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Laboratory preparation of hydrogen gas.

Answer Text:
Hydrogen
- An element that does not exist freely in nature;
- Generally exists in compounds such as water, sugars, fuels etc;
Laboratory preparation of hydrogen gas.
Note:
- Hydrogen gas is generally prepared by the reaction between dilute acids and metals;
- Most suitable acids are dilute hydrochloric acid and dilute sulphuric (VI) acid;
- Most suitable metal is zinc metal;
Apparatus:

Procedure:
- Zinc granules are added to dilute sulphuric (VI) acid;
- Small amounts of copper (II) sulphate are added to the zinc – acid mixture;
Reason: To act as a catalyst hence speed up the reaction;
- The resultant colourless gas is collected over water;
Reason: The gas is insoluble in water;
- If the gas is required dry, the gas is passed through concentrated sulphuric (VI) acid or a U-tube containing calcium chloride;
Diagrams for drying the
gas:
Zinc granules
- The dry gas is collected by upward delivery (downward displacement of air);
Reason: It is less dense than air (note that hydrogen is the lightest gas known);
Note:
- Nitric (V) acid is not used in preparation of hydrogen gas; except very dilute nitric (V) acid and magnesium
Reason: Nitric acid is a strong oxidizing agent hence the hydrogen formed is immediately oxidized to water
- Potassium, sodium, lithium and calcium are not used in laboratory preparation of hydrogen gas;
Reason: They react explosively with acids;
- Magnesium is not usually used for laboratory preparation of hydrogen;
Reason: It is expensive;
- Prior to using it for preparation of hydrogen; aluminium should be washed with concentrated hydrochloric acid;
Reason: To remove the protective oxide layer that usually forms on the aluminium surface on its exposure to air;
- Hydrogen gas produced from iron metal tends to have a foul smell;
Reason: Iron gives a mixture of gases due to impurities in the iron; the foul smell is usually due to production of hydrogen sulphide that results from sulphide impurities in the iron metal;
Reaction equations:
I. Iron + Sulphuric (VI) acid → Iron (II) sulphate + Hydrogen gas;
#Fe(s) + H_2SO_4(aq) → FeSO_4(aq) + H_2(g)#
II. Iron (II) sulphide + Sulphuric (VI) acid → Iron (II) sulphate + Hydrogen sulphide gas;
#FeS(s) + H_2SO_4(aq) → FeSO_4(aq) + H_2S(g)#;
Other sources of hydrogen.
- Cracking of alkanes;
- Fractional distillation of petroleum;