(a)(i) What is meant by hydrocarbons? (ii) A hydrocarbon consists of 92.3% carbon. If it's vapour density is 39, determine its molecular formula. (H = 1; C ...

Answer Details

(a)(i) Hydrocarbons are compounds that contain only carbon and hydrogen.

(ii) Molecular formula

Carbon = 92.3%, so hydrogen = 100 - 92.3 = 7.7%.

C: \( \dfrac{92.3}{12} = 7.69 \)    H: \( \dfrac{7.7}{1} = 7.70 \)   ratio \(1:1\), so empirical formula = CH (mass 13).

Molar mass \( = 2 \times \text{vapour density} = 2 \times 39 = 78 \)

\( n = \dfrac{78}{13} = 6 \), so molecular formula \( = \mathbf{C_6H_6} \) (benzene).

(b)(i) Ethanol from cassava

Cassava is rich in starch. Grate the tubers and mix with water; hydrolyse the starch to glucose (using malt/diastase enzyme or by boiling with dilute acid then neutralising). Cool, add yeast and allow to ferment anaerobically for a few days at about 35°C; the enzyme zymase in yeast converts glucose to ethanol and carbon dioxide. Finally distil (fractional distillation) to concentrate the ethanol.

\[ C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2 \]

(ii) Reagents for oxidising ethanol to ethanoic acid: acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄); acidified potassium tetraoxomanganate(VII) (KMnO₄/H₂SO₄).

(c) Names of processes

• (i) Conversion of alkanols to alkanoates: esterification.
• (ii) Breakdown of proteins to amino acids: hydrolysis.
• (iii) Conversion of oils to fats: hydrogenation.
• (iv) Alkaline hydrolysis of fats and oils: saponification.