. Every reaction that proceeds in a cell requires energy, and it is released during the transformation ATP → ADP. For example, the muscle protein myosin splits the ATP molecule and as a result the protein molecule becomes shorter and the muscle contracts. This is the mechanism by which the chemical energy of the carbohydrates contained in food is transformed into the motion of the muscles.

Pyroracemic acid, which is an intermediate in the oxidation reaction, can be completely oxidized to carbon dioxide and water when oxygen is in excess. On the other hand, if there is not enough oxygen, pyroracemic acid is reduced to a lactic acid, CH3CH(OH)COOH. This is why sportsmen have increased lactic acid levels in their muscles immediately after intense exercise, because there is too little oxygen in the organism to oxidize all the glucose and their energy requirements were large.

Finally fungus microorganisms (yeasts) transform glucose into pyroracemic acid, as in the case of breathing, and then the pyroracemic acid is decomposed into carbon dioxide and acetaldehyde CH3CHO. The acetaldehyde reacts with ethanol dehydrogenase and a special reducing agent to yield ethyl alcohol (Fig. 1).

Carbohydrates are not the only substances that provide energy for our body. The second type of "fuel" for a living organism is fat. Fats are esters of organic acids and trihydric alcohol - glycerin. They are deposited in body tissues and are similar in function to glycogen. When treated with alkalies, fats decompose into glycerin and the salts of organic acids, which are used as soaps. Add a small amount of an alcohol to a few drops of vegetable oil in a glass and place there a small quantity of sodium hydroxide. Heat the mixture

13

Fig. 1 The oxidation of hydrocarbons in organisms

in a hot water bath until it becomes homoge neous. You have thus obtained a soap solution. If you then add dilute sulphuric acid to the solution, drops of organic acids will appear on the surface of the solution. Vegetable oils contain glycerides of uns&t-urated acids, which can easily be identified using potassium permanganate. The permanganate reacts at the site of a double bond and becomes colourless. Add a small amount of washing soda solution and a few drops of potassium permanganate to a glass containing a few drops of vegetable oil. When the mixturetis stirred, the crimson colour of potassium permanganate vanishes.

Proteins constitute another component of food, which are used as a construction material for the body and to produce other proteins in the organism. When the organism needs more energy than is available, it "bums" the amino acids of proteins. This can be compared to kindling a fire with a costly species of wood. Of course, walnut or beech can be used as firewood, but it is more rational to use them in the manufacture of fine furniture.

There are a vast number of proteins, but they all consist of similar residues of ammo acids. An ammo acid molecule contains two groups, which have opposite properties; namely, a carboxyl group with acid properties and an amino group with basic properties. Due to these groups, amino acid residues can be connected into lone?

14

usuauy contains dozens of amino acid residues.

Isolate an egg white, beat it thoroughly and add a tenfold amount of water. Now filter the obtained mixture through a double layer of gauze. The filtrate contains a protein called albumin, while a protein called globulin remains in the precipitate. The globulin can be dissolved in a solution of common salt. Put a few drops of albumin solution in a glass or onto a glass slide and add a few drops of the alkali and copper sulphate solution: a violet colour appears. The oxygen and nitrogen atoms attached by peptide bonds to the protein react with the copper to form a violet complex.

The sequence of amino acid residues in a protein molecule is called its primary structure. Although scientists can now establish the primary structures for very intricate protein molecules, the procedure is tedious and timeconsuming. In order to establish the primary structure of a protein, special reagents are used to split the protein into residues containing only a few amino acids. The structure of these residues can then be identified more easily. Then, a map is compiled from which the structure of the whole protein molecule can be reproduced. Scientists can now also synthesize many proteins. This is achieved by gradually adding to a chain of amino acid until the required structure is produced.

The hydrogen atoms in the amino groups in a protein chain can form hydrogen bonds with oxygen atoms in the carbonyl groups C=О. Because each hydrogen in each NH group bonds to the CO group three amino acid residues along in the same chain, a long chain of amino acid coils into a regular helix. The general formula of an amino acid is H2NCH(R)COOH, where R stands for hydrogen, methyl, or more complex radical or group, for example, -CH2CH2CH2CH2NH2. This group distinguishes the amino acids. When the chain of a protein molecule coils into the helix, the radicals protrude from the spiral cylinder, and the resultant helix is called the secondary structure of the protein.

The radicals in one protein molecule can attract groups in another and thus the molecules interact. As a result, the whole protein helix can roll into a ball. However, this rolling is not random event, it follows a plan. This is why proteins need radicals and why a different protein is formed when only one radical is changed in a molecule. The way the molecule is rolled is called

15

the tertiary structure of the protein. But this is not all. Several protein balls can combine to form a quaternary structure. This is the case with the haemoglobin molecule. It is rather difficult to establish the tertiary and quaternary structures of a protein, and to do it protein crystals must be analyzed by X rays.

All four structures of a protein can be modelled. Take a long thin wire. This represents the primary structure of a protein molecule. To obtain the secondary structure, coil the wire into a spiral. The tertiary structure is produced when the spiral is rolled into a ball. Several balls combined together model a synthesized protein in its natural form. Indeed, a protein can only "work" in the organism if it is in its tertiary or quaternary structure. However, this structure can easily be destroyed. Everyone has done this when boiling an egg. What happens to the transparent, liquid, and water-soluble egg albumin when it is heated? It turns out that the hydrogen bonds between the - R groups, which keep the protein molecule in the form of a compact ball, break. After this, the tertiary structure is destroyed and the protein, whose molecules take on the form of long entangled threads, ceases to be soluble. A protein can be denatured, i.e. its tertiary structure destroyed, in other ways. If alcohol or acetone is added to an aqueous solution of egg albumin, the solution becomes turbid. In fact, the protein has not yet been denatured, rather the albumin has been precipitated. When more water is added to the turbid solution, the precipitated albumin redissolves. However, when the turbid solution is allowed to stand for a time, the tertiary structure of the protein gradually breaks down completely.

The proteins eaten as food are broken down in the stomach into separate residues, which are further degraded in the intestine into individual amino acids. Those can then penetrate the intestine walls and are carried by the blood to the body's cells. Once in the cells, the amino acids are combined into different proteins, i.e. the proteins needed by the cell. The synthesis of a protein in a cell requires energy, and therefore molecules that accumulate ATP, which acquire energy as a result of glucose oxidation, are involved in the synthesis. Proteins with constituents such as valine, leucine, and lysine must always be eaten. They are the essential amino acids (as they cannot be replaced by other ammo acids), and everyday some 30 g of these proteins should be consumed. The other ammo acids, i.e. alanine, asparagine and glutamine, are not obligatory components of food, because they can be synthesized in the organism if necessary.

16

The Rate of Chemical Transformations

A great number of chemical reactions are known that differ in reacting substances, reaction phase (gaseous phase, liquid, or at the phase interface), and in the heat effect (absorption or evolution of heat during the reaction). Inaddition, there is another parameter that distinguishes one reaction from another.

...A rifleman pulls the trigger of a rifle, the gunpowder in the cartridge catches fire, and the formed gases push the bullet out of the barrel. The chemical reaction between the components of the gunpowder occurred within fractions of a second. Another type of reaction is the conversion of wood - trunks of dead trees - into coal. This chemical process requires millions of years for its completion. Thus, chemical reactions differ in their rates. But how does one define the rate of a chemical reaction?

First, the velocity of a moving body should probably be defined. It is the path covered by the body divided by the time required by the body to cover this path. Certainly, this division yields the average velocity of the body over this path. In order to determine the instantaneous velocity at a given moment, the time interval over which the velocity is measured should be reduced as much as possible, i.e. it should approach zero. In this case, the velocity will be expressed mathematically as the derivative of the path with respect to time.

Now, imagine that a team of masons are building a brick house. How can one determine the velocity of the building process? One of the ways is to determine the velocity of "disappearance" of the bricks reserved for building. By dividing the total number of bricks by the time spent on building, one can find the average velocity. But the actual velocity varied over time: at first it was high, but then the workers had to carry bricks to higher floors, and the velocity decreased somewhat. The velocity of construction at a given moment can be found by dividing the number of bricks layedjust before, and right after, this moment by the time that elapsed between these two moments, and then by making this time interval approach zero.

The same technique is employed in chemistry when determining the rate of a chemical reaction; the only difference is that reacting molecules are counted instead of bricks. But since it is difficult to count molecules at each moment, scientists determine the concentration of substances, i.e. some great number of molecules per unit volume. When determining the rate of a chemical reaction, the concentration of a reacting substance at the initial moment t1 is determined (let it be c1), and then the concentration of this re-

17

agent is measured at time t2 (it turns out to be c2). The difference between the concentrations, c2 – c1 = ∆ c, is divided by the time interval, t2 — t1= ∆ t and then this interval is made approach zero. The reaction rate W is written as follows:

The expression dc/dt denotes the derivative of concentration with respect to time;

a minus sign is placed before the expression to make the rate positive (because the value Cg is less than Ci).

What does the rate of a chemical reaction depend on? Let's perform an experiment. It is known that sodium thiosulphate reacts with acetic acid to produce colloidal sulphur, which separates in the form of a white opalescent precipitate. The reaction equation is

Na2S2О3+2CH3COOH=H2O+SO2↑ +S↓ +2CH3COONa

Dissolve two teaspoons of sodium thiosulphate (also known as hyposulphite and widely used in photography as a fixing agent) in a glass of water at room temperature. Then, pour this solution into each of four empty glasses: 2 teaspoons of the solution into the first glass; 4 teaspoons into the second; 8 teaspoons into the third; and 16 teaspoons into the fourth glass. Add water to the first three glasses so that the volume of the solution in each of them equals the volume of the solution in the fourth glass. You have prepared four solutions with different concentrations. The concentration of the second solution is twice that of the first solution; the concentration of the third solution is four times greater than that of the first solution and two times greater than the concentration of the second solution. Quickly add a teaspoon of acetic acid to the first solution and record the time that elapses between the moment the solutions are mixed and the moment the turbidity appears. The mixture should be stirred with a spoon during the reaction. Assume that 90 seconds passed until the turbidity appeared. Add a teaspoon of acetic acid to each of the remaining solutions in sequence and record the time when the turbidity forms in these solutions. Assume that it was 40 seconds for the second glass, 22 seconds for the third, and 12 seconds for the fourth. Now take a sheet of graph paper and plot the time history of thiosulphate concentration. Plot the concentration (expressed in teaspoons of the initial solution) along the a:-axis, and the time elapsed ts (in seconds) along the y-axis. A curve drawn through the four points has the form of a hyperbola, and the latter is a graphic representation of an inverselye Unpropor-

18

tional function. Therefore, if you plot the value that is inverse to time tg (instead of ts itself) along the y-axis the experimental points will lie along a straight line passing through the origin. But the inverse of time tg is proportional to the reaction rate at the initial moment, i.e. W~ 1/tg. Hence, the greater the concentration of a reacting substance, the greater is the reaction rate. Generally, the reaction rate is described by the following equation:

W = kcAcB .

Here k is the proportionality factor: it does not depend on the concentrations of the reactants and is called the rate constant of a chemical reaction; cA and cB are the concentrations of reactants A and B; a and b are, simulta-

neously, the exponents at c and the coefficients showing the number of A and B molecules involved in the reaction. It is clear that the reaction rate decreases as the reactants are used up. Recall also the previous example with masons, where the velocity of construction decreased with time. However, this analogy is not accurate enough. The phenomenon under consideration can be illustrated more accurately using a billiard game as an example. First, assume that a chemical reaction occurs between two molecules only when they collide. Then, suppose we have two types of molecules, which are represented by white and black balls. The molecules are in constant thermal motion; we will hit the balls in any sequence or direction along the table. When the white and the black ball collide, they will be removed from the table, i.e. the reaction has occurred, and me molecules of reactants have transformed into me molecules of reaction products. Now, imagine that there are 100 white balls and 100 black balls on the table. It is obvious that they will frequently collide and will immediately be removed from the table. A different situation will happen when only one white ball and one black ball remain on the table: you will have to drive them for a good deal of time before they collide. Now it becomes clear why the reaction proceeds slowly when the concentrations of reactants are low.

The billiard game example gives the answer to the question why the equation for the reaction rate contains the product of concentrations instead of their sum. It was already mentioned that for the reaction to occur, two reacting particles must collide, i.e. they must be at the same place in a reaction mixture at the same time. The probability that a molecule of a given reactant will be at a given place is proportional to the reactant concentration. On the other hand, the probability that two molecules will simultaneously arrive at the same place is equal to the product of probabilities of meeting a molecule of each reactant at this place, i.e. it is proportional to the product of reactant

19

concentrations.

Let us consider other parameters that affect the reaction rate. Dissolve two teaspoons of sodium thiosulphate in a half glass of water. Pour two teaspoons of the solution into each of four empty glasses. Dilute the solutions with water to half a glass. Add a teaspoon of acetic acid (vinegar essence) to the first solution, which is at room temperature (about 20°C, measure the temperature with a thermometer). Record the time it takes for the turbidity to appear (constantly stir the solution). Assume that the time is 75 seconds. Now heat the second solution to 30°C, repeat the above procedure, and record the time of turbidity formation. You will see that the time has been reduced by half (in our case it has been reduced to 35 seconds). At 40°C, me turbidity appears in 20 seconds; and at 50°C, in 10 seconds.

Try to process the results mathematically. If you construct a graph by plotting the temperature and the reaction time along the corresponding axes,, you will obtain a curve whose physical meaning is difficult to understand. But if you plot the inverse to the absolute temperature along the a:-axis (this quantity is equal to 1/T, where T = 273 + t°C, and t°C is the temperature measured in degrees centigrade) and the natural logarithm of the reaction rate along the y-axis, the experimental points will fall in a straight line. (Natural logarithms can be found in reference tables or calculated on calculators.) The straight line is described by the mathematical equation

lnts = b / T + a

where a and b are constants. The equation can be transformed into

ts = a1eb / T

where e is the base of the natural logarithm, and 0,1= e°. The reaction time tg is inversely proportional to the reaction rate constant k. Taking this into account, the expression for k can be rewritten as follows:

k = Ae− b / T .

Here A is reaction, a new constant for the given

It is obvious that the molecules interact only when they collide. Experiments demonstrate, however, that not every collision results in a reaction. The reaction proceeds only when the energy of the colliding molecules is not less than a certain quantity Ea, which is specific for each reaction. If Ea and the absolute temperature T of the reaction mixture are known, one can calculate the number of molecules n that enter into a reaction upon col-