(a) What is the structure of:
(i) graphite:
(ii) diamond
(ii) Explain briefly why diamond is hard and a non-conductor of electricity while graphite is soft and an electrical conductor,
(b)i. State what is achieved at each of the following stages in the purification of town water supply:
- aeration;
- screening
- sedimentation.
(ii) Name two substances responsible for hardness in water.
(iii) State two methods for the removal of hardness in water.
(iv) Give one disadvantage of hard water
(c)i). Describe briefly the extraction of tin from its ore.
(ii) Write a balanced chemical equation for the reaction.
(iii) Write an equation for the reaction of tin with:
- oxygen;
- chlorine.
(a)(i) Structures
- Graphite: a giant covalent structure made of flat layers of carbon atoms. Within each layer every carbon is covalently bonded to three others in a hexagonal (honeycomb) arrangement, leaving one delocalized electron per atom. The layers are held together only by weak van der Waals forces.
- Diamond: a giant covalent structure in which every carbon atom is covalently bonded tetrahedrally to four other carbon atoms, forming a rigid three-dimensional network.
(a)(iii) Hardness and conductivity
Diamond is hard because its carbon atoms are held in a rigid three-dimensional network of strong covalent bonds, and it is a non-conductor because all four valence electrons of each carbon are used in bonding, leaving no free electrons to carry charge. Graphite is soft because its layers are held by weak forces and can slide over one another, and it conducts electricity because each carbon uses only three electrons in bonding, leaving one delocalized electron per atom free to move and carry charge.
(b)(i) Water purification stages
- Aeration: water is sprayed into air to add dissolved oxygen and drive off dissolved gases and unpleasant odours; it also oxidizes dissolved iron and manganese.
- Screening: large floating and suspended solids (leaves, sticks, debris) are removed by passing water through screens/gratings.
- Sedimentation: the water is left to stand (often with a coagulant such as alum) so that fine suspended particles settle out to the bottom.
(b)(ii) Substances causing hardness
Dissolved calcium and magnesium salts, that is, the hydrogencarbonates and sulphates of calcium and magnesium (\(Ca^{2+}\) and \(Mg^{2+}\) ions).
(b)(iii) Methods of removing hardness
Boiling (removes temporary hardness), adding washing soda (\(Na_2CO_3\)), ion exchange, or distillation.
(b)(iv) One disadvantage of hard water
It wastes soap by forming an insoluble scum, and it deposits scale (fur) in kettles, boilers and pipes.
(c)(i) Extraction of tin
Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is concentrated by washing and froth flotation to remove earthy impurities, then roasted in air to remove sulphur and arsenic. The concentrated oxide is then reduced with carbon (coke) in a reverberatory or blast furnace, and the molten tin is run off and refined.
(c)(ii) Equation for the reduction
\[SnO_{2(s)} + 2C_{(s)} \to Sn_{(l)} + 2CO_{(g)}\]
(c)(iii) Reactions of tin
\[\text{With oxygen: } Sn_{(s)} + O_{2(g)} \to SnO_{2(s)}\]\[\text{With chlorine: } Sn_{(s)} + 2Cl_{2(g)} \to SnCl_{4(l)}\]
(a)(i) Structures
- Graphite: a giant covalent structure made of flat layers of carbon atoms. Within each layer every carbon is covalently bonded to three others in a hexagonal (honeycomb) arrangement, leaving one delocalized electron per atom. The layers are held together only by weak van der Waals forces.
- Diamond: a giant covalent structure in which every carbon atom is covalently bonded tetrahedrally to four other carbon atoms, forming a rigid three-dimensional network.
(a)(iii) Hardness and conductivity
Diamond is hard because its carbon atoms are held in a rigid three-dimensional network of strong covalent bonds, and it is a non-conductor because all four valence electrons of each carbon are used in bonding, leaving no free electrons to carry charge. Graphite is soft because its layers are held by weak forces and can slide over one another, and it conducts electricity because each carbon uses only three electrons in bonding, leaving one delocalized electron per atom free to move and carry charge.
(b)(i) Water purification stages
- Aeration: water is sprayed into air to add dissolved oxygen and drive off dissolved gases and unpleasant odours; it also oxidizes dissolved iron and manganese.
- Screening: large floating and suspended solids (leaves, sticks, debris) are removed by passing water through screens/gratings.
- Sedimentation: the water is left to stand (often with a coagulant such as alum) so that fine suspended particles settle out to the bottom.
(b)(ii) Substances causing hardness
Dissolved calcium and magnesium salts, that is, the hydrogencarbonates and sulphates of calcium and magnesium (\(Ca^{2+}\) and \(Mg^{2+}\) ions).
(b)(iii) Methods of removing hardness
Boiling (removes temporary hardness), adding washing soda (\(Na_2CO_3\)), ion exchange, or distillation.
(b)(iv) One disadvantage of hard water
It wastes soap by forming an insoluble scum, and it deposits scale (fur) in kettles, boilers and pipes.
(c)(i) Extraction of tin
Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is concentrated by washing and froth flotation to remove earthy impurities, then roasted in air to remove sulphur and arsenic. The concentrated oxide is then reduced with carbon (coke) in a reverberatory or blast furnace, and the molten tin is run off and refined.
(c)(ii) Equation for the reduction
\[SnO_{2(s)} + 2C_{(s)} \to Sn_{(l)} + 2CO_{(g)}\]
(c)(iii) Reactions of tin
\[\text{With oxygen: } Sn_{(s)} + O_{2(g)} \to SnO_{2(s)}\]\[\text{With chlorine: } Sn_{(s)} + 2Cl_{2(g)} \to SnCl_{4(l)}\]
