(a)(i) Give the names of two allotropes of sulphur.
(ii) State and explain what is observed when hydrogen sulphide is bubbled through acidified potassium tetraoxomanganate (VII) solution
(iii) List one product of the reaction of sulphur (IV) oxide with hydrogen sulphide
(b)(i) What are the raw materials for the manufacture of tetraoxosulphate (VI) acid by the contact process?
(ii) Write an equation for the reaction that requires a catalyst in the contact process and state the catalyst used.
(iii) State the observation and the product formed when concentrated H\(_2\)SO\(_4\) reacts with each of the following:
I. Copper turnings Ii. A cube of sugar
(c)(i) Give three uses of sodium trioxocarbonate (IV).
(ii) What name is given to reactions of the following type?:
exposure to air
Na\(_2\)CO\(_3\) + 10H\(_2\)O\(_{(s)}\) \(\to\) Na\(_2\)CO\(_3\); H\(_2\)O\(_{(S)}\) + 9H\(_2\)O
(iii) Calculate the solubility of Na\(_2\)CO\(_3\) at 25°C, if 20 cm\(^3\) of its saturated solution at that temperature gave 1.75g of the anhydrous salt. [C = 12, O = 16, Na = 23].
Explanation
(a)(i) Allotropes of sulphur prismatic / monoclinic; Rhombic sulphur. Also plastic/amorphous sulphurs.
(ii) When H\(_2\)S is bubbled through acidified KMnO\(_4\), the solution turns from purple to colourless / is decolourised with yellow deposit of sulphur. H\(_2\)S is a reducing agent. It reduces manganese from oxidation state of + 7 to +2 and itself oxidized to sulphur.
(iii) The products are sulphur and water
(b)(i) The raw materials for the manufacture of H\(_2\)SO\(_4\) are sulphur and air or oxygen.
(ii) The equation of the reaction that requires a catalyst in the contact process is 2SO\(_2\) + O\(_2\) -> 2SO\(_3\). The catalyst used is V\(_2\)O\(_5\) (vanadium (V) oxide)
(iii) with copper turnings a blue solution and a gas (with irritating smell) are observed. Products are CuSO\(_4\), SO\(_2\) and water - with a cube of sugar, a black mass of carbon is obtained.
(c)(i) Uses of Na\(_2\)CO\(_3\)
- As a water softener
- In glass / paper manufacture
- As analytical reagent, or specific reaction.
(ii)
Exposure
Na\(_2\)CO\(_3\).10H\(_2\)O \(\to\) Na\(_2\)CO\(_3\) . H\(_2\)O + 9H\(_2\)O. This equation illustrates efflorescence in air
in air
(iii) Solubility calculation. Mass of salt per 20cm\(^3\) of saturated solution = 1.75g 1000cm\(^3\) of saturated solution contains
\(\frac{1000 x 1.75g}{20}\) = 87.5g
molar mass of Na\(_2\)CO\(_3\) = (2 x 23) + 12 + (3 x 16) = 106g
Solubility of Na\(_2\)CO\(_3\) = 87.5
= 0.825 mol /dm\(^{-3}\)