Ionic bonding and ionic latticesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student lays a strip of filter paper, soaked in aqueous sodium nitrate, between two electrodes connected to a 20 V d.c. supply. A single crystal of potassium manganate(VII), KMnO₄, is placed in the middle of the strip. After 20 minutes a purple band has spread along the paper towards the electrode connected to the positive terminal, while the rest of the paper remains colourless.

   (a)

   Which ion is responsible for the purple colour?

   [1 mark]

   • AK⁺
   • BNa⁺
   • CMnO₄⁻
   • DNO₃⁻

   (b)

   What can be deduced from the movement of the purple colour?

   [1 mark]

   • AThe coloured ion is negatively charged and is attracted to the positive electrode.
   • BThe coloured ion is positively charged and is attracted to the positive electrode.
   • CThe coloured particles are uncharged molecules carried along by the flow of water.
   • DElectrons from the crystal are flowing along the paper to the positive electrode.

   (c)

   Explain why this experiment provides evidence that potassium manganate(VII) contains ions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Electron density maps of sodium chloride, made using X-ray diffraction, show contour lines of electron density. The contours form separate, almost circular patterns around the sodium and chlorine nuclei, and the electron density falls to almost zero in the region between the two kinds of ion.

   (a)

   Which is the electronic configuration of the sodium ion in the lattice?

   [1 mark]

   • A1s²2s²2p⁶3s¹
   • B1s²2s²2p⁶3s²
   • C1s²2s²2p⁶3s²3p⁶
   • D1s²2s²2p⁶

   (b)

   Which statement best describes the ionic bonding in sodium chloride?

   [1 mark]

   • AThe sharing of a pair of electrons between a sodium atom and a chlorine atom.
   • BThe strong net electrostatic attraction between oppositely charged ions throughout the lattice.
   • CThe attraction between sodium ions and a sea of delocalised electrons.
   • DThe electrostatic attraction between each sodium ion and only one chloride ion.

   (c)

   Explain what the electron density map shows about the bonding in sodium chloride.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Calcium fluoride, CaF₂, occurs naturally as the mineral fluorite and is used to make windows for infrared spectrometers. Calcium has atomic number 20 and fluorine has atomic number 9. Solid calcium fluoride does not conduct electricity, but it does conduct when molten, and its melting point is 1418 °C.

   (a)

   Describe, in terms of electrons, how calcium atoms and fluorine atoms form calcium fluoride.

   [3 marks]

   (b)

   Explain why calcium fluoride has a high melting point and why it conducts electricity when molten but not when solid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A data book gives these ionic radii: Li⁺ 0.076 nm, Na⁺ 0.102 nm, K⁺ 0.138 nm, F⁻ 0.133 nm, Cl⁻ 0.181 nm and Br⁻ 0.196 nm. It also gives the melting points of four ionic compounds, all with giant cubic lattices: lithium fluoride 848 °C, sodium chloride 801 °C, potassium bromide 734 °C and magnesium oxide 2852 °C.

   (a)

   Explain the trend in the ionic radii of Li⁺, Na⁺ and K⁺, and the trend in the radii of the isoelectronic ions N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺.

   [6 marks]

   (b)

   Explain why the melting points decrease from lithium fluoride to sodium chloride to potassium bromide, and why magnesium oxide has a much higher melting point than all three.

   [6 marks]

   Total for question 4: 12 marks

End of questions