Preparation of halogens

Laboratory Preparation of Chlorine

Any of the methods listed below, can be used to prepare chlorine in the laboratory.

From concentrated hydrochloric acid by oxidation

The apparatus is set up as shown in figure 14.3.

Chlorine can be prepared by removing the hydrogen from hydrochloric acid using an oxidizing agent. Any oxidising agent such as manganese dioxide, lead dioxide, trilead tetroxide, potassium permanganate or potassium dichromate can be used. Firstly, the oxidising agents are taken in the round bottomed flask. Concentrated hydrochloric acid is then added through a thistle funnel. This mixture is then heated. The oxygen of the oxidizing agents combines with the hydrogen of the hydrochloric acid leaving behind chlorine i.e. hydrogen is removed from hydrochloric acid. The metallic ions of the oxidising agents combine with part of chlorine to form the respective chlorides.

Remember:-

No heating is required in when potassium permanganate is used as an oxidizing agent in the above method of preparing chlorine.

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• From sodium chloride, manganese dioxide and concentrated sulphuric acid

The apparatus is set up as shown in figure 14.4.

Chlorine can be prepared by the action of hot concentrated sulphuric acid on a mixture of a chloride salt and an oxidising agent. Sodium chloride, being the cheapest and the most easily available chloride salt, is used along with manganese dioxide as the oxidising agent.

A mixture of almost equal quantity of sodium chloride and manganese dioxide is taken in a round bottomed flask. Concentrated sulphuric acid is then poured through the thistle funnel. The reaction takes place in two stages. In the first stage sulphuric acid reacts with the chloride to form hydrochloric acid. In the second stage the hydrochloric acid so formed combines with the oxidising agent to liberate chlorine.

(1)

(2) Manganese (II) chloride, formed during the reaction, reacts with sulphuric acid to form manganese (II) sulphate and hydrochloric acid as under:

From Bleaching Powder [Ca(OCl)Cl]

The apparatus is set up as shown in figure 14.5.

Bleaching powder is taken in a round bottom flask. Any dilute mineral acid is poured through the thistle funnel. Chlorine can be prepared by dropping any acid on bleaching powder.

The chorine produced from hydrochloric acid by the above method, is passed through two wash bottles. The first wash bottle contains water, to remove traces of hydrogen chloride gas from chlorine. The second wash bottle contains concentrated sulphuric acid to dry the gas.

Industrial Preparation of Chlorine

Chlorine is mostly obtained as a by-product during the manufacture of caustic soda, by the electrolysis of brine or molten sodium chloride. Hence chlorine is rather prepared by cheap methods. During this electrolysis, chlorine is liberated at the anode.

Remember:-

As chlorine is denser than air it is collected by the upward displacement of air.