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10 Обзорное упражнение
1. Oppositely charged ions attract each other very strongly and usually require energies of 400 kg mol-1 or more in order to be separated.
2. In order for hydrogen bonding to occur, there must be a hydrogen atom connected to a small, highly electronegative atom in one molecule.
3. In the USA 03 and other strong oxidants from photo chemical smog are known to do more than 450 million dollars damage year to tires and other articles made of rubber.
4. The concentration of phosphorous compounds in the earth’s atmosphere is so small as to be negligible.
5. S03 dissolves in water to give а strong acid, while SO2 gives a weak one.
6. А Swedish chemist Svante Arrhenius was the first to define an acid as any substance which increases the concentration of hydrogen ions in aqueous solution.
7. If the pH is greater than 12 or 13, the solution attacks skin rapidly enough to be dangerous.
8. The concentrations of lead ions and chlorine ions in the resultant mixture are too low for a precipitate to from.
9. The first of carbide complexes to be dissolved was CFe5(CO)15.
10. In order to determine whether a reaction is spontaneous or not, it is not sufficient the entropy difference between products and reactants to be determined.
11. The liquid to be electrolysed must be able to conduct electricity.
12. A particularly interesting type of nuclear reaction in an accelerator is the production of the transuranium elements. These elements have atomic numbers greater than that of uranium and are too unstable to exist for long in nature.
13. The toxic effect of nonactin and several related antibiotics is the results of their ability to transport alkali metals ions to regions of a cell where they should not be.
14. The incessant motion of the very large number of gas molecules
causes them to со11ide with the container walls so frequently that a steady pressure on the walls results.
15. Throughout Nature there is a tendency for forces to balance one another.
16. In order to show а pure rotational spectrum, а molecule must possess а permanent dipole moment.
17. The fusion of nuclei to form heavier ones is capable of producing very large amounts of energy, such processes are believed to take place in the sun and starts.
18. For а vibration to be active in the infrared spectrum, the motion of the nuclei must result in a change in the dipole moment of the molecule.
19. Most elements form no compounds in which there can be assured to be single covalent bonds between two atoms of the element.
20. Thio-halides are likely to be similar in behaviour.
21. Structural studies of some of carbon compounds have shown the ligands to be bonded to boron atoms in the 6 and 9 positions.
22. The infrared spectrum shows S2N2 to form а four-membered ring.
23. Iodine heptaflueride is known to be much more reactive than the pentafluoride.
24. The largest quantity of salt are considered to occur in sea water.
25. In late years the chemical industries are estimated to have used 60% of all the salt produced in the USA.
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Catalysts are known to be used in a wide variety of industrial processes. Many catalysts are either transition metals or their compounds as they are able to show variable oxidation states. Although many other compounds are also found to posses catalytic properties.
We assume catalysts to work by providing an alternative reaction pathway with lower activation reaction than uncatalysed pathway.
In industry catalysts are chosen to maximise both yields and profits. Catalysts prove to become useless if they combine irreversibly with another substance so that the active sites become blocked .Such substances are known to be called catalytic poisons. Sulfur must be removed from crude oil before refining takes place as it has the ability to poison catalysts. Carbon monoxide and cyanide have also been observed to block the catalytic action ( 900 печатных знаков).
2. Liquid Crystals
Liquid crystals are a phase or state of matter that lies between solid and liquid states. The molecules in a liquid crystal tend to retain their orientation as in solid , but can also move to different positions as in a liquid. The physical properties of liquid crystals (such as electrical conductivity, optical activity and elasticity) are found to be dependent on the orientation of the molecules relative to some fixed axis in the material. The orientation of the liquid crystal molecules are shown to be controlled by applied field due to their polarity.
The orientation of the molecules affects the ability to transmit light. In liquid crystals displays used in digital watches, calculators and laptops a small voltage is applied across a thin film of the material. This controls the areas of the display that are light and dark, and
hence gives the characteristic reading of the time or letters. The great advantage of LCDs over other types of electronic displays is sure to be their ability to work on small currents.
The disadvantage is that they work only within a certain temperature range which is known to explain why a digital watch or laptop screen may give a strange display in very hot or cold temperatures ( 1307 печатных знаков ).
Much of excitement in nanochemistry has been in the manufacture and characterization of nanotubes. These are essentially made from just carbon atoms arranged into hexagonal rings with pentagonal rings to close the structure at the ends. Either single or multi-walled tubes are shown to be formed from concentric nanotubes They appear to have huge tensile strength .
Bundels of nanotubes may be shown to be in the region of 50 times stronger than steel. Reseach is currently under way to use carbon nanotubes to create transistors and other electronic devices much smaller than can be created using silicon chips. By inserting silver halides into singlewalled nanotubes and the decomposing them to silver, metallic nanowire of pure silver with diameter 2,0X10 m have been made, which are reported to be the thinnest electrical wires in existence.
Nanotechnology has the potential to solve many problems, such as increasing food production, preventing, monitoring and curing diseases, and improving information and communication technology, although most of these benefits still are likely to lie somewhere far in the future ( 1222 печатных знака).
4. Cross-linking in polymers
When polymers are able to form giant three-dimensional structures, as for example in phenol-methanal plastics, it makes the polymer extremely strong, because a large amount of energy is required to break it down. It also makes them insoluble and generally unreactive.
Another example of this is Kevlar, the material which is known to be used in production of lightweight bullet-proof vests, composites for motor-cycle helmets and armour.
Kevlar is a polyamide made by condensing 1,4,-diaminobenzene( para-phenylenediamine) with benzene-1,4-dicarbonyl chloride( terephthaloyl dichloride). A strong three-dimensional structure in this polymer is believed to be formed due to hydrogen bonding between the long rigid chains.
Polymers can be modified by various means during their manufacture. For example, air can be blown into polyurethane to make polyurethane foams for use as cushions and thermal insulation. The fibers of polyesters can be blended with other manufactured or natural fibers to make clothes such as skirts that are more comfortable, and which can hold dyes fast ( 1175 печатных знаков).
5. Silicon and photovoltaic cells
Metals are known to conduct electricity well when voltage is applied , because they contain delocalized electrons. Silicon is a semiconductor. A crystal of silicon is shown to contain a lattice of silicon atoms bonded to each other by share of paired electrons. These electrons are in fixed positions , so silicon is a poor conductor under normal conditions However, the energy required to excite an electron and free it from its bonding position is equivalent to the energy of light with a wavelength of 1.1x10m. So sunlight is able to excite an electron in silicon. The electron is then free to move through the crystal lattice when voltage is applied, making silicon an electrical conductor. In practice the process does not appear to be very efficient and the cost of purifying the silicon is high. To improve the efficiency of photoelectric effect doping need to be used. Adding very small amount of other elements one can produce n-type semiconductors as well as p- type semiconductors. The junction between the two different types of semiconductors allows the current to flow in one direction only. Such a junction is known to be a rectifier and can be used to convert alternating current into direct (1342 печатных знаков).
6. Research, development and testing of new pharmaceutical products
The research and development of new drugs is a long and costly process, but the financial rewards to pharmaceutical companies can be high. To test a new product it is necessary to isolate it from an existing species or synthesize it chemically and then subject to thorough laboratory and clinical pharmacological studies to demonstrate its effectiveness. It is tested on animals to determine the lethal dose required to kill 50 % of animal population. The effective dose required to bring about noticeable effect in 50 % of population is also obtained so that the safe dose to administer can be determined. Then the drug is likely to be used in initial clinical trial on humans. This is usually on consenting volunteers as well as patients, half of whom are given the drug and half of whom are given a similar-looking placebo. The volunteers do not know whether they have been taking the drug or placebo .This initial trial is closely monitored to establish the drug’s safety and possible side effects
(1237 печатных знаков).
Because alcohol is stated to impair judgment and slow down the body’s reflexes, most countries have strict legal limits for permissible amount of alcohol in the blood for motorists, airline pilots and people operating machinery. There are several ways to determine the amount of alcohol in the body. In past motorists might be asked to blow into a breathalyzer. This involved acidified potassium dichromate(VI) crystals turning green as they were reduced by the ethanol to chromium (III) ions, or use of a fuel cell, where the ethanol was oxidized to produce electricity. These methods are not sufficiently accurate to be used in court. At police station a blood sample may be taken and sent to a forensic science laboratory. Modern intoximeters can now be used to measure accurately the amount of alcohol in the breath. They are known to be based on the principle that the carbon –to-hydrogen bonds in the ethanol absorb infrared radiation of a particular wavelength. Ethanol can interact with and considerably enhance the effect of other drugs because it depresses the central nervous system . With aspirin it increases the risk of stomach bleeding, because the alcohol helps to transport aspirin through the protective mucus lining of the stomach wall (1484 печатных знаков).
8. Historical development of penicillin
The use of moulds to prevent infections has existed in folklore for a long time. A Costa Rican, Clodomiro Picardo Twight working at the Pasteur Institute in Paris was the first to record the action of the fungal genus Penicillium sp. on the growth of bacteria and the work was published in 1927. However, the discovery of penicillin is known to be attributed to the Scotsman Alexander Fleming.
Fleming, a bacteriologist, was working with cultures of Staphylococcus aureus, a bacterium that causes boils and other types of infection. In 1928 hе left an open petri dish containing one of the cultures in the laboratory while he went away on holiday. Upon his return he noticed a mould to have developed and inhibited growth of the bacterium. He deduced the mould produced a compound ( which he called penicillin ) that inhibited the growth of bacteria. The discovery of penicillin is often quoted as an example of serendipity-the accidental discovery of something useful while looking for something else. In the 1950s the structure of penicillin was determined, and this enabled chemists to synthesize different types of penicillin and other antibiotics ( antibacterials originating from moulds) (1222 печатных знаков).
9. Dissolved oxygen in water
When matter decomposes aerobically in water it uses up the dissolved oxygen. The biological ( or biochemical) oxygen demand (BOD) is a measure of the dissolved oxygen ( ppm) often measured over a set time period of 5 days. Water that has a high BOD without the means of replenishing oxygen will rapidly fail to sustain aquatic life. A fast –flowing river is likely to recover its purity, because the water becomes oxygetared through the mechanical action of its flow. Lakes have relatively little flow, and reoxygenation appears to be much slower or does not happen at all. Pure water has a BOD of less than 1ppm, water with a BOD above 5ppm is known to be regarded as polluted (744 печатных знаков).
10. Waste water treatment
Waste water treated at sewage works contains floating matter, suspended matter, colloid matter, dissolved ions and a range of micro-organisms. The aim of the treatment is to remove this additional material and recycle the fresh water. Primary treatment is supposed to remove about 60% of solid material and about a third of the oxygen-demanding wastes. The incoming sewage is passed through coarse mechanical filters to remove large objects such as sticks, paper, and rags. It is then passed into a grit chamber, where sand and small objects settle. From there it passed into a sedimentation tank, where suspended solids settle out as sludge. A mixture of calcium hydroxide and aluminium sulfate is added to aid this process.
The two chemicals combine to form aluminium hydroxide, which precipitates, carrying with it suspended dirt particles ─ a process which is known to be called flocculation. Secondary treatment is likely to remove up to 90 % of oxygen demanding wastes. The principle is to degrade the waste aerobically using oxygen and bacteria. Tertiary treatment is aimed to remove heavy metal ions, phosphates and nitrates. Nitrates are difficult to remove by chemical means as all common nitrates are soluble, so precipitation cannot be used.
Anaerobic denitrifying bacteria can reduce them to nitrogen, or the water can be passed through algal ponds, where the algae utilize the nitrate as a nutrient (1542 печатных знаков ).
Antioxidants are assumed to delay the onset of oxidation, or slow down the rate at which it occurs. Although some occur naturally , they are also added to extend the shelf life of food. Naturally occurring antioxidants are known to include vitamin C, vitamin E ,b- carotene, selenium. All of them are found to contain a phenolic group ,and many contain a carbon atom bonded directly to three methyl groups , which is known as a tertiary butyl group. Both the phenolic group and the tertiary butyl group are considered to be free radical scavengers. They are shown to react with and remove the free radicals involved in the oxidation of food and thus prolong the shelf life. They are believed to enhance the health effects of other foods, and boost overall health and resilience. However, synthetic antioxidants which are classed as food additives need to be regulated by policies and legislation to ensure their safe use in food (1122 печатных знаков).
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