Chemistry Practical Tips

PRACTICAL TIPS PROCEDURE Reflux

Reflux for 30 min Advantages of suction filtration over normal filtration. Aspects of the melting temperature determination that would indicate the crystals were pure. Adding drying agent

Adding anti-Bumping granules Constant stirring

A solution is added drop wise with cooling. Washing with NaHCO3

REASON

RESULT

 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. Reaction is slow / High activation energy  Faster  Dries solid  Melting temperature should be‘sharp’.  It should be consistent with the literature. To remove water Eg:- Anhydrous MgSO4 or Anhydrous Na2SO4 or Anhydrous CaCl2 To ensure even boiling of liquids 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) Reaction is exothermic / To avoid the temperature rising too much 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)

Solution becomes clear

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

Precautions 1) Fume cupboard 2) Wear gloves

3) Water bath Nitrous acid (HNO2) is made in the reaction mixture rather than being obtained from a chemical supplier. When Nitrous acid is used, the temperature must not be lower than 0°C nor higher than 10°C. 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.

i) ii)

Toxic Harmful by skin absorption / corrosive acid iii) Flammable liquid HNO2 is unstable at room temperature and hence, cannot be stored. Below 0oC, reaction is too slow and above 10oC, diazonium compound decomposes . 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) The solid is washed with a small amount of cold solvent.

4) To removes soluble impurities.

5) The solid is dried in a desiccator

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.

 An exact volume of solution is needed.

 More accurate

 To measure more than 25cm3, measuring cylinder is used.

 Only an approximate volume / excess is needed.

 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 =

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

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 hotter Parts (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 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.

ORGANIC COMPOUNDS 1 Solubility

2 Chemical tests

3 Ignition

Organic reactions

Phenol