1. 1
   Barium sulfate, BaSO₄, is used as a component of drilling mud because of its high density and very low solubility in water. It is formed from Ba²⁺ ions and the polyatomic sulfate ion, SO₄²⁻.

   (a)

   Which electron configuration correctly describes the Ba²⁺ ion, formed when a barium atom (Z = 56, [Xe]6s²) loses two electrons?

   [1 mark]

   • A[Xe]
   • B[Kr]4d¹⁰5s²5p⁴6s²
   • C[Xe]6s¹
   • D[Xe]5d²

   (b)

   Which is the correct formula for barium sulfate?

   [1 mark]

   • ABa₂SO₄
   • BBaSO₄
   • CBaSO₂
   • DBa(SO₄)₂

   (c)

   Explain, in terms of structure and bonding, why solid barium sulfate does not conduct electricity but molten barium sulfate does.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Nitrogen trifluoride, NF₃, is a colourless gas used in the semiconductor industry to clean chamber surfaces. In the NF₃ molecule, the central nitrogen atom is bonded to three fluorine atoms and has one lone pair of electrons. (Electronegativities: N 3.0, F 4.0)

   (a)

   What is the electron domain geometry and the molecular geometry of NF₃?

   [1 mark]

   • ATetrahedral electron domain geometry, tetrahedral molecular geometry
   • BTrigonal planar electron domain geometry, trigonal planar molecular geometry
   • CTetrahedral electron domain geometry, trigonal pyramidal molecular geometry
   • DTrigonal pyramidal electron domain geometry, tetrahedral molecular geometry

   (b)

   Is the N–F bond polar, and is the NF₃ molecule overall polar?

   [1 mark]

   • AThe N–F bond is polar but the molecule is non-polar overall because the three bond dipoles cancel
   • BThe N–F bond is non-polar and the molecule is non-polar overall
   • CThe N–F bond is non-polar but the molecule is polar overall because of the lone pair alone
   • DThe N–F bond is polar and the molecule is polar overall

   (c)

   State the type(s) of intermolecular force present between NF₃ molecules, and explain why NF₃ has a much lower boiling point (−129 °C) than ammonia, NH₃ (−33 °C), even though both molecules are polar and of similar molar mass.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The melting points of the group 1 metals are: sodium 98 °C, potassium 63 °C, rubidium 39 °C, caesium 28 °C. Their metallic (ionic) radii are: Na⁺ 102 pm, K⁺ 138 pm, Rb⁺ 152 pm, Cs⁺ 167 pm. All four metals delocalise one electron per atom.

   (a)

   Explain the trend in melting point from sodium to caesium.

   [3 marks]

   (b)

   A chemistry teacher tells the class that calcium (melting point 842 °C) has a much higher melting point than potassium (63 °C) mainly because calcium atoms are smaller. Evaluate this claim, and identify what you consider to be the more significant factor.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A ceramics company wants a material to line a high-temperature furnace. Two candidates are considered: magnesium oxide, MgO (melting point 2852 °C, formed from Mg²⁺ and O²⁻ ions), and sodium chloride, NaCl (melting point 801 °C, formed from Na⁺ and Cl⁻ ions). Ionic radii: Mg²⁺ 72 pm, O²⁻ 140 pm, Na⁺ 102 pm, Cl⁻ 181 pm. The furnace lining is held in place by a steel support frame made from an iron–carbon alloy.

   (a)

   Explain, in terms of ionic charge and radius, why magnesium oxide has a much higher melting point than sodium chloride, and hence which compound is more suitable for the furnace lining.

   [6 marks]

   (b)

   The furnace lining is held in place by a steel support frame made from an iron–carbon alloy. Explain, using the metallic bonding model, why this alloy is harder than pure iron, and why both the alloy and pure iron can conduct electricity.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Calcium fluoride, CaF₂, occurs naturally as the mineral fluorite. It is formed from Ca²⁺ ions and F⁻ ions, and is used industrially as a source of fluorine and in the manufacture of hydrofluoric acid.

   (a)

   Which electron configuration correctly describes the F⁻ ion, formed when a fluorine atom (Z = 9, 1s²2s²2p⁵) gains one electron?

   [1 mark]

   • A1s²2s²2p⁵3s¹
   • B1s²2s²2p⁶
   • C1s²2s²2p⁴
   • D1s²2s²2p⁶3s¹

   (b)

   What is the correct formula for calcium fluoride?

   [1 mark]

   • ACaF₂
   • BCaF
   • CCa₂F
   • DCa₂F₂

   (c)

   Predict, giving a reason, whether solid calcium fluoride conducts electricity, and whether molten calcium fluoride conducts electricity.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Boron trichloride, BCl₃, is a trigonal planar molecule with an incomplete octet on boron (six electrons only). When BCl₃ gas is mixed with ammonia gas, NH₃, a white solid forms in which each nitrogen atom becomes bonded to a boron atom.

   (a)

   What is the electron domain geometry around the boron atom in BCl₃?

   [1 mark]

   • ATetrahedral
   • BTrigonal pyramidal
   • CBent/angular
   • DTrigonal planar

   (b)

   What type of bond forms between the nitrogen atom of NH₃ and the boron atom of BCl₃ in the product?

   [1 mark]

   • AAn ionic bond, because nitrogen and boron have very different electronegativities
   • BA metallic bond, because boron is a metalloid
   • CA coordinate (dative) covalent bond, because both shared electrons come from the lone pair on nitrogen
   • DA hydrogen bond, because nitrogen has a lone pair

   (c)

   Explain why boron in BCl₃ is able to accept a lone pair of electrons from nitrogen to form this new bond.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Overhead power cables are usually made from aluminium (reinforced with a steel core) rather than copper, even though copper is a better conductor per unit cross-sectional area. Aluminium's density is 2.70 g cm⁻³ and copper's is 8.96 g cm⁻³; aluminium delocalises three electrons per atom ([Ne]3s²3p¹) and copper delocalises one electron per atom ([Ar]3d¹⁰4s¹).

   (a)

   Explain, using the metallic bonding model, why aluminium is able to conduct electricity.

   [3 marks]

   (b)

   Suggest, using the data given, why aluminium cables are used for long-distance power lines instead of copper cables, despite aluminium being a slightly poorer conductor. In your answer, evaluate the roles of density and the number of delocalised electrons per atom.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Diamond and graphite are both allotropes of carbon. In diamond, each carbon atom forms four covalent bonds to four neighbouring carbon atoms in a rigid three-dimensional network. In graphite, each carbon atom forms three covalent bonds to three neighbouring carbon atoms within flat layers, with the fourth outer electron delocalised across each layer; the layers are held together only by weak London (dispersion) forces and can slide over one another. A new addition polymer is separately made by polymerising the monomer CH₂=CCl₂ (1,1-dichloroethene), and a condensation polymer is made by reacting a diol, HO–(CH₂)₃–OH, with a diacid, HOOC–(CH₂)₄–COOH.

   (a)

   Explain why diamond is used as an abrasive/cutting material, while graphite is used as a lubricant and in pencils, in terms of their structures and bonding.

   [6 marks]

   (b)

   Deduce and describe the repeating unit formed from each monomer/pair of monomers, and explain one key difference between how addition and condensation polymers form.

   [6 marks]

   Total for question 8: 12 marks

End of questions