Trends in Lattice Enthalpies The strength of a enthalpy of lattice formation depends on the following factors 1. The sizes of the ions: The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them. 2. The charges on the ion: The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values). The lattice enthalpies become less negative down any group. e.g. LiCl, NaCl, KCl, RbCl e.g group 1 halides (eg NaF KI) have lattice enthalpies of around –700 to – 1000 group 2 halides (eg MgCl2 ) have lattice enthalpies of around –2000 to –3500 group 2 oxides eg MgO have lattice enthalpies of around –3000 to –4500 kJmol-1 Perfect Ionic Model Theoretical lattice enthalpies assumes a perfect ionic model where the ions are 100% ionic and spherical and the attractions are purely electrostatic. There is a tendency towards covalent character in ionic substances when •the positive ion is small •the positive ion has multiple charges •the negative ion is large •the negative ion has multiple negative charges. When the negative ion becomes distorted and more covalent we say it becomes polarised. The metal cation that causes the polarisation is called more polarising if it polarises the negative ion. + – Ionic with covalent 100% ionic character When 100 % ionic the ions are spherical. The theoretical and the born Haber lattice enthalpies will be the same The charge cloud is distorted .Differences between theoretical and Born Haber (experimental) lattice enthalpies The Born Haber lattice enthalpy is the real experimental value. When a compound shows covalent character, the theoretical and the born Haber lattice enthalpies differ. The more the covalent character the bigger the difference between the values. When a compound has some covalent character- it tends towards giant covalent so the lattice is stronger than if it was 100% ionic. Therefore the born haber value would be larger than the theoretical value.
18.104.22.168 Born–Haber cycles (A-level only)
Students should be able to:
compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds.