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Experimental Chemistry , an Introduction to ChemistryMeasurements –Physical QuantitiesThere are four basic physical quantities used in Chemistry experiments:1.Volume2.Temperature3.Time4.MassTo measure these quantities accurately, we use measuring instruments. Sometimes, also called measuring apparatus.Examples of Measuring InstrumentsFor example•To measure temperature, we use a thermometer. •To measure volume, we use a volumetric flask.•To measure mass, we use a mass balance.•To measure time, we use a stopwatch/clock.Data Logging•Measurements can also be made by data logging. That is, instead say of a thermometer, a temperature sensor connected to a data logger and a computer can measure the temperature change in an experiment.•Data logging is usually used in industry and hospitals.1. Volume measured in Experiments•The SI unit for volume is the cubic metre(mm3). As this is a large unit, we usually measure volumes of liquids in cubic centimetres(ccmm3). Larger volumes are occasionally measured in cubic decimeters (ddmm3)•There are apparatus used to hold and measure different volume of liquids.1.Reading the volume of liquid with different apparatus•The volume of gas produced in a chemical reaction is measured with a gas syringe. The gas can be collected in the syringe during an experiment on rate(speed) of reaction. The gas syringe measures gas on a graduated scale.•Type types of solidRegular Solids (Regular Geometry)Irregular SolidsFor regular type of solids, like rectangle, circle, spherical, conical solids, the various mathematical formulae may apply.For irregular type of solids, we will use the change in volume of fluids after addition of the given solid to determine the volume of solids.What is the volumeof the stone?2.Temperature•Temperature can be easily measured using a thermometer. The usual laboratory thermometer is a mercury in-glass or alcohol-in-glass type, in which the liquid expands with the rising temperature, resulting in an increase in the length of the mercury of alcohol thread.•SI Unit for temperature : Kelvin (KK)•However most thermometer has a non SI unit: °CC•Measurements (accuracy) = nearest 0.5°CCSo values from measurement ends with .0 or .5.2.Temperature (Unit Conversion)•Temperature in K and °CC are related.TTTTmmTTTTTTTTTT TT TTTT ii ii KK=TTTTmmTTTTTTTTTT TT TTTT ii ii°CC+273 Note that: ∆TT KK=∆TT°CC•The lowest possible temperature is zero on the Kelvin scale OOKK, that is equivalent to −273°CC. This temperature is known as absolute zero. •However, for very accurate measurement of temperature, a temperature sensor can be connected to a data logger. A data logger is useful when we need to measure temperature changes over a period of time.3.Time•SI Unit: second (s).•Other units are the hour (h) and the minute (min).1ℎ=60min=3600ss•Time is measured with a clock or digital stopwatch. For ‘O’ level Chemistry experiments, it is sufficient to measure time to the nearest second.Do you know ?Per day is how many hours?Per year is how many days/weeks ?4.Mass•The mass of a substance is the measure of the amount of matter contained in it. The SI unit for mass is the kilogram (kg). In Chemistry, smaller masses are measured in grams (g) or milligrams (mg).•In the chemistry laboratory, the mass of a chemical is usually measured with an electronic ‘top pan’ balance or a beam balance such as triple beam balance. Some electronic balances can read up to an accuracy of 0.001g while a triple beam balance can read up to an accuracy of only 0.1 g.•In the chemical industries, large quantitates of substance is used. Large masses can be measured in tonnes (t). (1 tonne = 1000 kg)Set-up for Simple Experiments•In many chemistry experiments, you will make measurements. You will also need to assemble suitable apparatus for experiments. Following are a few types of experiment that you may encounter:Collecting and Measuring Volume of GasesMeasuring change in mass during a Reaction.Use of Data Logging Apparatus•Used for gases that are insoluble or slightly soluble in water.•Examples are:HydrogenOxygenCarbon Dioxide•Used for gases that are soluble in water.•The gases must be DENSER than air.•Denser air will sink into the gas jar.•Example: Chlorine gas (CCll2)•Used for gases that are soluble in water.•The gases must be LESS DENSE than air. •Gases that are less dense will float into the gas jar. •Example: Ammonia andHydrogen (NNHH3,HH2)•Used for ANY gases.•Particularly useful if gases are to be collected and the volume measured over a period of time.•Gas produced from reaction is collected in the gas syringe.•Apparatus is useful in experiments on speed ofreactionMeasuring Change in Mass During a Reaction•An electronic balance can be used to measure the change in mass of a reaction mixture over a period of time.•Consider : NNTT2CCOO3(ss)+2HHCCll(TTaa)→2NNTTCCll(TTaa)+CCOO2(gg)+HH2OO(ll)•The mass decreases as the gas formed in the reaction escapes from the flask. This apparatus is also useful in experiments on speed of reaction.Use of Data Logging Apparatus•Data logging can be used to measure and record variables(or parameters)that change over time during an experiment.•The following figure shows a temperature sensor is used to study the temperature changes in a reaction. The main piece of apparatus used are the sensor (or probe), a connector (or interface), the computer and the display screen (or monitor). The computer collects the measurements and displays them on the screen as a graph.Concept Map。
ChemistrySummer Holidays Homework for Future Freshmen of High schoolClass: __________________________Chinese Name:______________________English Name:______________________Beijing#80 High School International DepartmentIntroduction to Chemistry 化学入门Definition:Chemistry is the study of the composition, structure, and properties of matter, the processes that matter undergoes, and the energy changes that accompany there processes.(化学的定义：化学是研究物质的组成，结构，性质，物质发生的变化，以及变化过程中涉及的能量变化。
)Branches of Chemistry: O rganic Chemistry；Inorganic Chemistry；Physical Chemistry; Analytical Chemistry;Biochemistry; Theoretical chemistry(化学的分支：有机化学；无机化学；物理化学；分析化学；生物化学；理论化学)Day 1【Task】Please put the Chinese name into the suitable chapter. Vocabulary about chapter name. 章节名称词汇（----What may we study about chemistry in the first year? 高一可能涉及哪些化学知识？）物质和变化；原子：构建物质的基本单元；酸和碱；氧化还原反应；气体；化学键；原子中的电子排布；称量和计算；有机化学；反应能量；元素周期律；化学方程式和化学反应；化学平衡；化学反应动力学；化学计量学；化学式和化学物质；物质的状态；生物化学；电化学；滴定与pH值；水溶液中离子和稀溶液的依数性；溶液；Chapter 1 Matter and Change ( )Chapter 2 Measurement and Calculation ( )Chapter 3 Atom-Building Block of Matter ( )Chapter 4 Arrangement of Electrons in Atoms( ) Chapter 5 The periodic Law ( )Chapter 6 Chemical Bonding ( )Chapter 7 Chemical Formulas and Chemical Compounds( )Chapter 8 Chemical Equations and Reactions ( )Chapter 9 Stoichiometry ( )Chapter 10 States of Matter ( )Chapter 11 Gas ( )Chapter 12 Solution ( )Chapter 13 ions in Aqueous Solution and colligative Properties( )Chapter 14 Acid and Base ( )Chapter 15 Acid-Base Titration and pH ( )Chapter 16 Reaction Energy ( )Chapter 17 Reaction Kinetics ( )Chapter 18 Chemical Equilibrium ( )Chapter 19 Oxidation-Reduction Reactions ( )Chapter 20 Electrochemistry ( )Chapter 22 Organic Chemistry ( )Chapter 23 Biology Chemistry ( )【Task】Identify the vocabularies and master as possible as you can. Matter and its properties物质及其特点Mass 质量Definition of Matter物质的定义States of Matter物质的状态solid 固体liquid液体gas气体plasma等离子Composition of Matter 物质的构成Chemical and Physical Properties化学性质和物理性质Chemical and Physical Changes 化学变化和物理变化Conservation of Mass 质量守恒atom 原子molecular 分子ion离子cation 阳离子anion阴离子element 元素/单质compound 化合物pure substance 纯物质mixture混合物reactant 反应物group 族元素周期表的纵行family 族元素周期表的纵行period 周期元素周期表的横行metal 金属nonmetal 非金属metalloid 准金属Noble Gas 稀有气体【Task】Identify the vocabularies and master as possible as you can.Scientific Method(科学方法)system 系统hypothesis 假设model 模型theory 理论weight 重量derived Unit 衍生单位Energy能量Definition of Energy能量的定义Forms of Energy能量的形式Types of Reactions反应类型Exothermic Versus Endothermic 放热对吸热Measurements and Calculations测量和计算Temperature Measurements温度测量Scientific Notation 科学记数法Method of Conversion 转换方法Precision, Accuracy, and Uncertainty精密度，准确度，不确定度Significant Figures有效数字Calculations with Significant Figures 有效数字的计算directly proportional 正比例inversely proportional 反比例Chemical Formulas 化学分子式Equation 化学方程式Writing and Balancing Simple Equations 写作和配平简单方程式Ionic Equations 书写离子方程式mass number 质量数average atomic mass 平均分子量mole摩尔Avogadro’s number 阿伏伽德罗常数molar mass 摩尔质量Day4【Task】Identify the vocabularies and master as possible as you can.Law of conservation of mass 质量守恒定律nuclear forces 原子核力atomic nuclei原子核proton 质子neutron 中子electron 电子charge电荷positive charge 正电荷negative charge 负电荷atomic number 原子序数isotope 同位素nuclide 核素particle 粒子Oxidation Number and Valence氧化数和化合价Reactivity反应Period Table of the Elements元素周期表Periodic Law周期律Properties Related to the Periodic Table元素周期表的性质Radii of Atoms原子半径Electro negativity电负性Electron Affinity电子亲和能Ionization Energy电离能Bonding 化学键Types of Bonds 化学键类型Ionic Bonds离子键Covalent Bonds共价键Metallic Bonds金属键Intermolecular Forces of Attraction 分子间的吸引力Hydrogen Bonds氢键Double and Triple Bonds双键和三键Resonance Structures共振结构Day 5【Task】Learn the apparatus vocabulary and match the vocabulary with the picture given. Ifnecessary, google the vocabularies on the baidu /google image and try your best to finish it.学习仪器词汇并完成后面的图片匹配题。
and two others with nodes (antibonding states), i.e., three molecular orbitals in all. (See fig. 3.)As the length of the chain is increased, the number of electronic states into which the atomic 2s state splits also increases, the number of states always equaling the number of atoms. The same occurs when lithium chains are placed side-by-side or stacked on top of each other, so that finally the space lattice of the lithium crystal is obtained. It is of great significance that these electronic states have energies which are bounded by an upper and lower limiting value (see fig. 4). Within these limits the states form an energy band of closely spaced values (one gram of lithium contains nearly 1023 atoms). Similarly, energy bands can also result from overlapping p and d orbitals. The electronic states (orbitals) within an energy band are filled progressively by pairs of electrons in the same way that the orbitals of an atom were filled in accordance with the Pauli principle. This means that for lithium the electronic states of the 2s band will be exactly half-filled.It is of interest to consider why lithium atoms or Li2 molecules combine to form a metal lattice. In the lithium lattice the smallest distance between neighboring atoms is3.03 x 10–10 m, which is larger than in the Li2 molecule. This reflects the fact that bonds between pairs of atoms in the metal are weaker than they are in the molecule. Nevertheless, the metallic form of lithium is more stable than the molecular form because in the metal one atom has many more neighbors than in the Li2 molecule. As a result, the binding energy per gram atom of lithium (i.e., per 6.92 g of lithium) is163 kJ for the metal lattice, but only 56 kJ for one mole of molecule.[The possibility of hybridization (first advanced by L. Pauling to explain metallic bonding) is also a likely factor for the formation of metallic bonds. Thus strong bonds can be formed when the valence electron clouds become concentrated along the direction in which the bonding partners are situated. According to Pauling the situationtable, can often replace each other in arbitrary proportions without altering either the lattice type or the structure of the energy bands. This explains why such metals tend to form a complete series of solid solutions. Metallic alloys consist of such solid solutions or of heterogeneous mixtures of such solutions. Within certain limits, even metal atoms of different valence can be interchanged in a lattice.Band Structure of MetalsAccording to the above considerations the band structure of Li metal can be represented as shown in fig. 5.According to previous reasoning, the 2s band has N states (N = number of atoms) and accommodates n 2s electrons (where n is the number of electrons per atom in the 2s state times N). Thus, this band has only half of the states filled since each state can accommodate two electrons of opposite spin (Pauli exclusion principle). In accordance with the Aufbau principle, the lowest energy states of the band are filled first and the upper states remain empty – but can readily be occupied by electrons upon thermal excitation or the application of an electric field. Since the width of the energy band is of the order of a few volts, spacings of states within the band are of the order of~10–20 eV (1 eV = 1.6 x 10–19 J), electrons can readily acquire the energy necessary to move into excited states, be accelerated, and move through the metal as conducting electrons. Partly filled bands thus constitute conduction bands.The conduction mechanism in Mg, for example, appears complicated by the fact that each 3s state of the valence shell in the atoms is doubly occupied (3s2). Thus the 3s band must be filled completely and no electronic conduction would in principle be expected. Electronic conduction, however, is observed because of a partial overlap of the 3s and the empty 3p bands. With this overlap, electrons can be activated into empty 3p states and exhibit conduction, as in the partly filled s band in Li.filledsolidfilled levelsFig. 6 Band structure of insulators and semiconductors (molecular crystals); the conditions depicted reflect a molar crystal of carbon (diamond).Both insulators and semiconductors have the same basic band structure – the primary) between the valence and the, generically, are materials with very high resistivity (see T able I), comprising glasses, polymers, refractories, composites, liquids and gases. In the present context, an insulator is a molecular crystal, such as diamond (C) or sapphire, with a band gap ) in excess of 4 eV (arbitrary value). Generally such materials will not conduct electricity since their valence band is filled and the energy required to transfer electronsfrom the valence band to the empty conduction band is far in excess of both the thermal energies at room temperature and the energy provided by radiation of the visible spectrum (~2 eV). Therefore, insulators (in single crystal form) are normally transparent (colorless); however, if light is excessively or totally scattered at internal heterogeneities (such as grain boundaries), they may be translucent and even opaque. It should also be recognized that impurities (Cr3+ in Al2O3) or particular point defects (color centers) may impart a color to the transparent insulator crystals. The color arises because of partial absorption of white light and selective transmission of the other portions of the visible spectrum.Semiconductors: The conventional semiconductors, silicon (Si) and germanium (Ge), have a band gap (E g) of 1.1 and 0.7 eV respectively and therefore absorb visible radiation; they are opaque (fig. 7). Considering the statistical nature of the thermal energy distribution in the solid matrix (Maxwell-Boltzmann), a significant number of electrons in the valence band will, at room temperature, acquire sufficient energy to cross the existing energy gap and thus provide for semiconductivity. The conductivity will therefore increase with temperature, contrary to metallic systems, until electron scattering effects, due to increased lattice vibrations (which decrease the mobility of electrons), begin to dominate.The value of semiconductors for solid state device fabrication lies in the fact that the number and type of conducting electric charge carriers [electrons are n-type (negative), holes are p-type (positive)] can be controlled through incorporation of appropriate dopant elements. Thus the substitutional incorporation of Group V elements (Sb, As, P) provides for shallow donor levels in the band gap at about 0.01 eV from the conduction band. The substitutional incorporation of Group III elements (B, Al) generates acceptor levels in the band gap at about 0.01 eV from the valence band. The two types of impurities are almost completely ionized at room temperature and give rise to extrinsicE h (transparent to light)Insulators (carbon)Impurity and defect levels in the energygap may give rise to selective absorptionof light (colour the object).E h (visible light is absorbed)(-)(+)excited electron holeSemiconductor (Si)Fig. 7 Optical bebavior of insulators and semiconductorsn-type and p-type conductivity – the basis for the formation of diodes and transistors (fig. 8).Of increasing importance are compound GaAs, InSb, InP and GaP (compounds of Group III and Group V elements). Together these compounds provide eight valence electrons and, by sp form a diamond-like, covalent crystal structure with semiconductor properties. These* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *Table I. ELECTRICAL RESISTIVITIES OFMETALS AND NONMETALS AT 20_C*______________________________________________________________________________Resistivity,Resistivity, Metals10–8 ohm–m**Nonmetals ohm–m**______________________________________________________________________________Silver 1.6SemiconductorsCopper 1.67Silicon1000.0Gold 2.3Germanium0.09Aluminum 2.69InsulatorsMagnesium 4.4Diamond1010–1011Sodium 4.61Quartz 1.2 x 1012Tungsten 5.5Ebonite 2 x 1013Zinc 5.92Sulfur 4 x 1013Cobalt 6.24Mica9 x 1013Nickel 6.84Selenium 2 x 1014Cadmium7.4Paraffin wax 3 x 1016Iron9.71Tin12.8Lead20.6Uranium29Zirconium41Manganin44Titanium55Lanthanum5996%Iron–4%Si62Cerium78Nichrome100*From American Institute of Physics Handbook, Dwight E. Gray, ed. McGraw-Hill, New York(1963), pp. 4–90; 9–38.**Note the different units in the two columns.* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *EXERCISE FOR THE IDLE MIND1. A solid is found to have an energy band gap (E g) of 3 eV. What is the likely colorof this solid in transmitted sunlight?2.An optically transparent solid appears green in transmitted sunlight. What do youexpect the band gap (E g) of this solid to be (in eV)?3.AlN and GaSb are compounds, solid at room temperature. On the basis ofbonding considerations and data provided in the P/T, attempt to predictdifferences in the properties of these solids.4.Account for the conductivity of (a) Na (metallic) and (b) Mg (metallic) on the basisof appropriate energy diagrams.5.What is the radiation of longest wavelength which is still capable of beingtransmitted through:(a)Si with E g = 1.1 eV(b)Ge with E g = 0.7 eV, and(c)the compound GaAs with E g = 1.43 eV?6.Explain the difference between extrinsic and intrinsic semiconductors.7.Semiconductors in single crystal form are usually produced by solidification ofmelts. To achieve extrinsic semiconductivity it is customary to add to the melt“doping elements” which are substitutionally incorporated [replace, for example, a silicon atom in the crystal (ordered structure)]. If the doping element is P, which has 5 valence electrons, and it replaces a silicon atom (whose 4 valence electrons are normally immobilized because of bond formation), each P atom will be able to contribute one electron to conduction; its other four valence electrons take part in bond formation. Assume you add 3 mg P to 50 g silicon and form a crystal from it in which the P atoms are uniformly distributed; what is the number of conduction electrons/cm3 in the doped crystal? (You may neglect the volume of thesubstitutionally replaced Si atoms and assume that only electrons from P atoms contribute to conduction.)8.Potassium (K) and beryllium (Be) are metals which exhibit good electricalconductivity. Explain for both elements the reasons for the observed conductivity on the basis of the band structure.9.The energy gap (E g) in zinc oxide (ZnO) is 3.2 eV.(a)Is this material transparent to visible radiation?(b)Do you expect this material to be a conductor at room temperature? (Givethe reasons for your answer.)18. A chemical analysis indicates that a silicon crystal weighing 100 g contains 33 mgof aluminum (Al) which is substitutionally incorporated (the Al atoms replace some Si atoms in the crystal).(a)Is this crystal n-type or p-type? (Explain in one sentence.)(b)What is the number of extrinsic charge carriers (per cm3) in this crystal?19.An unknown material is transparent to light of frequencies ($) up to 1.3 x 1014 s–1.Draw a meaningful schematic band structure for this material.20.We know that, in semiconductors, charge carriers can be thermally activated fromthe valence band into the conduction band. The number of thermally activatedelectrons (n e) per cm3 is given by:n e+A T3ń2e*E gńkT(where A = 5 x 1015 cm–3 for silicon). Determine for pure silicon (Si) the number of electrons/cm3 in the conduction band at 500°C.21. A crystal of germanium (E g = 0.7 eV) is found to be n-type with 5 x 1018 mobilecharge carriers/cm3 (at room temperature).(a)Draw a schematic energy band diagram that reflects the indicatedproperties. (Label pertinent features in the diagram.)(b)Do you expect this crystal to be transparent or opaque to radiation of $=1 x 1015 s–1?22.Draw three energy band structures representing respectively (and identifiably) an:(1)extrinsic n-type semiconductor(2)an insulator(3)and a metal.23. A 50 kWatt radio transmitter emits radio waves with a wavelength of 300m. Howmany photons does it emit per minute (1 Joule = 1 Watt per sec)?24. A material exhibits an “optical band edge” (transition from absorption of light totransmission) at $ = 5 x 1014 Hz (s–).(a)Draw a diagram which reflects the indicated optical behavior.(b)What do you expect the color of this material to be when viewed in daylight?(c)What is the band gap (E g) of this material?25. A sample of germanium (Ge), weighing 30 g, is found to contain 54 mg of arsenic(As). Determine for this sample the mobile charge carrier density (carriers/cm3) at room temperature. (Assume As to be substitutionally incorporated in Ge and that all As atoms are ionized at room temperature; you may neglect any intrinsiccharge contributions.)。
Introduction to Organic ChemistryOrganic chemistry was born in 1828 when Friedrich Wohler attempted to synthesize ammonium cyanate (氰酸铵), NH4CNO, and obtained urea (尿素)O‖NH2 CNH2Instead, Wohler, who had studied to be a doctor of medicine before he decided to become a chemist, discovered that the compound he had made was identical with urea recovered from urine. Up to that time, scientists had thought that the compounds present in living plants and animals could not be synthesized in the laboratory from inorganic reagents. Wohler recognized the importance of his experiment and wrote to a friend, "I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea. "Wohler's discovery was important because it gave impetus to a long series of experiments in which chemists probed the nature of the chemical substances that exist in living organisms and in petroleum and coal, which are formed from the rem ains of certain plants and animals that lived in the distant past. As early chemists struggled to isolate and purify the components of plants, animals, coal, and petroleum, they observed that many of the compounds that they isolated were composed of carbon and hydrogen. Many contained, in addition, nitrogen(氮), oxygen, sulfur(硫), and phosphorus(磷). Chemists quickly recognized that the chemistry of carbon was associated with life in a special way that distinguished that elem ent from all others. Compounds of carbon were called organic compounds to reflect their origin in living systems and to distinguish them from the inorganic compounds, the acids, bases, and salt, derived from the other elements on the periodic table (元素周期表).Organic chemistry is recognized today as an area of study central to many disciplines. Life processes are supported by the chemical reactions of complex organic compounds such as enzymes(酶), hormones, proteins, carbohydrates(碳水化合物), and lipid(脂). The lipid cholesterol(胆固醇); hormones such as cortisone(皮质酮), and testosterone(睾丸激素); glucose(葡萄糖), the most important source of energy in our bodies; and the pancreatic(胰腺的) enzyme chymotrypsin(胰凝乳蛋白酶), which is essential to our digestion, are among the compounds the structures and chemistry of which we will explore. Cells divide and grow in part according to signals carried and transmitted by giant organic molecules called deoxyribonucleic acids(脱氧核糖核酸), DNA, and ribonucleic acids(核酸), RNA(核糖核酸). An understanding of organic chemistry is essential to the study of the biological and medical sciences.Chemists, in attempts to improve on nature, have created millions of organic compounds that did not exist in nature originally. The local anesthetic Novocain(奴佛卡因局部麻醉剂), for example, was developed to mimic the numbing effects of the natural alkaloid(生物碱) cocaine. The search continues for a synthetic painkiller that gives the merciful relief from pain affordedby natural opiates(麻醉剂) such as morphine without having the undesirable side effect of being addictive. The discovery in recent years that the brain manufactures endorphins(多肽，内啡肽), compounds that interact with the same sites in the brain as morphine does, is an exciting new development in our understanding of how the body copes with pain. Industrial chemists, on the other hand, have developed synthetic rubber, Neoprene(氯丁橡胶), and synthetic silks, such as rayon(人造纤维) and nylon, to improve on the proper ties of the natural substances and to meet shortages of natural supplies.Crude petroleum is converted by organic reactions into fuels that supply ene rgy for heat, transportation, and industry. Petroleum is also the chemical basis for giant molecules engineered to have properties that are useful.Food additives, dyes, artificial flavorings, artificial sweeteners, preservatives, and pesticides, most of the organic compounds, make the headlines in newspapers with regularity. The boxes our crackers and cereals come in carry the abbreviations BHT(丁化羟基甲苯) andBHA(丁化羟基茴香醚) to designate organic chemicals that keep the food from becoming rancid. The carcinogenicity(致癌力) of saccharin (糖精)and whether the use of it should be banned is a major political as well as scientific issue.The early organic chemists were confronted with many puzzles. They determined molecular formulas for the compounds that they had recovered from natural sources. As early as 1824 they discovered that several compounds with very different properties might have the same molecular formula. In 1830, the Swedish chemist Jakob Berzelius named such compounds "isomeric bodies" from the Greek word isos, meaning "equal" and meros, meaning "part". This discovery intensified investigations into the nature of the forces that held atoms togethe r and the ways in which chemical bonds could be symbolized in writing. A lively debate over these issues continued through the middle of the nineteenth century. Many different ways of representing the structures of molecules were proposed, disputed, and discarded.Analyses of organic compounds provided empirical ratios of the weights of carbon, hydrogen, oxygen, and nitrogen. However, exact molecular formulas could not be assigned until a uniform and correct set of atomic weights was adopted by chemists. The Italian chemist, Stanislav Cannizzaro, proposed a system of experimentally accurate atomic weights in 1860. The gradual acceptance of his system made it possible for chemists to assign correct and universally recognized molecular formulas. This advance in turn allowed ideas about bonding and structure to develop.In the early nineteenth century, chemists also experimented with the transformation of one chemical substance into another. As a result of these experiments, they observed that there were some structural units that were carried unchanged from one reaction to another. The recognition of these intact structural units, which they called radicals, enabled chemists to manipulate chemical substances with more precision. The art of organic synthesis was born. Chemists could use new pathways to construct compounds that already existed in nature, or they could create entirely new compounds.By the end of the nineteenth century, the science of organic chemistry was flourishing. Serious study of the structural and chemical properties of the compounds of carbon had resulted in important discoveries about the chemistry of carbohydrates and proteins. Chemists had tackled the problems of representing the structures of organic compounds. They had wondered how molecules looked in three dimensions and had arrived at pictures remarkably similar to the ones we still use. Many organic compounds that did not exist in nature had been synthesized.The next significant step forward in organic chemistry took place in the 1920 sand 1930s. During these years, chemists explored the exact details of chemical transformations. They began to ask questions about the relative timing of the breaking and forming of bonds during chemical reactions. Chemists were also concerned with how the shapes of molecules, that is, their spatial properties, affected chemical reactivity. The question of three-dimensionality in reactions turned out to be a central one in biochemistry, the study of chemical transformations in living organisms.While chemists were asking more and more subtle questions about chemical reactions, new instrum ents that extended their powers of observation were invented. These newinstrum ents and techniques made it possible to probe more deeply into the processes of nature. The number of experimental observations that could be collected multiplied rapidly.The progress of chemistry as a science depends on the experimental manipulations of substances in the laboratory leading to the observation of new phenom ena. In thinking about these phenomena chemists arrive at ideas about the nature of the chemical substances under investigation. New experiments and new observations test these ideas. The range of manipulations and observations available to the chemist has expanded enormous ly in recent years. The power that chemists have to transform chemical compounds into new ones, som etimes useful and sometimes harmful, has also increased.Chemists have refined the way they visualize and think about the submicroscopic units called atoms and molecules. They have created models that help them to picture and understand experimental facts about chemical substances. Human beings are constantly creating and refining the models they use in dealing with the physical world. Some models are pictorial; others are more abstract and mathematical. Some models are widely adopted because they are useful; others are quickly modified on the basis of new observations.Read the text quickly and answer the following questions and choose the best answer from the four choices marked A), B), C) and D).1. What does the author mean by saying that "organic chemistry was born in 1828when Friedrich Wohler attempted to synthesize ammonium cyanate, NH4CNO, and obtained urea"?A. F riedrich Wohler finally obtained urea.B. U rea only exists in human body.C. C hemists were inspired to probe the nature of the chemical substances thatexist in living organisms and in petroleum and coal.D. L ife processes are supported by the chemical reactions of complex organiccompounds.2. According to the text, which of the following is essential to our digestion?A. t estosteroneB. e nzyme chymotrypsinC. p roteinsD. g lucose3. According to the text, which of the following is true of synthetic painkiller?A. T hey are organic compounds which exist in nature.B. T hey are developed to mimic the numbing effects which can give the mercifulrelief from pains.C. T he employment of them will cause undesirable side effect of being addictive.D. T hey are regarded as exciting new development in our understanding of howthe body deals with pain.4. Which of the following scientists proposed the system of accurate atomic weightswhich made it possible for chemists to assign correct and universally recognized molecular formula?A. A lbert EinsteinB. F riedrich WohlerC. J akob BerzeliusD. S tanislav Cannizzaro5. Which of the following discoveries intensified investigations into the nature o f theforces that held atoms together and the ways in which chemical bonds could be symbolized in writing?A. T he discovery that several compounds with very different properties mighthave the same molecular formula.B. c holesterol and cortisoneC. W ohler's discovery of synthetic ureaD. t he carcinogenicity of saccharin。
