4.2. Bonding and structure
A subsection of Chemistry, 9701, through 4. States of matter
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The arrangement of the anions around a cation is called the geometry of the cation; e.g. in [CuCl 4]2– the geometry of copper is tetrahedral and the co-ordination number of copper is 4. The geometry of a cation in an ionic compound can be predicted from the ratio of the ionic radii of the cation and anion involved. cation radius anion radius geometry of cation 0.155 – 0.225 trigonal planar 0.225 – 0.414 tetrahedral 0.414 – 0.732 octahedral Use data from the Data Booklet to predict the geometry of, and hence the co-ordination number of, the cation for ● sodium chloride, NaCl, geometry of Na+ = co-ordination number of Na+ = ● magnesium chloride, MgCl 2. geometry of Mg2+ = co-ordination number of Mg2+ = Magnesium(chloride, MgCl, is an unstable compound and readily decomposes as shown. 2MgCl Mg+ MgCl 2Use the following data to calculate the enthalpy change of this reaction. MgCl = –106 kJ mol–1 MgCl 2= – 642 kJ mol–1 enthalpy change = kJ mol–1 The equation for which ΔH is the lattice energy for MgCl is shown. Mg++ Cl –MgCl Use the equation, the following data, and relevant data from the Data Booklet to calculate a value for the lattice energy of MgCl. You might find it helpful to construct an energy cycle. electron affinity of Cl = –349 kJ mol–1 enthalpy change of atomisation of Mg= +147 kJ mol–1 enthalpy change of formation of MgCl = –106 kJ mol–1 lattice energy MgCl = kJ mol–1 Suggest how the lattice energies of MgCl 2 and NaCl will compare to that of MgCl. Explain your answers. MgCl 2 and MgCl NaCl and MgCl Define the term electron affinity.
9701_w17_qp_42
THEORY
2017
Paper 4, Variant 2
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