CBSE Class 10 Chemistry Laboratory Preparation of Gases and Acids Notes Set 15

Preparation of Chlorine, Sulphur Dioxide and Hydrogen Sulphide

Chemical Behaviour Seen in the Experiments

1. Chlorine is a very active non-metal. A candle burns in it with a red smoky flame because the hydrogen of the wax forms HCl and carbon is left over. Powdered antimony, Dutch metal and heated sodium burn in it spontaneously, and yellow phosphorus burns to give its chlorides.
2. Chlorine is displaced in the order Cl, Br, I: chlorine water turns potassium bromide orange-red (bromine) and potassium iodide violet-brown (iodine). It oxidises hydrogen sulphide to yellow sulphur.
3. Chlorine water contains HCl and hypochlorous acid. The unstable HOCl gives nascent oxygen, which bleaches coloured matter permanently by oxidation.
4. Sulphur dioxide puts out a burning candle and does not burn. Burning magnesium continues to burn in it, forming white magnesium oxide and yellow sulphur. It is an oxidising agent towards hydrogen sulphide (yellow sulphur forms) but a reducing agent towards acidified permanganate (purple becomes colourless) and acidified dichromate (orange becomes green). Its bleaching is by reduction and is not permanent.
5. Hydrogen sulphide burns with a blue flame (sulphur in limited air, sulphur dioxide in excess) but puts out a candle. It forms black sulphides with copper, silver, lead and mercury salts, and it is a strong reducing agent: it reduces ferric chloride to pale green ferrous chloride with a yellow deposit of sulphur, and it reduces sulphuric and nitric acids and permanganate and dichromate.

Nitric Acid and Sulphuric Acid

1. Nitric acid is made by heating potassium or sodium nitrate with concentrated sulphuric acid in an all-glass retort and condensing the vapour in a cooled receiver. KNO\( _3 \) + H\( _2 \)SO\( _4 \) \( \rightarrow \) KHSO\( _4 \) + HNO\( _3 \). The yellow colour is dissolved nitrogen dioxide from decomposition.
2. Nitric acid is a strong oxidising agent. Copper gives blue copper nitrate with reddish-brown NO\( _2 \) from the concentrated acid and colourless NO from the dilute acid. Zinc gives mainly N\( _2 \)O with dilute acid, and only magnesium gives hydrogen with very dilute acid. Hot turpentine bursts into flame and leaves carbon.
3. Concentrated sulphuric acid is a dehydrating agent (it chars sugar and paper), a drying agent for gases and, when hot, an oxidising agent: copper, lead, tin and zinc give sulphates with sulphur dioxide (zinc also gives some H\( _2 \)S and S).
4. Dilute sulphuric acid acts on zinc, iron and magnesium to give hydrogen, but not on copper, lead and tin. Barium chloride gives a white precipitate of barium sulphate that does not dissolve in hydrochloric acid, which is the test for sulphate.
5. Always add concentrated sulphuric acid slowly to water, stirring, never water to acid.

Safety Points

1. Chlorine, sulphur dioxide and hydrogen sulphide are poisonous; prepare them in a fume cupboard and never inhale them directly.
2. Wear goggles and handle concentrated acids with care; wash splashes at once with plenty of water.
3. Test for hydrogen by a burning splinter on a small sample, never near a large volume of gas.