Modern inorganic chemistry

266 GROUP VI

which almost certainly contain Se 8 ring structures. Selenium, how-

ever, also has a grey allotrope which is metallic in appearance. It is

stable at room temperature and is made up of extended spiral chains

of selenium atoms.

TELLURIUM

Only one form of tellurium is known with certainty. It has a silvery-

white metallic appearance.

CHEMICAL REACTIVITY

1. REACTIONS WITH AIR

At high temperatures oxygen reacts with the nitrogen in the air form-

ing small amounts of nitrogen oxide (p. 210). Sulphur burns with a

blue flame when heated in air to form sulphur dioxide SO 2 , and a

little sulphur trioxide SO 3. Selenium and tellurium also burn with a

blue flame when heated in air, but form only their dioxides, SeO 2

and TeO?.

2. REACTIONS WITH ACIDS

Oxygen

Oxygen is unaffected by aqueous acids unless they have powerful

reducing properties when the acid is oxidised*. For example

2HNO 2 + O 2 -> 2HNO 3

4HI + O 2 -> 2I 2 + 2H 2 O

However, hydrogen chloride gas, obtained as a by-product in

chlorination reactions, is commercially converted to chlorine by

passing the hydrogen chloride mixed with air over a copper catalyst

at a temperature of 600-670K when the following reaction occurs:

4HC1 + O 2 ^ 2H 2 O 4- 2C1 2

• The redox half-reaction O 2 (g) + 4H 3 O+ + 4e" - 6H 2 O has E = + 1.23 V
  suggesting that oxygen is a good oxidising agent in acid solution. However, when
  oxygen gas is passed into a solution where oxidation might be expected, the reaction
  is often too slow to be observed —there is an adverse kinetic factor.