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.

Quicklime

Coal and limestone are heated in a furnace. Quicklime is formed, but it contains ash. To get ash-free quicklime, limestone is heated with producer gas.

Uses

1.        To make slaked lime and limewater.
CaO + H₂O → Ca(OH)₂

2.        To dry ammonia.

3.        To produce limelight, an intense white light created by heated CaO.

4.        To make mortar. A thick paste of slaked lime, 3-4 parts sand, is used as mortar. On drying, CaO remains, which reacts with carbon dioxide to form calcium carbonate.
Ca(OH)₂ → CaO + H₂O
CaO + CO₂ → CaCO₃

Oxygen

Occurrence

1.      In air, 21% by volume.

2.      In water, 89%, and dissolved in it.

3.      In metal and non-metal oxides.

Preparation

Lab method

1.      Heating of metal oxides

2Ag₂O Δ→ 4Ag + O_2↑

2HgO Δ → 2Hg + O_2↑

2Pb₃O₄ Δ → 6PbO + O_2↑

2PbO₂ Δ → 2PbO + O_2↑

2.      Heating hydrogen peroxide
2H₂O₂ Δ (MnO₂) → 2H₂O + O_2↑

3.      Heating of potassium chlorate
2KClO₃ Δ → 2KCl + 3O₂

4.      Reaction of water and sodium peroxide.
2Na₂O₂ + H₂O → 4NaOH + O_2↑

Industrial

1.      From air – dust is removed by filters, CO₂ is removed by potassium hydroxide and water is removed by a drying agent. Air is liquefied by compression and cooling. When warmed, liquid nitrogen boils away first (-196°C) and liquid oxygen next (-183°C). It is stored in cylinders.

2.      From water – the electrolysis of water yields oxygen at the anode.

Properties

Physical

1.      It is a colorless, odorless, tasteless, neutral gas.

2.      It is slightly soluble in water, and slightly heavier than air.

3.      It boils at -183°C.

Chemical

1. It does not burn, but supports combustion.
2. It forms oxides.
1. Acidic oxides – or acid anhydrides, combine with water to form acids.

S + O₂ → SO₂ then SO₂ + H₂O → H₂SO₃

C + O₂ → CO₂ then CO₂ + H₂O → H₂CO₃

4P + 5O₂ → 2P₂O₅ then P₂O₅ + 3H₂O → 2H₃PO₄

These react with bases:

SO₂ + NaOH → Na₂SO₃ + H₂O

CO₂ + NaOH → Na₂CO₃ + H₂O

P₂O₅ + 6NaOH → 2Na₃PO₄ + 3H₂O

2. Basic oxides – are oxides of metals – they react with acids

MgO + H₂SO₄ → MgSO₄ + H₂O

Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O

CaO + H₂SO₄ → CaSO₄ + H₂O

Some basic oxides react with water to form alkalis

K₂O + H₂O → 2KOH

Na₂O + H₂O → 2NaOH

CaO + H₂O → Ca(OH)₂

3. Neutral oxides – for example, H₂O, CO, NO, etc.
4. Amphoteric oxides – act as both acids and bases.

ZnO + H₂SO₄ → ZnSO₄ + H₂O

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

3.  Oxygen oxidizes lower oxides                    

2NO + O_{2 (Pt)}→ 2NO₂
2SO₂ +  O₂ → 2SO₃
2CO + O₂ → 2CO₂

4.  Oxidation

2H₂S + 3O₂ → 2H₂O + 2SO₂

CH₄ + 2O₂ → CO₂ + 2H₂O

2ZnS + 3O₂ → 2ZnO + 2SO₂

4NH₃ + 5O_{2 (Pt, 800°C)} → 4NO + 6H₂O

Confirmatory Tests

1. It rekindles a glowing splint
2. In contact with colorless nitric oxide, it produces reddish-brown fumes of nitrogen dioxide
3. It is absorbed by a solution of pyrogallol, which turns brown

Uses

1. For respiration
2. In medicine
3. In oxy-acetylene torch
4. In iron and steel production, to remove impurities
5. In explosives
6. In rockets, liquid oxygen is used for combustion