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Summary Required Practicals-AQA A Level Chemistry
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Summaries of each required practical for AQA A level Chemistry
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8) Measuring the EMF of an electrochemical cellMethod: Salt bridge:
1) Clean the Zn & Cu foils with emery before use. Used to connect up the circuit, free
Degrease the metal using some cotton wool & moving ions conduct the charge
propanone Usually made from a piece of filter
2) Place the Cu strip into a 100cm3 beaker with about paper (or material) soaked in a salt
50cm3 of 1moldm-3 CuSO4 solution solution usually potassium nitrate,
3) Place the Zn strip into a 100cm3 beaker with about can also be a glass U tube containing
50cm3 of 1moldm-3 ZnSO4 solution a salt solution plugged with cotton
4) Use a strip of filter paper soaked in saturated wool
potassium nitrate solution for the salt bridge Salt should be unreactive with the
5) Connect the Cu(s)|Cu2+(aq) and Zn(s)|Zn2+(aq) electrodes & electrode solutions e.g
half-cells by connecting the metals using crocodile potassium chloride would not be
clips & leads provided to the voltmeter suitable for copper systems because
Cl- ions can form complexes with
Method- using a platinum electrode: Cu2+ ions
If one or both of the half-cells don`t contain a A wire isn`t used because the metal
conducting metal, we must use an inert platinum wire would set up its own electrode
electrode system without the solutions
Set up a copper half-cell using a similar
arrangement to the previous one. Combine it with
a Fe2+/Fe3+ half-cell with a platinum electrode
Half cell should have a mixture of acidified 1.0M
iron (II) sulphate solution & an equal volume of
0.5M iron (III) sulphate solution as the electrolyte.
Use a fresh salt bridge
Pt electrode is used because it is unreactive & can
conduct electricity
, Purifying an organic solid
9) Titration Curves
10) Preparation of a pure organic solid & test of its purity
Reason Loss of yield in this process:
(recrystallisation) method Constructing a pH curve:Calibrating pH meters:
Crystals lost when filtering or
washing
Calibrate first by measuring known
1) Dissolve the impure Appropriate solvent= one 1) whichTransfer 25cm3 of acid to a conical flask with a volumetric
pHSome
of a product
buffer stays in solution
solution, necessary
compound in a minimum will dissolve both compounds & pipette
after recrystallisation
as pH
volume of hot (near impurities when hot & one2)in Measure initial pH of acid withmeters can lose accuracy on
a pH meter
Other side reactions occurring
boiling) solvent which the compound itself3)doesn`t Add alkali in small amounts storage
If (2cm3) noting
crystals are the volume
not properly dried
dissolve when cold. added Most pH probes are calibrated by
the mass will be larger than
Minimum volume used to 4) obtain
Stir mixture to equalise putting pH probe
theexpected in acan
setmean
buffer (often
which
saturated sol. & to enable
5) Measure & record pH topH4) 1dp & pressing
percentage a calibration
yield is larger than
crystallisation on cooling
6) Repeat steps 3-5 but button/setting
when approaching
100% for
endthat pH. add in
point
2) Hot filter solution through Removes any insoluble impurities
Sometimes this is repeated with a
(fluted) filter paper & heat will prevent crystals smaller volumes of alkali
quickly reforming during filtration 7) Add until alkali in excess second buffer at a different pH
3) Cool filtered solution by Crystals will reform but soluble Can also improve accuracy by
inserting a beaker in ice impurities will remainpH graphs:
in sol. Form maintaining constant temperature
as they are present inStrong
small acid- strong base:
quantities so solution is not Long steep part around 3-9, pH at equivalence
Half neutralisation volume: point=7
saturated The equivalence point liesFor at the
weak acids: of the
mid-point
Ice will increase yield of crystalsextrapolated vertical portion of the curve
Ka= [H+(aq)][A-(aq)]
4) Suction filtrate with a Water pump connected to
[HA(aq)]
Buchner flask to separate Buchner flask reducesWeakpressure & strong base:
acid-
out crystals speeds up filtration
At the start the pHAtrises quickly & then
½ neutralisation levels off
volume, [HA] = [A-]
5) Wash crystals with To remove soluble impurities
distilled water
The flattened part= Sobuffer region
Ka = [H+] and&pKa=Is formed
pH because a
6) Dry crystals between buffer solutionPurification
is made
If a pH curve
stage:isrecrystallisation
plotted then the pH of a
absorbent paper Equivalence pointweak >1)7, With
steep part
acidaat halfofneutralisation
measuringcurve >7 (around
cylinder, (1/2 7-9)
measure V) =
pKa out 15cm3 ethanol into a boiling tube
Preparation of aspirin:
Strong acid- weak base: 2) Prepare a beaker half-filled with hot
1) Add to a 50cm3 pear-shaped flask 2g of 2-hydroxybenzoic acid & 4cm3
Steep part
ethanoic of curve <7 water
anhydride (aroundfrom a kettle at approx. 75
4-7)
2) Equivalence
Add 5 drops ofpoint <7 degrees
85% phosphoric acid & swirl
3) 3) Use a&spatula
Fix flask with a reflux condenser to addoncrude
heat mixture aspirin
a boiling waterto
bathweak
for about boiling tube with ethanol & place
5 mins, without cooling mixture carefully add 2cm3 tube
Weak acid- base:
water in 1 portion down the incondenser
beaker of hot water
4) No
Whensteep partreaction
vigorous of the has
curve
ended, pour mixture intotube
40cm3 of all
cold
4) Stir contents of boiling until
water in a 100cm3 beaker, aspirin
stir % rub sides of beaker
has dissolved with a stirring rod
necessary to induce recrystallisation
5) Pour the hot solution containing
5) Allow mixture to stand in ice bath to complete crystallisation, collect
dissolved aspirin through a warmed
product by suction filtration & wash it with a little water
filter funnel & fluted filter paper to hot
filter
Notes for nitration process: 6) Pour filtrate into 40cm3 water in a
Conc. acids= corrosive, wear conicalgloves
flask
Reaction= exothermic 7) Allow are
so acids keptflask
conical coolto
& cool
acid slowly
added &
dropwise white needles of aspirin should
Temp. = kept low at addition stage to prevent multiple
separate
substitution of nitro 8)groups on benzene
Cool whole ring in an ice bath
mixture
9) Filter off purified solid under reduced
Nitration method: pressure & allow it to dry on filter
1) Measure 2.5cm3 paper methyl benzoate into a small conical
flask & dissolve it
10) Record in 5cm3
massofofconc. H2SO4,solid
dry purified when
liquid has dissolved, cool mixture in ice
2) Prepare nitrating mixture by adding drop by drop
Notes:
2cm3 of conc. H2SO4 to 2cm3 conc. HNO3, cool this
In reaction phenol group on aspirin is turned into
mixture in ice as well
an ester by reacting it with the reactive ethanoic
3) Add nitrating mixture drop by drop from a dropping
anhydride
pipette to methyl benzoate sol., stir mixture with
Ethanoic anhydride used as its cheaper, less
thermometer & keep temp. below 10 degrees
corrosive, less vulnerable to hydrolysis & less
4) When addition is complete, allow mixture to stand at
dangerous to use
room temp for 15 mins
Excess ethanoic anhydride will hydrolyse &
5) Pour reaction mixture onto about 25g crushed ice &
contents of flask will boil
stir until all ice has melted & crystallise methyl3-
Avoid naked flames dur to flammability of ethanol
nitrobenzene has formed
Hot filtering solution during recrystallisation will
6) Use same purification method as in aspirin
remove any insoluble impurities & heat will
prevent crystal reforming during filtration
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