A technician prepared two solid compounds for a water-treatment plant. Fig. 1 shows particles in molten potassium chloride and molten silicon carbide. The p...
Question 1 Report
A technician prepared two solid compounds for a water-treatment plant. Fig. 1 shows particles in molten potassium chloride and molten silicon carbide. The plant melts potassium chloride in a furnace, then uses the liquid in an electrolysis process. Silicon carbide is used as a hard lining because it remains solid at the furnace temperature. In the first box, ions can move. In the second box, atoms are held in a network by covalent bonds.
(a) Name the particles that carry charge in molten potassium chloride. [1]
(b) Use Fig. 1 to describe one difference between the structures. [1]
(c) Explain why silicon carbide has a high melting temperature. [2]
(d) What happens to the potassium and chloride ions when the potassium chloride cools and solidifies? [1]
Table 1 shows data collected while a student compares substances used in electrical equipment. The student measures the mass of equal-sized pieces before testing them. Copper is used in cables, graphite is used in some electrodes, and sulfur is stored as a molecular solid. The results help the student relate bonding and structure to conductivity and melting temperature. All tests are carried out at room temperature except the melting temperature measurement.
| substance | mass of piece in g | melting temperature in degrees C | conductivity as a solid |
|---|---|---|---|
| copper | 8.9 | 1085 | high |
| graphite | 2.2 | 3650 | medium |
| sulfur | 2.1 | 115 | zero |
(a) Name the substance with metallic bonding. [1]
(b) Use Table 1 to state the melting temperature difference between copper and sulfur. [1]
(c) Explain why graphite conducts but sulfur does not. [2]
(d) Describe why sulfur has the lowest melting temperature. [1]
Answer Details
Potassium chloride and silicon carbide
(a) Charge in molten potassium chloride is carried by potassium ions and chloride ions. [1]
(b) Potassium chloride contains oppositely charged ions, whereas silicon carbide contains atoms joined in a covalent network. [1]
(c) Silicon carbide contains many strong covalent bonds. A large amount of energy is needed to break these bonds, giving it a high melting temperature. [2]
(d) As potassium chloride cools and solidifies, its ions become fixed in position in a lattice and can no longer move freely. [1]
Copper, graphite and sulfur
(a) The substance with metallic bonding is copper. [1]
(b) \[1085-115=970\text{ °C}\] [1]
(c) Graphite conducts because it has delocalised electrons that can move through its structure. Sulfur has no mobile charged particles or electrons, so it does not conduct. [2]
(d) Sulfur has weak intermolecular forces between its molecules, so little energy is needed to overcome them. [1]
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