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Fundamentals of Physical Chemistry – III 1. Explain the extraction of aluminium from its one. Aluminium ( /ˌæljuːˈmɪniəm/ AL-ew-MIN-ee-əm) or aluminum (American English; /ˌəlˈuːmɪnəm/ ə-LOO-mi-nəm) is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal, in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is too reactive chemically to occur natively. Instead, it is found combined in over 270 differentminerals.[4] The chief ore of aluminium is bauxite. Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates. Despite its prevalence in the environment, aluminium salts are not known to be used by any form of life. In keeping with its pervasiveness, it is well tolerated by plants and animals. [5] Because of their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest. Aluminium is a soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. Aluminium is nonmagnetic and does not easily ignite. A fresh film of aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, whilealuminium alloys have yield strengths ranging from 200 MPa to 600 MPa.[6] Aluminium has about one-third thedensity and stiffness of steel. It is easily machined, cast, drawn and extruded. Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.[6] This corrosion resistance is also often greatly reduced when many aqueous salts are present, particularly in the presence of dissimilar metals. 2. Give the preparation and uses of Thio and Potassium dichromate. Chemistry Potassium dichromate is an oxidant (oxidizing agent). The reduction half-equation can be seen: Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O (E = +1.23 V) In organic chemistry, potassium dichromate is a mild oxidizer compared with potassium permanganate. It is used to oxidize alcohols. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under reflux. In contrast, with permanganate, carboxylic acids are the sole products. Secondary alcohols are converted into ketones — no further oxidation is possible. For example, menthone may be prepared by oxidation of menthol with acidified dichromate.[2] Tertiary alcohols are not oxidized by potassium dichromate. In an aqueous solution the color change exhibited can be used to test whether an aldehyde or ketone is present. When an aldehyde is present the chromium ions will be reduced from the +6 to the +3 oxidation state, changing color from orange to green. This is because the aldehyde can be further oxidized to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidized further, and so the solution will remain orange. [edit]Uses [edit]Cleaning Like other chromium(VI) compounds (chromium trioxide, sodium dichromate), potassium dichromate may be used to prepare "chromic acid", which can be used for cleaning glassware and etching materials. [edit]Construction It is used as an ingredient in cement in which it retards the setting of the mixture and improves its density and texture. This usage commonly causescontact dermatitis in construction workers.[3] [edit]Ethanol determination The concentration of ethanol in a sample can be determined by back titration with acidified potassium dichromate. Reacting the sample with an excess of potassium dichromate, all ethanol is oxidized to acetic acid: C2H5OH + [O] → CH3COOH The excess dichromate is determined by titration against sodium thiosulfate. Subtracting the amount of excess dichromate from the initial amount, gives the amount of ethanol present. Accuracy can be improved by calibrating the dichromate solution against a blank. One major application for this reaction is in old police breathalyzer tests. When alcohol vapor makes contact with the yellow dichromate-coated crystals, the color changes from yellow to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath. 3. 1. Explain the various types of crystals with examples. Cubic or Isometric - not always cube shaped! You'll also find octahedrons (eight faces) and dodecahedrons (10 faces). 2. Tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms. 3. Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together). 4. Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon. 5. Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division. 6. Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes. 7. Monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids. This is a very simplified view of crystal structures. In addition, the lattices can be primitive (only one lattice point per unit cell) or non-primitive (more than one lattice point per unit cell). Combining the 7 crystal systems with the 2 lattice types yields the 14 Bravais Lattices (named after Auguste Bravais, who worked out lattice structures in 1850). The structure of real crystals is pretty complicated! You can read about crystallography and mineral structures hereand here. 4. Discuss the structure of various forms of silicates. Structural principles In the vast majority of silicates, including silicate minerals, the Si occupies a tetrahedral environment, being surrounded by 4 oxygen centres. In these structures, the chemical bonds to silicon conform to the octet rule. These tetrahedra sometimes occur as isolated SiO44- centres, but most commonly, the tetrahedra are joined together in various ways, such as pairs (Si2O76-) and rings (Si6O1812-). Commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All such species have negligible solubility in water at normal conditions. [edit]Occurrence in solution Main article: Sodium silicate Silicates are well characterized as solids, but are less commonly observed in solution. The anion SiO44- is the conjugate base of silicic acid, Si(OH)4, and both are elusive as are all of the intermediate species. Instead, solutions of silicates usually observed as mixtures of condensed and partially protonated silicate clusters. The nature of soluble silicates is relevant to understanding biomineralizationand the synthesis of aluminosilicates, such as the industrially important catalysts called zeolites.[1] [edit]Silicates with non-tetrahedral silicon Although the tetrahedron is the common coordination geometry for silicon compounds, silicon is well known to also adopt higher. A well known example of such a high coordination number ishexafluorosilicate (SiF62-). Octahedral coordination by 6 oxygen centres is observed. At very high pressure, even SiO2 adopts this geometry in the mineral stishovite, a dense polymorph of silica found in the lower mantle of the Earth. This structure is also formed by shock during meteorite impacts. Octahedral Si in the form of hexahydroxysilicate ([Si(OH) 6]2−) is observed in thaumasite[citation needed]a mineral occurring rarely in nature but sometimes observed amongst other calcium silicate hydrate artificially formed in cement and concrete submitted to a severe sulfate attack. A silicate (SiO44-) is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate ([SiF6]2−) and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other terrestrial planets, rocky moons, and asteroids. Sand, Portland cement, and thousands of minerals are examples of silicates. Silicate compounds, including the minerals, consist of silicate anions whose charge is balanced by various cations. Myriad silicate anions can exist, and each can form compounds with many different cations. Hence this class of compounds is very large. Both minerals and synthetic materials fit in this class. 5. Illustrate the isomerism in Tartanic acid. 6. What is conformation? Explain the conformes of n-butane. In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds. [1] Such isomers are generally referred to as conformational isomers or conformers and specifically as rotamers [2] when the rotation leading to different conformations is restricted (hindered) rotation, in the sense that there exists a rotational energy barrier that needs to be overcome to convert one conformer to another. Conformational isomers are thus distinct from the other classes of stereoisomers for which interconversion necessarily involves breaking and reforming of chemical bonds. The rotational barrier, or barrier to rotation, is the activation energy required to interconvert rotamers. Types of conformational isomerism Butane has three rotamers: two gauche conformers, which are enantiomeric and an anti conformer, where the four carbon centres are coplanar. The three eclipsed conformations with dihedral angles of 0°,120° and 240° are not considered to be rotamers, but are instead transition states. Some important examples of conformational isomerism include: 1. Linear alkane conformations with staggered, eclipsed and gauche conformers, and 2. Ring conformation Cyclohexane conformations with chair and boat conformers. Carbohydrate conformation 3. Atropisomerism- due to restricted rotation about a bond, a molecule can become chiral 4. Folding of molecules, where some shapes are stable and functional, but others are not. [edit]Equilibrium population of conformers The population of different conformers follows a Boltzmann distribution: The left hand side is the equilibrium ratio of conformer i to the total. Erel is the relative energy of the i-th conformer from the minimum energy conformer. Ek is the relative energy of the k-th conformer from the minimum energy conformer. R is the molar ideal gas constant equal to 8.31 J/(mol·K) and T is the temperature in kelvins (K). The denominator of the right side is the partition function. [edit]Isolation or observation of the conformational isomers Atropoisomers can be quite stable depending on the steric effects and were the first conformational isomers to be identified.[3] In thebiphenylic system atropisomerism is especially prevalent, e.g. binaphthol. In cyclohexane derivatives, the two chair conformers interconvert with rates on the order of 105ring flips/sec, which obviously precludes their separation.[3] The equatorial conformer crystallizes selectively, and when these crystals are dissolved at very low temperatures, one can directly monitor the approach to equilibrium by NMR spectroscopy.[4] 7. State the preparation and uses of saecharin and aspartic acid. Aspartic acid was first discovered in 1827 by Plisson, synthesized by boiling asparagine (which had been isolated from asparagus juice in 1806) with a base.[4] [edit]Forms and nomenclature The term "aspartic acid" refers to either of two forms or a mixture of two.[3] Of these two forms, only one, "L-aspartic acid", is directly incorporated into amino acids. The biological roles of its counterpart, "D-aspartic acid" are more limited. Where enzymatic synthesis will produce one or the other, most chemical syntheses will produce both forms, "DL-aspartic acid," known as a racemic mixture. [edit]Role in biosynthesis of amino acids Aspartate is non-essential in mammals, being produced from oxaloacetate by transamination. It can also be made in the Urea Cycle from Ornithine andCitrulline. In plants and microorganisms, aspartate is the precursor to several amino acids, including four that are essential for humans: methionine,threonine, isoleucine, and lysine. The conversion of aspartate to these other amino acids begins with reduction of aspartate to its "semialdehyde," O2CCH(NH2)CH2CHO.[5] Asparagine is derived from aspartate via transamidation: -O2CCH(NH2)CH2CO2- + GC(O)NH3+ O2CCH(NH2)CH2CONH3+ + GC(O)O (where GC(O)NH2 and GC(O)OH are glutamine and glutamic acid, respectively) [edit]Other biochemical roles Aspartate is also a metabolite in the urea cycle and participates in gluconeogenesis. It carries reducing equivalents in the malate-aspartate shuttle, which utilizes the ready interconversion of aspartate and oxaloacetate, which is the oxidized (dehydrogenated) derivative of malic acid. Aspartate donates one nitrogen atom in the biosynthesis of inosine, the precursor to the purine bases. [edit]Neurotransmitter Aspartate (the conjugate base of aspartic acid) stimulates NMDA receptors, though not as strongly as the amino acid neurotransmitter glutamate does.[6] [edit]Sources [edit]Dietary sources Aspartic acid is not an essential amino acid, which means that it can be synthesized from central metabolic pathway intermediates in humans. Aspartic acid is found in: Animal sources: luncheon meats, sausage meat, wild game Vegetable sources: sprouting seeds, oat flakes, avocado, asparagus[citation needed], young sugarcane, and molasses from sugar beets.[1] Dietary supplements, either as aspartic acid itself or salts (such as magnesium aspartate) The sweetener aspartame (NutraSweet, Equal, Canderel, etc.) [edit]Chemical synthesis Racemic aspartic acid can be synthesized from diethyl sodium phthalimidomalonate, (C6H4(CO)2NC(CO2Et)2).[7] The major disadvantage of the above technique is that equimolar amounts of each enantiomer are made, the body only utilises L-amino acids. Using biotechnology it is now possible to use immobilised enzymes to create just one type of enantiomer owing to their stereospecificity. Aspartic acid is made synthetically using ammonium fumarate and aspartase from E.coli, E.coli usually breaks down the aspartic acid as a nitrogen source but using excess amounts of ammonium fumarate a reversal of the enzyme's job is possible, and so aspartic acid is made to very high yields, 98.7 mM from 1 M. 8. Explain the mechanism of Frieldal crafts Acylation reaction. Friedel-Crafts alkylation Friedel-Crafts alkylation of benzene What is alkylation? Alkylation means substituting an alkyl group into something - in this case into a benzene ring. A hydrogen on the ring is replaced by a group like methyl or ethyl and so on. The facts Benzene reacts at room temperature with a chloroalkane (for example, chloromethane or chloroethane) in the presence of aluminium chloride as a catalyst. On this page, we will look at substituting a methyl group, but any other alkyl group could be used in the same way. Substituting a methyl group gives methylbenzene. or: Note: The methylbenzene formed is more reactive than the original benzene, and so the reaction doesn't stop there. You get further methyl groups substituted around the ring. You can improve your chances of just getting monosubstitution by using a large excess of benzene. You won't have to worry about this for UK A level purposes. You will find the mechanism for this reaction in the mechanisms section of this site. Use the BACK button on your browser to return to this page later. Friedel-Crafts alkylation of methylbenzene (toluene) Again, the reaction is just the same with methylbenzene except that you have to worry about where the alkyl group attaches to the ring relative to the methyl group. Unfortunately this time there is a problem! Where the incoming alkyl group ends up depends to a large extent on the temperature of the reaction. At 0°C, substituting methyl groups into methylbenzene, you get a mixture of the 2-.3- and 4- isomers in the proportion 54% / 17% / 29%. That's a higher proportion of the 3- isomer than you might expect. At 25°C, the proportions change to 3% / 69% / 28%. In other words the proportion of the 3- isomer has increased even more. Raise the temperature some more and the trend continues. The reason for this is again beyond UK A level. Note: The problem in this case lies in the fact that the methyl groups attaching to the ring can fall off again and reattach somewhere else in the presence of the aluminium chloride. You can get equilibria set up between the various isomers. The reason for the 2,4- directing effect of the methyl group in methylbenzene lies in the fact that the 2- and 4- isomers form faster than the 3- isomer. However, in this case, the 3- isomer is the most thermodynamically stable of the three. If you raise the temperature, or allow more time, the equilibria set up favour the most stable product. You do NOT have to worry about this for A level purposes. Friedel-Crafts alkylation industrially The manufacture of ethylbenzene Ethylbenzene is an important industrial chemical used to make styrene (phenylethene), which in turn is used to make polystyrene poly(phenylethene). It is manufactured from benzene and ethene. There are several ways of doing this, some of which use a variation on Friedel-Crafts alkylation. What follows is the method required by the UK A level Exam Board, AQA. The reaction is done in the liquid state. Ethene is passed through a liquid mixture of benzene, aluminium chloride and a catalyst promoter which might be chloroethane or hydrogen chloride. We are going to assume it is HCl (because that's what AQA want!). Promoters are used to make catalysts work better. There are two variants on the process. One (the Union Carbide / Badger process) uses a temperature no higher than 130°C and a pressure just high enough to keep everything liquid. The other (the Monsanto process) uses a slightly higher temperature of 160°C which needs less catalyst. (Presumably - although I haven't been able to confirm this - the pressure would also need to be higher to keep everything liquid at the higher temperature.) or: Again, the aluminium chloride and HCl aren't written into these equations because they are acting as catalysts. If you wanted to include them, you could write AlCl3 and HCl over the top of the arrow.