Hydrogen

Occurrence

In water, organic compounds, etc.

Preparation

Lab method

1. Action of active metals on water.

2Na + 2H₂O → 2NaOH + H_2↑

2K + 2H₂O → 2KOH + H_2↑

Ca + 2H₂O → Ca(OH)₂ + H_2↑

2. Action of less active metals on steam –

Zn + H₂O → ZnO + H_2↑

Mg + H₂O → MgO + H_2↑

Over red hot iron, steam forms hydrogen

3Fe + 4 H₂O → Fe₃O₄ + 4H_2↑

3. From acids

Zn + H₂SO₄ → ZnSO₄ + H_2↑

Zn + 2HCl → ZnCl₂ + H_2↑

                    The intensity with which metals displace hydrogen from acids depends on a series

K, Ca, Na,	Mg, Al, Zn, Fe,	Ni, Sn, Pb,	H,	Cu, Hg, Ag, Au, Pt
Displace hydrogen  violently	Displace hydrogen  vigorously	Displace hydrogen quietly		Do not displace hydrogen

4. From alkalies

Zn + 2NaOH → Na₂ZnO₂ + H_2↑

Zn + 2KOH → K₂ZnO₂ + H_2↑

2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H_2↑

2Al + 2KOH + 2H₂O → 2KAlO₂ + 3H_2↑

The gas is dried by CaCl₂

Industrial method

1. Steam is passed over heated coke to produce water gas.

               C + H₂O → CO + H_2↑

Water gas and excess steam are passed over a catalyst, iron (III) oxide.

               CO + H₂ + H₂O → CO₂ + 2H_2↑

Carbon dioxide is absorbed by caustic soda or potash to obtain hydrogen.

2. If water is electrolyzed, hydrogen can be collected at the cathode.

Properties

Physical

1. It is a colorless, odorless gas.
2. It is 0.069 times as heavy as air.
3. It is almost insoluble in water.
4. B.P = -253°C, M.P. = -259.4°C.
5. Noble metals readily absorb hydrogen when finely divided.

Chemical

1. It is combustible, but does not support combustion.

2H₂ + O₂ → 2H₂O

2. Hydrogen and chlorine react explosively in direct sunlight.

H₂ + Cl₂ → 2HCl

3. If hydrogen is passed through boiling sulfur, it forms hydrogen sulfide

H₂ + S → H₂S

4. Combines with nitrogen in the presence of catalysts to form ammonia.

N₂ + 3H₂ → 2NH₃

5. It reduces metal oxides

H₂ + CuO → H₂O + Cu

H₂ + PbO → H₂O + Pb

Fe₂O₃+ 3H₂ → 3H₂O + 2Fe

Uses

1. As a fuel – in rockets, in torches. In electric arcs, it absorbs energy to split to atoms, and outside the arc it forms molecules releasing energy.
2. In self-lighting jets – hydrogen is absorbed by finely divided metals, and heat is released, which is used to ignite fuel.
3. In meteorological balloons.
4. In the presence of catalysts, it is used to make petrol from coal.
5. In the extraction of metals

WO₃ + 3H₂ → W + 3H₂O

6. Hydrogenation of oils – If heated with oils to 150° - 200°C at 5 atmos. pressure in the presence of nickel, oils are hydrogenated to a semi-solid state.
7. To make ammonia and hydrochloric acid by the direct combination of elements.