(a)(i) Give three characteristics of homologous series. (ii) Name two groups of compounds which form such a series. (b) A saturated organic compound A conta...

(a)(i) Characteristics of a homologous series

• Members have the same general formula.
• Successive members differ by a \(\ce{-CH2-}\) unit.
• They have the same functional group and therefore similar chemical properties.
• They show a gradual change in physical properties, such as boiling point and density, as molecular mass increases.

(ii) Examples are alkanes and alkanols (alcohols).

(b) Compound A is ethanol, because a two-carbon alcohol reacts with ethanoic acid to form a sweet-smelling ester.

(i) The functional group in A is the hydroxyl group, \(\ce{-OH}\).

(ii) Structure of A, ethanol

(iii)

\[\ce{CH3COOH + CH3CH2OH <=>[conc. H2SO4][heat] CH3COOCH2CH3 + H2O}\]

(iv) B is ethyl ethanoate.

(c)(i)

\[\ce{HC#CH + 2Br2 -> CHBr2CHBr2}\]

(ii) The IUPAC name of the product is 1,1,2,2-tetrabromoethane.

(iii) Conditions for cracking

• High temperature, usually about \(500\text{ to }700\,^\circ\mathrm{C}\).
• Use of a suitable catalyst, such as silica-alumina or zeolite.

(d)(i) Preparation of ethanol from starch

1. Warm starch with malt containing the enzyme diastase. The starch is hydrolysed to maltose:
   \[\ce{(C6H10O5)_{n} + \tfrac{n}{2}H2O ->[diastase] \tfrac{n}{2}C12H22O11}\]
2. Add maltase to convert maltose to glucose:
   \[\ce{C12H22O11 + H2O ->[maltase] 2C6H12O6}\]
3. Add yeast and keep the mixture warm, about \(30\,^{\circ}\mathrm{C}\), under anaerobic conditions. Zymase in the yeast ferments glucose to ethanol and carbon(IV) oxide:
   \[\ce{C6H12O6 ->[zymase] 2C2H5OH + 2CO2}\]
4. Distil the fermented liquid to obtain ethanol.

(ii) Two properties of starch

• It is a white, tasteless solid and is insoluble in cold water.
• When warmed with water, it forms a colloidal paste or solution.

(iii) Starch does not reduce Fehling's solution because it has no free reducing carbonyl group.

(iv) Add iodine solution to the sample. A blue-black colour confirms the presence of starch.