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Brown's Chemistry Clinic
My unofficial
support for Salters
A2
Advanced Chemistry
Salters A2 Chemistry - 'exam
bashing' thoughts for
Unit CD
"Colour by Design" -
part of module 2854
CD unit map-learning
objectives * extra CD stuff *
My revision index *
My Salters A2 homepage *
Email
At the moment the AS/A2 links
are for the old syllabus *
My NEW
Salters AS Chemistry page
PLEASE REMEMBER, THESE ARE NOT 'STAND
ALONE' NOTES, and were designed for my classes for use alongside the Salters
resources - Chemical Ideas, Chemical Storylines, Practical
Activities-Investigations and the AS-A2 Revision guides all published by
Heinemann Secondary Series, to reduce the reading workload and offer a study
strategy. From your teacher (not
me!), its handy to have the answers to the Chemical Ideas, Storylines
Assignments and Activities Questions side by side with the texts and these
strategy pages. You haven't time to redo the Q's but a quick read of the Q's and
connecting with the official answers is valuable revision - there is too much
hit and miss revision from doing past papers in my opinion.
Chemical
Storylines CD1 Ways of Making colour
Most colour due to coloured compounds and
how they interact with light. [green box on Pigments and dyes]
Ancient use of mineral pigments eg Red
Ochre (iron(III) oxide) but no good for colouring cloth!
Cloth must be coloured by using solutions
of a dye. Assignment 1 worth revising.
Original dyes from natural materials eg
blue woad/Indigo from plants and red Cochineal from insects.
However dyes and pigments were expensive
so there was scope for commercial and technical breakthrough's 100 years ago
for the development of
synthetic dyes which could be mass produced reasonably cheaply.
Raw material was coal tar and
many dyes developed
from extensive research but some found by accident eg the iron version of Monastral
Blue (another case of serendipity!).
Chemical
Ideas 6.7 Where does colour come from? (revision)
How colours arise through total
transmittance, partial transmittance (some wavelength’s absorbed), what
isn't absorbed constitutes the colour you see.
Reflection - what isn’t absorbed you
see as the complementary colour.
Fluorescence: what happens when molecule
absorbs a uv photon of radiation?
After the increase in vibrational energy and
excitation
to a higher electronic level, the excited molecule loses energy partly by molecular
collision, but mostly by re-emitting radiation of photons of lower frequency or longer
wavelength eg as ir or visible.
Activity
CD1 Changing colours chemically
All relevant chemistry
covered elsewhere BUT a quick glance through results to make sure you can
judge whether a reaction is redox, ionic precipitation, acid-base, ligand
exchange or a polymorphic change wouldn't go amiss.
Chemical
Storylines CD2 The Monastral Blue story
Accidental discovery of the dye Monastral
blue, turned out to be a phthalocyanine complex of iron.
Ligands bond via 4 nitrogen atoms,
conjugated system of bonds, large delocalised electron structure.
Other d-block metals also gave similar
coloured complexes. Revise assignment 2.
Copper phthalocyanine of commercial value,
known as Monastral blue, led to availability of a variety of phthalocyanine pigments.
All have good 'dying'
characteristics eg high colour strength, very stable
to heat, excellent fastness to light (ie don’t fade), relatively unreactive towards
acids and alkalis.
Chemical
Storylines CD3 Chrome Yellow
Limited pigments available upto middle of
18thC and artists looking for new and better pigments.
No bright lemon yellow available, but early
in 19thC lead chromate(VI) made by ionic precipitation, this is chrome yellow pigment
still used today. [green box - what
is paint?] and revise Assignment 3.
Pigments used in ancient civilisations were
often based on arsenic and lead etc. which can be poisonous, toxic, carcinogenic etc.!
Modern paints tend to be less toxic as
inorganic pigments are replaced by organic compounds.
Examination of van Gogh pictures to reveal
the materials and techniques he used.
The analysis uses uv and ir photography and
fluorescent techniques (ZnO fluoresces in uv, Emerald Green pigment absorbs in ir so shows
up as dark area)
Element analysis using atomic emission
spectroscopy of tiny samples and a scanning electron microscope is used to look at the pigment crystal
structure. See also [green box p220]
Chemical
Ideas 5.1 Ions in solids and solutions (revision)
Activity
CD3 Seeing colours
Chemical
Storylines CD4 Chemistry in the art gallery
Problems to solve in restoring paintings eg
paint flaking off, what pigments were used, binding agent stability etc.
Analysis of Cima painting using reflectance
spectrum to identify the pigments he used.
What were the binding medium, properties
required? [green boxes Emulsions, what are they? and binding
medium]
What medium did Cima use? - natural oils
like linseed and walnut dry and harden slowly which contain triesters of palmitic and
stearic acid with the triol glycerol but in differing ratios.
-
If we can measure the ratio
we can tell which oil Cima used to bind the pigments
-
This is found from gas-liquid
chromatography (g.l.c.) which gives the ester ratio,
-
but synthetic polymers in hydrocarbon
solvents were used in the restoration paints.
Element composition of Cima’s pigments
was done via [laser microspectral analysis]
in which microscopic amounts are vaporised by laser energy and atomic emission spectroscopy used
to identify and measure the amounts of each element.
Chemical
Ideas 6.8 Ultraviolet and visible spectroscopy
Reminder of colour you see is what is not
absorbed.
Spectrometer measures light absorption
versus wavelength, the resulting absorption spectrum eg Fig 57.
A uv or visible spectrometer works on the
same principles as an ir spectrometer BUT three differences from ir spectra:
-
peaks
rise from baseline to show absorption
-
units of wavelength are nanometre
(nm)
-
you can’t assign specific peaks to groups because you get broad absorption bands
A colorimeter is simple type of visible
spectrophotometer and is a useful way to measure the intensity of colour in a solution
(See previous Activity SS1.1).
Colour chemists interested in 3 features of
spectrum:
-
wavelength of the radiation absorbed (colour not
seen!)
-
intensity of
absorption (relate to quantity of pigment needed)
-
shape of absorption band
(relate to shade and purity of colour).
Reflectance spectra (fig 59), useful when
the substance cannot be dissolved in a colourless solvent, produced by shining
‘white’ light onto surface and the spectrum of the light not absorbed is
measured
Chemical
Ideas 13.6 Oils and fats
Be familiar with the
structure of
-
glycerol
(propane-1,2,3-triol)
-
long chain carboxylic (fatty)
acids
-
triesters of glycerol (may be mixed).
Fatty acids usually have an unbranched
hydrocarbon chain of 16-18 C’s and the alkyl chain may/may not have C=C double bonds
ie saturated or unsaturated, the latter may be mono or polyunsaturated if 1 or more
>C=C< double bonds.
Count carefully when doing the skeletal
formula of acids or oils and Know the conditions and equations for the alkaline hydrolysis
of fats.
Oils to fats by hydrogenation:
equation, catalyst and reasons why saturated fats may be converted into saturated fats.
Q's to the point and CI13.6 Q5 is a
'beauty'!
Chemical
Ideas 7.6 Chromatography
General principles: mobile phase,
stationary phase, partition of mixture of compounds between the two phases.
Paper or thin-layer chromatography
(t.l.c.): set-up and Rf values (white box! p185). Theory of separation (figs 13/14).
Gas-liquid chromatography (g.l.c.): main
components of the chromatograph (Figs 15/16), carrier gas carrying the vaporised sample
for analysis, interpreting gas chromatograms, retention time (dependent on several
factors), technique very sensitive.
Chemical
Ideas 6.1 Light and electrons (revision)
Activity
CD4.1 Using reflectance spectra to identify pigments
Activity
CD4.2 What factors affect the drying potential of an oil?
Activity
CD4.3 Investigating paint media
Activity
CD4.4 Identifying a pigment
Activity
CD4.5 Finding a perfect match
Chemical
Storylines CD5 At the Start of the rainbow
19thC saw development of
synthetic organic dyes - several based on phenylamine (aniline) originally from coal tar
(know structure)
Mauve made in large commercial quantities
from coal tar benzene => nitrated to nitrobenzene => reduced to phenylamine =>
oxidised to Mauve.
Molecular structure was eventually worked
out, needed ‘aromatic’ molecule ideas of Kekule', structure of Alizarin , only
sticks to cloth via impregnated metal compound such as aluminium sulphate, process called
mordanting (know the details of Fig 26 as an example, complex formation or chelation)
Synthesis routes can be very complex eg Fig
28 p2126, should be able to identify the types of reaction in each stage of a given
method, in this case to synthesise Alizarin.
Note the commercial impact of synthetic
dyes on traditional industries in poor countries
Revise assignments 7,8 and 9.
Chemical Ideas 12.3 Arenes
Details of the
‘special’ structure of benzene, electron delocalisation, all C-C bonds the same
(midway between single and double), hexagonal planar ring, all bond angles 120o
deg.
Benzene much more
stable than expected from Kekule' structure of alternate single and double bonds.
Evidence for this comes from enthalpy changes
(actual v theoretical), substitution reactions predominate which preserve the stable
aromatic ring (rather than addition like alkenes).
Examples of
structures of arenes (aromatic hydrocarbons) to recognise , multiple or fused ring systems.
A wide variety of
compounds derived from benzene, note all the familiar functional groups and their names, C6H5- is called the
phenyl group (see use in names on p290-291), other names based directly on the benzene ring.
CI 12.3 problems: naming and drawing
structures of aromatic compounds, investigating actual and Kekule' structures of arenes,
aromatic structures and enthalpy changes, relative stability
Chemical
Ideas 12.4 Reactions of arenes
Electron ‘cloud’ structure of
benzene, attacked by electrophiles in substitution reactions NOT addition reactions.
Need to know all the reaction
conditions, reagents, molecular equations
and mechanisms (where required) for the following electrophilic substitutions:
bromination using iron(III) bromide
catalyst
nitration with conc. Nitric/sulphuric
acids
Sulphonation with conc. Sulphuric acid
to make sulphonic acids
Chlorination with chlorine and
aluminium chloride catalyst (note in a CI 12.4 Q why benzene reacts with ICl but not I2)
Friedel-Crafts alkylation using
haloalkane and aluminium chloride catalyst
Friedel-Crafts acylation using acyl
chloride or acid anhydride and aluminium chloride catalyst to make aromatic ketones
Activity
CD5 Comparing hydrocarbons
Quick look over results
(chemistry below or on sheet).
Useful reminders on practical techniques used
or should know about eg
-
crude solid product filtered by reduced pressure
(Buchner/Hirsch funnel job!)
-
recrystallisation method
-
filter
recrystallised product, wash with a little solvent and leave to dry
-
determine melting point as a simple test of purity, check experiment value
with data book.
Scribble
on reactions 1 and 2. The
chemistry of reaction 3 is on the sheet.
Chemical
Storylines CD6 Chemists Design colours
You need to know relationship between colour
and molecular structure (aminobenzene good example).
Dyes made by diazo reaction, first azo dyes
made coupling a diazonium salt with another reagent such as an aromatic amine (or phenol),
example of colour linked to number of amine groups attached to the ring .
Consider a dye molecule as being built up
of a group of atoms called a chromophore, which is largely responsible for the colour .
For many molecules the delocalised aromatic
system is the chromophore and attached to the chromophore are other interacting functional
groups which can change the colour .
These, and other added functional groups
can modify/enhance colour, make the dye more soluble in water (sulphonate group), attach
the dye molecule to cloth fibres.
All azo dyes have the basic structure
X-N=N-Y, all sorts of X,Y combinations investigated giving a huge range of colours, BUT
mostly yellow-orange-red with a few blues and greens.
Assignments 9 and 10 good revision.
Chemical
Ideas 13.10 Azo compounds
Azo compounds contain the –N=N-
grouping - in R-N=N-R’ molecules R and R’ are usually arene groups and are more
stable than if R and R’ were alkyl.
Azo dyes formed by coupling reactions
between diazonium salts and a coupling reagent (eg aromatic amine or phenol).
Preparation of diazonium salt from primary
aromatic amine and nitrous acid, note reaction conditions, equations and structure of
diazonium cations and apply to diazo coupling reaction with amines or phenols.
Chemical
Ideas 6.9 Chemistry of colour
Electronic theory of colour, coloured
substances absorb in the visible region, absorption causes electronic changes from ground
state to excited state.
Many molecules absorb in uv, so seem
colourless to us.
Energy required is called the excitation
energy.
Coloured compounds often contain
unsaturated groups such as C=C, C=O or N=N - and usually part of extended delocalised
system eg arene system of delocalised electrons, which are more easily excited than in
single bond systems
Other added functional groups change the
electronic levels, changing the excitation energy, ie changing the colour.
Many dye molecule colours are pH dependent
because they undergo acid-base reactions involving cationic, neutral or anionic forms
Also note the origin of colour due to
electronic changes in transition metal compounds and the effect of ligand change.
Activity
CD6 Making azo dyes
Chemical
Storylines CD7 Colour for Cotton
The search for fast dyes ie those that stay
attached after washing, rubbing, exposure to light.
Some dyes bound well to some materials, but
not others,
Vat dye like indigo where colourless
soluble reduced form is oxidised to insoluble coloured form
direct dyes held onto cotton
(cellulose based) by hydrogen bonding, must be long/straight to maximise weak
inter-molecular forces (H-bonding weak compared to covalent), see Figs 34/35 and
Assignment No 11.
Development of dyes that will strongly
covalently bond with the fibres can be eventually using trichloroazine (has reactive
–Cl groups) as an ‘intermediate’ to link the dye and fibre molecules,
Still some problems with acid/alkali
reactions with fibres such as cotton, problem overcome by controlling the pH of the
reaction medium with buffers (details later in O unit).
Activity
CD7.1 Dyeing with a direct dye and a reactive dye
Activity
CD7.2 Different dyes for different fibres
Chemical
Storylines CD8 High-Tech colours
Development of high-tech dyes for ink jet
printers and electronic photography, but early ink jet printers had a ‘smudge’
problem because print wasn’t very permanent. For the method to work well the ink must
be soluble in water in the print head BUT insoluble on the paper.
The problem was solved by replacing some of
the sulphonic acid groups with carboxylic acid group,
-
the arene-COOH form is insoluble, but
the anionic salt form of the arene-COO – anion is soluble,
-
so by using the soluble ammonium salt in
print head cartridge and heating on printing, the ammonium salt decomposes to the
insoluble acid form, is it the ultimate dye?
The dye diffusion thermal transfer process
for converting electronic camera images to a colour print (Fig 43). Assignment 15.
Activity CD9
Check your notes on Colour by design and Chemical Storylines CD9 summary are
incorporated in the learning objectives list, and don't forget to revise your CD UNIT TEST
and note what you got wrong!
GENERAL
REVISION
NOTES
* Salters
Advanced Level Chemistry * Salters Advanced Level Chemistry * Salters
Advanced Level Chemistry * Salters Advanced Level Chemistry * Salters
Advanced Level Chemistry *
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