Chemistry Practical Tips
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Transcript of Chemistry Practical Tips
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PRACTICAL TIPS
PROCEDURE REASON RESULT
Reflux As liquid(s) vaporize, theyare cooled and condense backto iquid(s)
Used to prevent the escapeof volatile substances and
thus increases the yield.
Reflux for 30 min Reaction is slow / High activation
energy
Advantages of suction
filtrationover normal filtration.
Faster Dries solid
Aspects of the melting
temperature determinationthat would indicate the crystals
were pure.
Melting temperature
should besharp. It should be consistent withthe literature.
Adding drying agent To remove water
Eg:- Anhydrous MgSO4orAnhydrous Na2SO4or Anhydrous
CaCl2
Solution becomes clear
Adding anti-Bumping granules To ensure even boiling of liquids
Constant stirring Two possible answers Reactants are immiscible
and they form separate
layers To make sure that
temperature is uniform
throughout the solution
(Note: plan your answeraccording to the situation)
A solution is added drop wise
with cooling.
Reaction is exothermic / To avoid
the temperature rising too much
Washing with NaHCO3 Two possible answers
To neutralize excess acid(when a pure sample is
prepared)
To quench the reaction(when order of the reactionto be found)
(Note: plan your answeraccording to the
situation)
The pressure in the
funnel increases due to
the formation of CO2
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Precautions
1) Fume cupboard2) Wear gloves
3) Water bath
i) Toxicii) Harmful by skin
absorption / corrosive
acid
iii) Flammable liquidNitrous acid (HNO2) is made inthe reaction mixture rather than
being obtained from a chemicalsupplier.
HNO2is unstable at roomtemperature and hence, cannot be
stored.
When Nitrous acid is used, the
temperature must not be lowerthan 0C nor higher than 10C.
Below 0oC, reaction is too slow
and above 10oC, diazonium
compound decomposes .
Recrystallization1) Impure solid is dissolved
in a minimum volume of
hot solvent.2) The solution is filtered
hot through a pre-heated
funnel.
3) The solution is cooledand filtered using aBuchner funnel.
4) The solid is washed with
a small amountof coldsolvent.
5) The solid is dried in adesiccator
1) To prevent the solidremaining in solution on
cooling.2) To remove insoluble
impurities.Pre-heated funnel is used
to prevent crystallization ofthe solid.
3) Impurities remain solubleand the pure solidrecrystallizes.
4) To removes soluble
impurities.
5) To prevent decomposition.Decomposition could occurif the compound is
heated in an Oven.
Volume measurements
To measure 10-25cm3
solution, pipetteis used.
To measure more than25cm
3,measuring
cylinderis used.
An exact volume ofsolution is needed.
Only an approximatevolume / excess is needed.
More accurate
Faster/Moreconvenient.
It is incorrect to use HClinstead
of H2SO4in KMnO4titrations.HClwill be oxidized to chlorine
by the manganate(VII)
Titre value will be too
high.
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I2Vs Na2S2O3titration
In burette - Na2S2O3
In pipette - I2solution
IndicatorStarch
Starch is added when the solution turns pale yellow.
Starch is not added initiallybecause it forms an insoluble complex so that titre valuereduces and hence, the percentage error would be high(er).
When starch is added, solution becomes blue-black which turns colourlessat the endpoint.
Percentage yield.
Percentage yield =
x 100%
Usually percentage yield is less than 100% because of,
i) Transfer losses
ii) Escape of volatile liquids
iii) Impurities in the reactants
Sometimes, percentage yield is greater than 100% because The product is not dry / is damp.
Transfer losses
Solid (product) remains on the filter paper / is deposited on the sides of the glassware,
which is not be recovered by filtration.
Accuracy
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Errors
Inorganic compounds and elements
1 Appearance
Coloured crystalline substances are usually hydrated salts of transition metals.
The colours of transition metal ions in dilute, aqueous solution are shown in the tablebelow.
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7 Tests for oxidizing and reducing agents
PRECIPITATES
1 Barium chloride solution
NOTE:- If dilute hydrochloric acid is added to the anion solution beforeaqueous barium chloridethen only the sulfatewill form as a precipitate.
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2 Sodium hydroxide solution
When dilute sodium hydroxide (NaOH) solution is added to a solution containing a metalion a precipitate of the insoluble hydroxide, eg Mn(OH)2, is usually formed.
Precipitates which are amphoterichydroxides will dissolve in excess sodium hydroxideto give a solution containing a complex ion, eg [Cr(OH)6]
3.
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3 Ammonia solution
Dilute aqueous ammonia (NH3), when added to a solution containing a cation, will formthe same hydroxide precipitate as dilute sodium hydroxide solution, eg Mn(OH)2.
Excess aqueous ammonia may dissolve the precipitate to form a complex ion,eg [Cu(NH3)4(H2O)2]
2+.
4 Silver nitrate solution
Aqueous silver nitrate is commonly used to test for the presence of halide ionsinsolution. Anions which would interfere with the test (eg carbonate) are removed byadding dilute nitric acidbefore the aqueous silver nitrate.
Silver halides which dissolve in ammonia do so to form a colourless solution of the
complex ion, [Ag(NH3)2]+.
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5 Concentrated sulfuric acid
When a few drops of concentrated sulfuric acid (H2SO4) are added to a solid halide theobserved reaction products may be used to identify the particular halide ion present.
This is a potentially hazardous reaction.
It must be carried out on a small scale and in a fume cupboard.
The products in brackets will not be observed since they are colourless gases.
ORGANIC COMPOUNDS
1 Solubility
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2 Chemical tests
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3 Ignition
Organic reactions
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Phenol