SS2 Chemistry Lesson Note on Physical and Chemical Properties of Hydrogen

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PHYSICAL AND CHEMICAL PROPERTIES OF HYDROGEN

PHYSICAL PROPERTIES

1. It is a colourless, odourless and tasteless gas which burns in air with a high pitch sound.
2. It is combustible and when it burns in the absence of gas, it burns quietly with pale blue flame.
3. It is the highest known gas.
4. It is 14.4 times less than air.
5. It exists ad diatomic molecule [H₂ ]
6. At high pressure, hydrogen can be liquefied at a critical temperature of -235
7. It is neutral to litmus and is insoluble in water
8. It does not support combustion.

CHEMICAL PROPERTIES

The chemical reactions of hydrogen arise from the fact that it can donate its single electron to form a positive ion [H⁺]; it can accept electron [to give a doublet structure] to form a negative hydride ion [H^–] and it can share electron with another atom to form a covalent molecule.

1. Hydrides of hydrogen: An important reaction of hydrogen is the formation of hydride with metals and non-metals.

• Metallic hydrides: Hydrogen is slightly electro negative and forms ionic hydrides e.g. NaH (Na⁺ + H^-) with alkali metals which are strongly electro positive. These hydrides contain the hydride ion H^–

2Li + H₂   →  2LiH (Lithium hydride)

Ca + H_{2    →}    CaH₂ [Calcium hydride]

The hydrides react with water with the liberation of hydrogen.

CaH_2(s)  + 2H₂O_{(l)        →}   Ca[OH]_2(aq) + 2H_2(g)

(ii)   Non-metallic hydrides: Hydrogen forms mainly covalent compounds with less electropositive metals [e.g. aluminium] and with non-metals. These are covalent hydrides. The reaction is spontaneous as shown in the reaction with fluorine and chlorine.

F_2(g) + H_{2(g)      →}      2HF    ——————-(1)

Cl_2(g) + H_2(g)  _→   2HCl _(g) —————–(2)

The reaction with chlorine is explosive in the presence of sunlight. Reactions with oxygen, sulphur and nitrogen require the use of catalyst.

Hydrogen reacts with molten sulphur to form hydrogen sulphide which smells like a rotten egg and it also reacts with nitrogen to form ammonia.

H_2(g) + S_{(s)  →}      H₂S_(g) ————(3)

3H_2(g) + N_{2(g)      →}        2NH_{3(g)———(}4)

1. Hydrogen is strong reducing agent when it is passed over the heated oxides as shown below.

PbO_(s) + H_{2(g)    →}          Pb_(s) + H₂O_(g)

FeO_4(s) + 4H_2(g)      ⇌          3Fe_(s) + 4H₂O_(g)

The reaction of tri-iron tetraoxide and hydrogen is a reversible reaction, other reduction reaction are

1. CuO_(s) + H_{2(g) →}   Cu_(s) + H₂O_(g)
2. ZnO_(s) + H_{2(g) →} Zn_(s) + H₂O_(g)

• Fe₂O₃ + 3H_{2(g) →} 2Fe(s) + 3H₂O_(g)

2. Hydrogen burns in air with pop sound and with formation of water [steam] as shown in the equation below.

2H_2(g) + O_{2(g)     →}   2H₂O_(g)

The reaction is explosive and gives out a large amount heat.

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