1. CO2 and SiO2 are oxides of Group IV elements.
(i) Account for the fact that CO2 is a gas while SiO2 is a high melting solid under room temperature and atmospheric pressure.
(ii) Give the hybridization state of carbon in CO2 and of silicon in SiO2.

(02-I 3 / 4 marks)

2. Account for each of the following facts:
(i) There is a significant difference in the melting point between silicon (m.p.1410°C) and lead (m.p. 328°C).
(ii) Silicon forms one oxide with formula SiO2, while lead forms three oxides with formulae PbO, PbO2 and Pb3O4.
(93-II / 5 marks)

3. Carbon, germanium and lead are elements in Group IV of the Periodic Table.
(a) Diamond and graphite are allotropes of carbon.
(i) Draw their three-dimensional structures.
(ii) With reference to their structures, compare the hardness of       diamond and graphite.
(b) Germanium has the same structure as diamond. Which of these substances has a higher melting point? Explain.
(c) Suggest why the density of lead (11.3 g cm-3) is much higher than that of germanium (5.3 g cm-3)

(03-AS Q. 7 / 7 marks)


5. Carbon, silicon and lead are group IV elements in the Periodic Table. They form different oxides.
(i) Suggest a chemical test to distinguish between carbon monoxide and carbon dioxide.
(ii) At room temperature and pressure, carbon dioxide is a gas whereas silicon dioxide exists as a solid. Explain this difference.
(iii) A reaction occurs when lead(IV) oxide is heated. State the observation and write a balanced equation for the reaction. Briefly explain why silicon(IV) oxide does not produce a similar reaction.
(95-AS 12 / 8 marks)



1. (i) CO2 exists as simple molecules and the intermolecular attraction is weak van der Waals’ forces 1
SiO2 has a giant covalent network structure, attraction between Si and O atoms is strong covalent bond 1
(ii) C in CO2 : sp hybridized; Si in SiO2 : sp3 hybridized

2. (i) Si has a giant covalent network structure
melting involves breaking a large no. of strong Si-Si covalent bonds between atoms 1Pb has a closely packed metallic structure melting involves breaking a small proportion of metallic bonds between Pb2+ and delocalized electrons 1
(ii) Si forms compounds with O.S. = +4 1 whereas Pb cpds exist in+2 and +4 1 For Pb, more cpds are formed in O.S. +2 rather than +4 (Pb3O4 ≡ 2 PbO⋅PbO2)

3. (a) diamond is harder than graphite

In graphite, hexagonal layers of C atoms are held by weak van der Waals’ forces
 the layers can easily slip over each other

In diamond, each C atom is bonded strongly to other C atoms by strong covalent bonds
 relative motion of atoms is restricted
(b) Diamond. C-C bond is stronger/shorter than Ge-Ge bond
(c) relative atomic mass of Pb is much higher than that of Ge atoms in lead is more closely packed

4. (i) Treat the two gases with citrated chicken blood: CO turns blood colour to cherry red; CO2 no observable change
or, Burn each of the gases: CO burns with a blue flame; CO2 does not burn
or, Pass each gas into NaOH/KOH: CO2 dissolves (therefore acidic); CO does not
or, Pass each gas into limewater: CO2 gives a milky solution; CO gives no observable change
(ii) CO2: exists as discrete molecules. Intermolecular attraction is weak van der Waals’ force SiO2: exists as giant covalent network with strong covalent C-C bonds between C atoms
(iii) (brown) PbO2 decomposes to give a solid PbO which is brown/orange when hot and yellow when cold
2 PbO2(s) → 2 PbO(s) + O2(g)
descending group IV, the +2 O.S. becomes more stable  SiO2 is thermally more stable than SiO

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