Question

How is hydrogen prepared in the laboratory? How is the gas collected ? What are the precautions to be adopted during the preparations of the gas?

Answer 1

(i) Principle: Hydrogen is prepared in the laboratory by the action of dilute sulphuric acid on impure granulated zinc at ordinary temperature. As a result of the reaction zinc sulphate and hydrogen are produced.
Chemical reaction:$\mathrm{Zn}\left(s\right)+H_2{\mathrm{SO}}_4\left(l\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+H_2\left(g\right)\uparrow$

(ii) Precautions: During the preparation and collection of hydrogen by this process, some precautions are to be taken:

• The end of the thistle funnel and granulated Zinc must remain under the surface of the liquid in the Woulff's bottle.

• Gas-tight corks must be used. The apparatus must be completely air tight. To test for it, air is blown from the mouth through the free end of the delivery tube when water rises up the stem of the thistle funnel. The end of the delivery tube is then closed by the finger. If the apparatus is fully air tight, the surface of water in the funnel will remain stationary and will show no tendency to descend gradually.

• Before collecting hydrogen, the gas must be allowed to escape for some time so that whole air inside the apparatus is driven out.

• The gas-jar used in collecting the gas must not contain any air bubble.

• No flame should be kept nearby as hydrogen explodes violently with air if a frame reaches the mixture.