Cambridge International AS Level Chemistry
4 The diagram below shows part of a giant metallic structure. –
–
+e + +e + e– e– e– e– + + + + + e– e– e– e– e– + + + + + e– e– e– e– e– + –+ + –+ + – e e e +
a b c d
e–
Use this diagram to explain the main features of metallic bonding. Explain why metals are good conductors of electricity. Explain why, in general, metals have high melting points. Suggest why potassium is a better conductor of electricity than lithium.
[3] [2] [2] [4] Total = 11
5 Methane, CH4, is a gas at room temperature. a Explain why methane is a gas at room temperature. b Draw a diagram to show the shape of a molecule of methane. On your diagram show a value for the
70
C H
H
[2]
bond angle.
[3]
c Perfumes often contain molecules that have simple molecular structures. Explain why. d When a negatively charged rod is held next to a stream of propanone, CH3COCH3, the stream of propanone is attracted to the rod. Draw the full structure of a molecule of propanone and use your diagram to explain why the stream of propanone is attracted to the rod.
[2]
[3]
Total = 10 6 Sodium iodide and magnesium oxide are ionic compounds. Iodine and oxygen are covalent molecules. a Draw dot-and-cross diagrams for: i magnesium oxide ii oxygen. b How do sodium iodide and iodine differ in their solubility in water? Explain your answer. c Explain why molten sodium iodide conducts electricity but molten iodine does not. d The boiling point of sodium iodide is 1304 °C. The boiling point of iodine is 184 °C. Explain this difference.
[2] [3] [2] [5] Total = 12
7 Hydrogen sulfide, H2S, is a covalent compound. a Draw a dot-and-cross diagram for hydrogen sulfide. b Draw a diagram of a hydrogen sulfide molecule to show its shape. Show on your diagram: i
c
the value of the
S
H
H
[2]
bond angle
ii the partial charges on each atom as δ+ or δ– iii an arrow showing the exact direction of the dipole in the molecule as a whole. Oxygen, O, sulfur, S, and selenium, Se, are in the same group in the Periodic Table. i Explain why hydrogen selenide, H2Se, has a higher boiling point than hydrogen sulfide, H2S. ii Explain why the boiling point of water is so much higher than the boiling point of hydrogen sulfide.
[4] [2] [5] Total = 13