chemistry practical tips

8/11/2019 Chemistry Practical Tips

1/18

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


2/18

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.


3/18

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


4/18

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.


5/18


6/18


7/18

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.


8/18

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.


9/18

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]+.


10/18

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


11/18

2 Chemical tests


12/18

3 Ignition

Organic reactions


13/18


14/18


15/18


16/18


17/18


18/18

Phenol

chemistry practical tips

Documents