Английский. пособие Химики АЯ. Introduction
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4. Chemical Bond In chemistry, a chemical bond is the force, which holds together atoms in molecules or crystals. In many simple compounds, valence bo__ __1) theory and the concept of oxid__ __ __ __ __2) number can be used to pre__ __ __ __3) molecular structure and composition. Similarly, theo__ __ __ __4) from classical physics can be us__ __5) to predict many ionic structures. Wi__ __6) more complicated compounds, such as me__ __ __7) complexes, valence bond theory fails a__ __8) a more thorough understanding based on qua__ __ __ __9) mechanics is necessary. The spatial charact__ __ __ __ __ __ __ __10) and range of energies encompassed b__11) chemical forces span a continuum, so t__ __12) terms for the different types o__13) chemical bond overlap in their applic__ __ __ __ __ __ __14), but the types include ionic bo__ __15), covalent bond, coordinate covalent bond, meta__ __ __ __16) bond, hydrogen bond. All chemical bon__ __ __ __17) arises from the energetically favourable (th__ __18) is, low-energy) interaction between electrons o__19) different atoms. The types of bon__ __ __ __20) are distinguished by the extent t__21) which electron density is localized o__22) delocalized among the atoms of t__ __23) substance. In the case of io__ __ __24) bonding, electrons are mainly associated wi__ __25) individual atoms, and an overall elec__ __ __ __26) charge is assigned to discrete const__ __ __ __ __ __27) atoms throughout the substance. The nat__ __ __28) of the interatomic (or in fa__ __29) interionic) forces is largely characterized b__30) isotropic continuum electrostatic potentials. By cont__ __ __ __31), in covalent bonding the electron den__ __ __ __32) distributions within bonds are not assi__ __ __ __33) to individual atoms, but are ins__ __ __ __34) delocalized across the molecule in struc__ __ __ __ __35) which are described by the mo__ __36) common contemporary theory as molecular orbi__ __ __ __37). Unlike pure ionic bonds, these m__ __38) have directed anisotropic properties. Intermediate situa__ __ __ __ __39) certainly exist, in which bonds sh__ __40) a mixture between polarized ionic character a__ __41) electron-delocalized covalent character. Ionic bonding c__ __42) largely be described by classical phy__ __ __ __43), but the complexity of covalent bon__ __ __ __44) relies more heavily on concepts fr__ __45) quantum mechanics. Aside from the intramolecular bonds which hold molecules together, intermolecular forces also act to provide an attraction between the molecules of a substance. 5. Laws The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it i__1) actually energy that is conserved, a__ __2) that energy and mass are rel__ __ __ __3); a concept which becomes important in nuc__ __ __ __4) chemistry. Conservation of energy leads t__5) the important concepts of equilibrium, thermod__ __ __ __ __ __ __6), and kinetics. Further laws of chem__ __ __ __ __7) elaborate on the law of conser__ __ __ __ __ __8) of mass. Joseph Proust’s l__ __9) of definite composition says that pu__ __10) chemicals are composed of elements i__11) a definite formulation; we now know th__ __12) the structural arrangement of these elem__ __ __ __13) is also important. Dalton’s l__ __14) of small whole numbers says th__ __15) these chemicals will present themselves i__16) proportions that are small whole num__ __ __ __17) (i. e. 1 : 2 O : H in water); although for biomacromolecules a__ __18) mineral chemistry the ratios tend t__19) require large numbers. More modern la__ __20) of chemistry define the relationship bet__ __ __ __21) energy and transformations. In equilibrium, mole__ __ __ __ __22) exist in mixture defined by t__ __23) transformations possible on the timescale o__24) the equilibrium, and are in a ra__ __ __25) defined by the intrinsic energy o__26) the molecules -- the lower the intr__ __ __ __ __27) energy, the more abundant the mole__ __ __ __28). Transforming one structure to another requ__ __ __ __29) the input of energy to cr__ __ __30) an energy barrier; this can co__ __31) from the intrinsic energy of t__ __32) molecules themselves, or from an exte__ __ __ __33) source which will generally accelerate transfo__ __ __ __ __ __ __ __34). The higher the energy barrier, t__ __35) slower the transformation occurs. There i__36) a hypothetical intermediate, or transition structure, th__ __37) corresponds to the structure at t__ __38) top of the energy barrier. T__ __39) Hammond-Leffler Postulate states that this stru__ __ __ __ __40) looks most similar to the pro__ __ __ __41) or starting material which has intr__ __ __ __ __42) energy closest to that of t__ __43) energy barrier. Stabilizing this hypothetical interm__ __ __ __ __ __44) through chemical interaction is one w__ __45) to achieve catalysis. All chemical processes are reversible (law of microscopic reversibility) although some processes have such an energy bias, they are essentially irreversible. 6. Reversibility Every chemical reaction is, in theory, reversible. In a forward reaction the reactants a__ __1) converted to products. In a reverse reac__ __ __ __2) products are converted into reactants. Chem__ __ __ __3) equilibrium is the state in wh__ __ __4) the forward and reverse reaction ra__ __ __5) are equal, thus preserving the amo__ __ __6) of reactants and products. However, a reac__ __ __ __7) in equilibrium can be driven i__8) the forward or reverse direction b__9) changing reaction conditions such as tempe__ __ __ __ __ __10) or pressure. Le Chatelier’s prin__ __ __ __ __11) can be used to predict whe__ __ __ __12) products or reactants will be for__ __ __13). Although all reactions are reversible t__14) some extent, some reactions can b__15) classified as irreversible. An irreversible reac__ __ __ __16) is one that "goes to compl__ __ __ __ __17)." This phrase means that nearly a__ __18) of the reactants are used t__19) form products. These reactions are ve__ __20) difficult to reverse even under ext__ __ __ __21) conditions. Law of mass action T__ __22) concentrations of reactants and products dete__ __ __ __ __23) the rate of forward and rev__ __ __ __24) reactions. Catalyst A catalyst increases the sp__ __ __25) of a reaction by lowering the activ__ __ __ __ __26) energy needed for the reaction t__27) take place, and supplies enough ene__ __ __28) for the reaction to happen. A catalyst is not destroyed or changed during a reaction, so it can be used again. 7. Separation Separation processes extract the desired product from the output stream of a reactor. The pharmaceutical and biotech industries typi__ __ __ __ __1) require extreme purification of the pro__ __ __ __2) in order to meet federal regul__ __ __ __ __ __3), making the separation process the mo__ __4) expensive part of a production plant. Comm__ __ __ __5), the output stream of a reactor i__6) passed through multiple separation processes, ea__ __7) designed to either remove a fraction o__8) the waste from the stream wh__ __ __9) leaving the product or to rem__ __ __10) only the product. There are nume__ __ __ __11) types of separation proceses, including Filtr__ __ __ __ __12). The stream of fluid containing t__ __13) product is passed through a membrane, conta__ __ __ __ __14) holes of varying sizes, which th__ __15) prevent the product from passing thr__ __ __ __16) the membrane or allow only t__ __17) product to pass through. Multiple si__ __ __18) of holes and designs exist, incl__ __ __ __ __19) direct flow and tangential flow. Chromat__ __ __ __ __ __ __20). The stream is passed through a lo__ __21) hollow column, which has its inte__ __ __ __22) edges coated with a specific chemical spe__ __ __ __23) that will interact with the str__ __ __24) is a desired manner. Depending upon t__ __25) type of coating used, either t__ __26) product or the waste chemical spe__ __ __ __27) may bind or adhere to t__ __28) inside of the column walls. On__ __29) bound, the product or waste m__ __30) be unbound, in a process called elu__ __ __ __31), by changing the pH or tempe__ __ __ __ __ __32) of the column, destabilizing the fo__ __ __33) which keeps the product or wa__ __ __34) from binding to the coating. T__35) separate the waste from the pro__ __ __ __36), one changes the containers in wh__ __ __37) the outflow enters prior to elu__ __ __ __38). The different types of chromatography a__ __39) named based on the type o__40) coating they employ. Some types inc__ __ __ __41): i) Affinity chromatography. It uses antibodies to bi__ __42) to product molecules; ii) Ionic chromat__ __ __ __ __ __ __43). It uses negative or positive ions t__44) either repulse or attract product o__45) waste molecules. Products may be specif__ __ __ __ __ __46) altered to make the separation pro__ __ __ __47) easier by including ’tags’ that bi__ __48) to specific coatings in the chromat__ __ __ __ __ __ __49) column. Centrifugation. The stream is pas__ __ __50) into a centrifuge that, while spinning, us__ __51) centrifugal force (centripetal acceleration) to pu__ __52) molecules with higher molecular weights out__ __ __ __53). Centrifugation is often used to remove excess water and concentrate the outflow of a reactor. 8. What Is Chemical Engineering? Chemical engineering basically is applied chemistry. It is the branch of engin__ __ __ __ __ __1) concerned with the design, construction, a__ __2) operation of machines and plants th__ __3) perform chemical reactions to solve prac__ __ __ __ __4) problems or make useful products. Wh__ __5) Is a Chemical Engineer? Like all engi__ __ __ __ __6), chemical engineers use math, physics, a__ __7) economics to solve technical problems. T__ __8) difference between chemical engineers and ot__ __ __9) types of engineers is that th__ __10) apply a knowledge of chemistry in addi__ __ __ __11) to other engineering disciplines. Chemical engi__ __ __ __ __12) sometimes are called ’universal engineers’ bec__ __ __ __13) their scientific and technical mastery i__14) so broad. What Do Chemical Engi__ __ __ __ __15) Do? Some chemical engineers make des__ __ __ __16) and invent new processes. Some cons__ __ __ __ __17) instruments and facilities. Some plan a__ __18) operate facilities. Chemical engineers have hel__ __ __19) develop atomic science, polymers, paper, dy__ __20), drugs, plastics, fertilizers, foods, petrochemicals... pre__ __ __21) much everything. They devise ways t__22) make products from raw materials a__ __23) ways to convert one material in__ __24) another useful form. Chemical engineers c__ __25) make processes more cost effective o__26) more environmentally friendly or more effi__ __ __ __ __27). As you can see, a chemical engineer can find a niche in any scientific or engineering field. Appendix 2 LIST OF CHEMICAL ELEMENTS
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