Download [edit]Occurrence in solution

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.