PRACTICAL TIPS
PROCEDURE
REASON
RESULT
Reflux As liquid(s) vaporize, they are cooled and condense back to iquid(s)
Used to prevent the escape of volatile substances and thus increases the yield. Reflux for 30 min Reaction is slow / High activation
energy Advantages of suction
filtration over normal filtration.
Faster Dries solid Aspects of the melting
temperature determination that would indicate the crystals
were pure.
Melting temperature should be‘sharp’.
It should be consistent with the literature.
Adding drying agent To remove water Eg:- Anhydrous MgSO4 or
Anhydrous Na2SO4 or 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 answer according to the situation) A solution is added drop wisewith 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 reaction to be found)
(
Note
: plan your answer according to thesituation)
The pressure in the funnel increases due to
Precautions 1) Fume cupboard 2) Wear gloves
3) Water bath
i) Toxic
ii) Harmful by skin absorption / corrosive acid
iii) Flammable liquid Nitrous acid (HNO2) is made in
the reaction mixture rather than being obtained from a chemical
supplier.
HNO2 is unstable at room
temperature and hence, cannot be stored.
When Nitrous acid is used, the temperature must not be lower than 0°C nor higher than 10°C.
Below 0oC, reaction is too slow and above 10oC, diazonium
compound decomposes . Recrystallization
1) 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 cooled and filtered using a Buchner funnel.
4) The solid is washed with a small amount of cold solvent.
5) The solid is dried in a desiccator
1) To prevent the solid remaining in solution on cooling.
2) To remove insoluble impurities.
Pre-heated funnel is used to prevent crystallization of the solid.
3) Impurities remain soluble and the pure solid
recrystallizes. 4) To removes soluble
impurities.
5) To prevent decomposition. Decomposition could occur if the compound is
heated in an Oven. Volume measurements
To measure 10-25cm3
solution, pipette is used. To measure more than
25cm3, measuring cylinder is used.
An exact volume of solution is needed. Only an approximate
volume / excess is needed.
More accurate Faster/More
convenient.
It is incorrect to use HCl instead of H2SO4 in KMnO4 titrations.
HCl will be oxidized to chlorine by the manganate(VII)
Titre value will be too high.
I
2Vs Na
2S
2O
3titration
In burette - Na2S2O3
In pipette - I2 solution
Indicator – Starch
Starch is added when the solution turns pale yellow.
Starch is not added initially because it forms an insoluble complex so that titre value reduces and hence, the percentage error would be high(er).
When starch is added, solution becomes blue-black which turns colourless at the end point.
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.
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 table below.
2 Flame tests
To carry out a flame test, a clean nichrome wire is used to mix a sample of a solid with one drop of concentrated hydrochloric acid. The wire is held in the hotterParts (blue) of a non-luminous Bunsen flame.
3 Heating
Gases or vapours may be evolved on heating a solid compound.
4 Action of dilute acids
When dilute sulfuric or hydrochloric acid is added to a substance a gas may be evolved or there may be a colour change in the solution.
5 Recognition and identification of common gases
6 Hydrogen peroxide solution
Aqueous hydrogen peroxide (H2O2) can act as both an oxidizing and a reducing agent
7 Tests for oxidizing and reducing agents
PRECIPITATES
1 Barium chloride solution
NOTE:- If dilute hydrochloric acid is added to the anion solution before aqueous barium chloride
2 Sodium hydroxide solution
When dilute sodium hydroxide (NaOH) solution is added to a solution containing a metal ion a precipitate of the insoluble hydroxide, eg Mn(OH)2, is usually formed.
Precipitates which are amphoteric hydroxides will dissolve in excess sodium hydroxide to give a solution containing a complex ion, eg [Cr(OH)6]3−.
3 Ammonia solution
Dilute aqueous ammonia (NH3), when added to a solution containing a cation, will form
the 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 ions in solution. Anions which would interfere with the test (eg carbonate) are removed by adding dilute nitric acid before the aqueous silver nitrate.
Silver halides which dissolve in ammonia do so to form a colourless solution of the complex ion, [Ag(NH3)2]+.
5 Concentrated sulfuric acid
When a few drops of concentrated sulfuric acid (H2SO4) are added to a solid halide the
observed 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.
