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 withthe literature.

Adding drying agent To remove waterEg:- Anhydrous MgSO4 or

Anhydrous Na2SO4 or Anhydrous CaCl2

Solution becomes clear

Adding anti-Bumping granules To ensure even boiling of liquidsConstant 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 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 reaction to be found)(Note: plan your answer according to the situation)

The pressure in the funnel increases due to the formation of CO2

Precautions1) Fume cupboard2) Wear gloves

3) Water bath

i) Toxicii) Harmful by skin

absorption / corrosive acid

iii) Flammable liquidNitrous 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.

I2 Vs Na2S2O3 titration 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 = Experimental valueTheoreticalvalue

x 100%

Usually percentage yield is less than 100% because of,i) Transfer lossesii) Escape of volatile liquidsiii) 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

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 often with the evolution of oxygen, although this may be unreliable.

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

then only the sulfate will form as a precipitate.

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 hydroxideto 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 byadding dilute nitric acid before the aqueous silver nitrate.

Silver halides which dissolve in ammonia do so to form a colourless solution of thecomplex 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.

ORGANIC COMPOUNDS

1 Solubility

2 Chemical tests

3 Ignition

Organic reactions

Phenol