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    Option A Modern Analytical Techniques

    Modern Analytical Chemistry

    OPTION A

    A1 Analytical Techniques (SL/HL)

    Analytical techniques are an important and useful tool for the following reasons:

    a) They allow the exact composition of substances/compounds/elements to be determined.

    b) The formula of the compounds can be deried.

    c) They allow a detailed structure to be deduced.

    d) They are able to determine the purity of the substances inestigated.

    The compositionof substances can be determined by separation techniques such as chromatography.

    !"aper# thin layer# column !$%/&%)# gas'liquid# high performance liquid !&%).

    The concentrationof substances can be deduced by process such as atomic absorption !AA)

    spectroscopy !$%/&%)# ( and isible spectroscopy !&%).

    The formulaof a compound can be determined from a ariety of techniques.

    *. Chemical testswill determine the functional groups present.

    +. mpirical formula can be wor,ed out by combustion with oxygen and then determining the mass

    of -O+and &+O gien.

    The structureof a compound requires a com!inationof modern instrumental methods.

    1" Mass Spectrometry"

    +. #nfra $ed Spectroscopy"!..)

    0. %uclear Magnetic $esonance !1M)

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    A& 'rinciples of Spectroscopy (SL/HL)

    lectromagnetic Spectrum

    adiation is related by the equation c2 3f. The greater the energy# the higher the frequency# the lower the

    waelength.

    %abel the main regions of the spectrum and the waelengths/frequencies.

    T*&ow is the electromagnetic spectrum used to transfer information4 5iscuss how using this energy has

    affected our lies. 6hat are the limits to its usefulness# if any4

    A!sorption and mission Spectra

    7mission $pectroscopy inoles analysis of light gien out by atoms/molecules as they return to their

    ground state.

    Absorption $pectroscopy inoles energy being absorbed by the atoms/molecules to promote into excited

    states.

    The energy absorbed/emitted is characteristic to the atom/molecule inoled.

    The most energetic absorptions inole electron transitions. These ta,e place in the 8'ray# 9'ray# ( and

    isible regions# depending on which electrons moe.

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    Types of .i!ration

    Only ibrations that inole a change in polarity will absorb infrared radiation.

    Molecules that are perfectly nonpolar!O+# &+etc.) 6ill not therefore gie absorptions.

    6ith non'polar molecules that contain polar bonds !-O+# --l;) the type of ibration is important.

    Symmetrical stretchingwould not cause absorption since the dipoles would cancel. Asymmetrical

    stretchingwould cause absorption# as would any type of !endingof the bonds.

    7.g. -O+ symmetrical stretch asymmetrical stretch bending

    7.g. $O+

    7.g. &+O

    7xtension: 5raw out some of the possible bending and stretching ibrations in

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    Analysing #$ Spectra

    Mainly used to identify functional groups.

    *. H !ondin carboxylic acids 2 0+0='0>>= cm'*!waenumber)

    +. H !ondin alcohols 2 +>=='00== cm'*

    0. C0 !ond 2 *?@='*>= cm'*

    ;. C0C !ond 2 *?*='*?@= cm'*

    >. ingerprint region" 2 ;=='*;== cm'*

    This region is too complicated to analyse# but since each molecule will gie a characteristic pattern# it can be

    used to positiely identify a compound by comparison with other ,nown spectra.

    A2 Mass Spectrometry (SL/HL)

    Brom Topic +

    *.aporisation

    +.onisation

    0.Acceleration

    ;.5eflection

    >.5etection

    n addition to determining the molar massof the substance# mass spectrometry will gie some information of the

    structure of the molecule.

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    The Molecular #on

    The ionised molecule is detected as the molecular ion (M3)with the largest mass and this determines the

    molar massof the molecule.

    ragments

    onisation of the molecule will often lead to the molecular ion splittinginto fragments. These fragments

    gie clues its structure.

    7.g. A difference in mass between + fragments of CC

    14suggests the loss of a

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    $ome of the most common positions are:

    & atoms -hemical shift

    '-&0 2 =.D

    '-&+' 2 *.0

    '-&' 2 +.=

    -&0'-2O 2 +.*

    'O& 2 ;.>

    -?&? 2 .0

    &-2O 2 D.

    &O'-2O 2 **.

    Eenerally# the more electronegatie the chemical enironment# the greater the chemical shift.

    The num!er of hydrogen atomscan be determined by the areaunder each pea, on the spectrum.

    This is useful information as it shows how many hydrogen atoms are experiencing the same chemical

    enironment.

    8ses of %M$ Spectroscopy in 9ody Scanners

    1M has some ery important medical applications. One reason is that the low energy radio waes used

    are harmless and hae no ,nown side effects.

    M !magnetic resonance imaging) is one application that can monitor and ealuate arious medical

    conditions.

    t inoles the whole body of the patient being placed inside the magnet of a large 1M machine.

    The protons !&Fs) in water# lipids# carbohydrates etc in the body gie different signals that allow an

    accurate 05 image of the body to be obtained.

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    A7 %M$ Spectroscopy (High $esolution) (HL %L:)

    Tetramethylsilane (TMS)

    This is used as a reference sample for the following reasons.

    t is non toxic and unreactie.

    t produces a ery clear single pea, since all *+ &Fs are equialent.

    t is ery unpolar and so occurs upfield# well away from any other possible pea,s.

    t has a ery low bp and so can therefore be easily remoed from the sample.

    Splitting 'atterns

    The absorption pea,s gien in *& spectra are influenced by adacent hydrogens.

    These can be analysed when using high resolution 1M to show a pea, splitting patterns.

    $inglet: f there are no adacent &Fs.

    5oublet: f there is one & atom on an adacent carbon.

    Triplet: f there are two & atoms on adacent carbons.

    Guartet: f there are 0 &Fs on adacent &Fs.

    This is called spin'spin coupling. 7xact details of how these patterns arise is not required.

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    A; Atomic A!sorption Spectroscopy (AA) (SL/HL)

    8ses of AA Spectroscopy

    AA measures the amount of energy that is absorbed by electrons as they are promoted from lower to

    higher energy leels.

    t is the reerse of emission spectra that measure the energy released when electrons drop down into

    lower energy leels and gie rise to characteristic colours of flames in the isible region !potassium 2 lilac)

    A light source of a particular frequency/waelength is selected to complement the colour of the sample.

    (sing the amount of energy absorbed# the concentrationof the sample can be accurately determined.

    t can therefore be used to determine the concentration of metal ionsin samples such as water# blood#

    food# soils etc.

    'rinciples of AA Spectroscopy

    AA machines wor, on the same double beam principle outlines for spectra.

    5raw a simplified labelled diagram:

    The sample is turned into a fine mist and then burnt in a fuel/air mixture. This happens in the atomiHer.

    A monochromatic light source is then passed through the resulting flame. The light source needs to contain

    the metal under inestigation so that the correct frequency of light will be used.

    The amount of light absorbed by the atoms in the sample is detected and conerted into an electrical

    signal.

    Iy changing the light source# the concentration of different metals in the same sample can be determined.

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    inding Concentrations

    The amount of light absorbed by a sample !Absorbance) is directly proportional to the concentration of

    metal ions in the sample.

    This is called the Ieer %ambert %aw.

    A 2 J-l

    A 2 Absorbance J 2 molar absorbtiity coefficient !constant)

    - 2 concentration l 2 path length of sample cell.

    7.g. A metal solution in a cell of length * cm. The absorbance of the solution at a particular waelength is *.D+.

    The molar absorptiity is *D;== for that waelength. !from a data boo,)

    Cali!ration Cur-es

    (sing this relationship# if a calibration cure is determined for a particular metal using arious ,nownconcentrations# the concentration of an un,nown solution of that metal can be easily determined.

    Absorbance

    -oncentration

    1ote: The calibration cure and Ieer %ambert %aw becomes less linear at higher concentrations of solution.

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    A5 Chromatography

    8ses of Chromatography

    -hromatography can be used to separate mixtures containing ery small amounts of substances.

    t can be used to identify how pure a substance is.

    Iy comparisons to ,nown samples it can be used to identify components and also determine the

    concentration/amount present.

    'rinciples of Chromatography

    All types of chromatography inole separation inoling two phases. The stationary phasestays fixed.

    The mo!ile phasemoes.

    $eparation occurs due to the difference in tendency to absorb onto the stationary phase compared to its

    solubility in the mobile phase.

    Adsorptioninoles a solid stationary phaseand a liquid mo!ile phase. The distance a solute moes

    depends on the relationship between how quic,ly it absorbs onto the solid phase compared to its solubilityin the liquid phase.

    The faster it absorbs# the less distance it moes.

    'artition inoles a liquid stationary phaseand a liquid/gas mo!ile phase" f the mobile phase is a

    liquid#the distance moed by the solute depends upon its difference in solubility in the two phases. f the

    mobile phase is a gas it depends on the olatility of the solute.

    The more soluble/olatile# the greater distance moed.

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    c) Thin Layer Chromatography (TLC)

    This wor,s by the same principle as paper chromatography# except instead of the paper# a thin layer of

    solid !silica) is used.

    n the same way# water acts as the stationary phase and the mobile phase is another solent.

    T%- allows the component to be scraped off at the end and re'dissoled. This method is used for some

    pregnancy sets.

    d) =asLiquid Chromatography (=LC) HL %L:

    This technique is used to analyse a mixture of -olatile liquids.

    The liquids need to be stable at temperatures close to the boiling point.

    The stationary phase consists of a liquid suspended on a solid in a long thin tube.

    The mobile phase is an inert gas such as helium or nitrogen.

    The sample is inected and then aporised by heat.

    The arious components in the aporiHed mixture are then carried through the column by the inert gas.

    5epending on the solubility in the stationary solent# the components hae different retention times and

    therefore reach the detector at different times.

    This method also allows the measurement of the relatie concentrations using the area under the pea,s.

    This allows for use in detecting the alcohol leel in blood during drin, driing tests.

    e) High 'erformance Liquid Chromatography (H'LC) HL %L:

    This method wor,s in a ery similar way to E%-. nstead of allowing graity to moe the components

    through the column# they are forced through much faster under pressure.

    This means that the column used doesnFt hae to be as long as the process is ery efficient.

    As well as separation# &"%- can also be used for identification of the components present.

    &"%- is used for non -olatile su!stances# and also for -olatile su!stances that tend to decompose

    near to their !oiling points. t can also be used to separate and identify optically acti-e compoundsifthe column is prepared using another optically actie stationery phase.

    Other uses include pollutants# food# alcoholic drin,s# insecticides/herbicides# biochemical research and

    pharmaceuticals.

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    A6 .isi!le and 8ltra-iolet Spectroscopy (HL %L:)

    Transition Metal Comple> Colours

    Transition metals hae incomplete d shells. This is the main reason why they form coloured compounds.

    6hat colour are -u# $c0and Nn+4

    The different colours obsered by arious transition metal compounds depends on ; factors.

    *. The type of transition metal.

    6hat are the electron configurations of Mn+and Be04 6hat are the colours4

    +. The oxidation state of the transition metal.

    6hat is the electron configuration of Be+

    4 &ow does the colour compare to Be0

    4

    0. The type of ligand.

    6hat colours is -u!&+O)?P+compared to -u!1&0);P

    +compared to -u!-l);P+'4

    6hat colour is -o!&+O)?P+compared to -o!1&0)?P

    +4

    ;. The shape of the complex.

    6hat shape is Be!&+O)?P+4 6hat shape are -u!-l);P

    +'and Ag!1&0)+P4

    Splitting of d r!itals in Transition Metal Comple>es

    There are > d orbitals of equal energy but different shapes# aligned along different axis. $,etch and label

    below.

    5uring complex formation# the non'bonding electrons on the ligand are donated into empty d'orbitals.

    n an octahedral complex# the ? ligands approach the metal ion along the x# y and H axis.

    6hich + of the d orbitals are arranged on these axix4

    The extra repulsion between the non bonding electrons in the ligands and these electron orbitals causes

    two of the d orbitals to split to a higher energy. 5raw out the d orbital split for an aqueous Be+

    %ight can be absorbed by these complexes causing electron transition from the lower to the higher energy

    d orbital energies.

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    %abel the amount of splitting as Q7. This amount of energy can be affected by the type of ligand# the

    oxidation state of the metal# the type of metal ion and the shape of the complex.

    1&0liagnds cause more splitting than &+O# -l'cause een less.

    The colour seen is the complementary colour to the one absorbed. 7.g. -u!&+O)?P+ absorbsred/orange

    light and so appears light blue. 6hen some of the ligands are replaced by 1&0 to gie -u!1&0); !&+O)+P+

    thesplitting increases and the complex absorbs energy in the yellow region. This causes the colour to

    appear iolet/blue.

    8..is Spectroscopy in rganic Molecules

    Organic molecules containing alternate double bonds are able to absorb energy in the isible and (

    range. These include long chain al,enes and arenes.

    7xamples include retinal !itamin A) chlorophyll and I carotene. 5raw out the structure of one of these.

    This type of structure is said to contain con?ugation.

    The part of the molecule where conugation occurs is called the chromophore. %abel the chromophore on

    the molecule aboe.

    %ight energy can be absorbed causing the pi electrons to become delocaliHed and moe to different

    orbitals. The amount of energy absorbed depends on the amount of conugation in the molecule.

    More conugation# more delocaliHation# the less the amount of energy required to excite the

    electrons. This means that absorption is more li,ely to occur in the isible region.

    7.g. Al,enes hae only * double bond# no conugation or delocaliHation. The energy required to excite the

    electrons into unfilled !molecular) orbitals is greater leading to absorption in the u part of the spectrum.

    This is why al,enes appear colourless. !As do aldehydes and ,etones).

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    6hat colour is chlorophyll or I carotene4 5o they absorb u or isible light4 6hat about their structure

    allows this to happen4 7xtension: 6hat actual !complementary) colour must they absorb4

    5raw out the dissociation of the wea, acid phenolphthalein.

    7xplain why in acid it is colourless# but in al,ali it becomes a colourful pin,.

    One of the main ingredients in suntan lotions is ;'aminobenHoic acid. 5raw the structure below.

    7xplain why this molecule is able to absorb harmful high energy u light.

    8sing the 9eer Lam!ert La, to orm Cali!ration Cur-es

    The amount of light absorbed by a sample !Absorbance) is directly proportional to the concentration of

    metal ions in the sample.

    This is called the Ieer %ambert %aw.

    A 2 J-l

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    f seeral samples of transition of arying concentration are prepared# the absorbance can be measured

    using a spectrometer/colorimeter.

    Measurements are made using !3max) by selecting the waelength of light that is the complementary colour

    using an appropriate filter.

    "lotting absorbance against concentration will gie a straight line graph.

    This allows the concentration of an un,nown sample to be measured by recording the absorbance andcomparing to the graph.

    7.g. f a number of solutions of the compound are prepared of accurately ,nown concentration. Those

    concentrations should brac,et the concentration to be determined.

    Bor each solution# measure the absorbance at the waelength of strongest absorption !3max). Then plot a

    graph of absorbance against concentration. This is a calibration cure.

    According to the Ieer'%ambert %aw# absorbance is proportional to concentration# producing a straight line.

    That is true as long as the solutions are dilute# but the %aw brea,s down for solutions of higher

    concentration.

    1o attempt is made to force the line bac, through the origin. f the Ieer'%ambert %aw wor,ed perfectly# it

    wouldpass through the origin# but this canRt be guaranteed for all concentrations.

    Measure the absorbance of the solution with the un,nown concentration at the same waelength. f# for

    example# it had an absorbance of =.?==# record the corresponding concentration from the graph as shown.