1. 1
   A diving instructor checks a cylinder filled with pure hydrogen gas, H2, before a technical dive, and compares it with a cylinder of water vapour, H2O, used in a separate humidity test.

   (a)

   Each hydrogen molecule is formed when two hydrogen atoms share a pair of electrons. What type of bond holds the two hydrogen atoms together in an H2 molecule?

   [1 mark]

   • AA covalent bond
   • BAn ionic bond
   • CA metallic bond
   • DA hydrogen bridge bond

   (b)

   In a separate humidity test, the instructor also examines water vapour, H2O, in which each oxygen atom forms covalent bonds with two hydrogen atoms. Why does the oxygen atom in a water molecule share electrons with two hydrogen atoms rather than just one?

   [1 mark]

   • AOxygen needs to lose two electrons to complete its outer shell
   • BOxygen needs to gain two more electrons to complete its outer shell and achieve a noble gas configuration
   • CHydrogen atoms are too large to share electrons with only one oxygen atom
   • DWater molecules are ionic, not covalent, so sharing does not apply

   (c)

   Describe, in terms of electron sharing, the formation of the covalent bonds in a molecule of water, H2O, stating how many electrons each atom contributes and what electronic configuration each atom achieves.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A materials tester compares two substances at room temperature: substance P, a simple molecular compound with a low melting point that does not conduct electricity, and substance Q, an ionic compound with a very high melting point.

   (a)

   Which property would you expect substance P to show, given that it is a simple molecular compound?

   [1 mark]

   • AGood electrical conductivity in all states, due to free ions
   • BVery high melting and boiling points, due to strong forces between molecules
   • CLow melting and boiling points, due to weak intermolecular forces between molecules
   • DA giant lattice structure extending in three dimensions throughout the solid

   (b)

   The tester also finds that substance P does not conduct electricity in any state (solid, liquid or gas). Which statement best explains this observation?

   [1 mark]

   • ASimple molecular compounds are always metals, which do not conduct when molten
   • BSimple molecular compounds contain free ions that are too heavy to move
   • CSimple molecular compounds always have very high melting points, which prevents conduction
   • DSimple molecular compounds have no overall charge and no free ions or delocalised electrons to carry current

   (c)

   Explain, in terms of structure and bonding, why substance P has a much lower melting point than the ionic substance Q, even though the covalent bonds within each molecule of P are themselves strong.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An agricultural chemist studies ammonia, NH3, which is used in fertiliser manufacture, and methane, CH4, which is used as a fuel.

   (a)

   Describe, in terms of electron sharing, the covalent bonding in a molecule of ammonia, stating the electronic configuration achieved by the nitrogen atom.

   [3 marks]

   (b)

   The chemist then studies methane, CH4. Describe, in terms of electron sharing, the covalent bonding in a molecule of methane, and explain why methane's low melting and boiling points make it suitable for use as a fuel that is easily transported and burned as a gas.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical engineer compares carbon dioxide, CO2, used in fizzy drinks, with nitrogen, N2, the major component of air. In a CO2 molecule, the carbon atom forms two double covalent bonds with oxygen atoms, while in an N2 molecule, the two nitrogen atoms are joined by a triple covalent bond.

   (a)

   Explain fully, in terms of electron sharing, how each of these molecules achieves stable electronic configurations, and describe the general properties (melting point, boiling point and electrical conductivity) that both CO2 and N2 are expected to show as simple molecular substances, giving reasons for these properties.

   [6 marks]

   (b)

   The engineer also examines ethene, C2H4, a raw material used in the plastics industry, in which the two carbon atoms are joined by a double covalent bond, and each carbon atom also forms single covalent bonds with two hydrogen atoms. Explain fully, in terms of electron sharing, how the carbon atoms in ethene achieve a stable electronic configuration, and explain why ethene, like the other simple molecular substances studied, is expected to have a low boiling point and poor electrical conductivity.

   [6 marks]

   Total for question 4: 12 marks

End of questions