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Perfect ionic model.

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Presentation on theme: "Perfect ionic model."— Presentation transcript:

1 Perfect ionic model

2 Lattice dissociation enthalpy
Definition: The lattice dissociation enthalpy, ΔHϴL (diss) is the enthalpy change when 1 mole of an ionic solid is separated into its gaseous ions Example: Sodium chloride (NaCl) NaCl (s) → Na+ (g) + Cl- (g) ΔHϴL (diss) = kJ mol-1 Energy must be put in to break the strong ionic bonds in the lattice; therefore it is an endothermic process

3 Factors affecting ΔHϴL
The two factors which affect the value of ΔHϴL (diss) are: 1) The size of the ions The halide ions increase in size in the order F-, Cl-, Br- and I- The distance between the positive ion and negative halide ion increases Therefore the electrostatic force of attraction between the ions is weaker and the ΔHϴL (diss) values decrease Compound ΔHϴL (kJ mol-1) NaF 918 NaCl 780

4 Factors affecting ΔHϴL
2) The charges on the ions The greater the charges on the ions, the stronger the electrostatic force of attraction between the ions Therefore more energy is required to separate the ions and the ΔHϴL (diss) value increases Compound ΔHϴL (kJ mol-1) NaF 918 MgO 3791 Na+ F- Mg2+ O2-

5 Born-Haber cycles Born-Haber cycles are used to calculate lattice enthalpies, ΔHϴL from experimental data (ΔHϴf, ΔHϴdiss, ΔHϴi, ΔHϴea and ΔHϴat) We can also calculate lattice enthalpies, ΔHϴL using a model called the perfect ionic model

6 Perfect ionic model Definition: The lattice dissociation enthalpy, ΔHϴL (diss) is the enthalpy change when 1 mole of an ionic solid is separated into its gaseous ions The perfect ionic model assumes: The ions are perfect spheres The ions are held together only by electrostatic forces (pure ionic bonding)

7 ΔHϴL Born-Haber (kJ mol-1) ΔHϴL Perfect ionic model (kJ mol-1)
We can compare the ΔHϴL (diss) values from the Born- Haber cycle (calculated from experimental data) with those from the perfect ionic model (theoretical) Compound (Alkali halides) ΔHϴL Born-Haber (kJ mol-1) ΔHϴL Perfect ionic model (kJ mol-1) NaCl 787 769 NaBr 747 732 NaI 704 682 KCl 701 690 KBr 670 665 KI 629 632

8 Perfect ionic model For the alkali halides there is good agreement between the ΔHϴL (diss) values from the Born-Haber cycle and the perfect ionic model We can conclude the structure of alkali halides is just like the perfect ionic model (ions are perfect spheres and held together only by electrostatic forces) This provides evidence to support the model works

9 Perfect ionic model We can conclude the structure of alkali halides is just like the perfect ionic model (ions are perfect spheres and held together only by electrostatic forces) The type of bonding in alkali halides is ionic bonding

10 ΔHϴL Born-Haber (kJ mol-1) ΔHϴL Perfect ionic model (kJ mol-1)
For the silver halides there is less agreement between the ΔHϴL (diss) values from the Born-Haber cycle and the perfect ionic model Compound (Silver halides) ΔHϴL Born-Haber (kJ mol-1) ΔHϴL Perfect ionic model (kJ mol-1) AgF 955 870 AgCl 915 864 AgBr 904 830 AgI 889 808

11 Ionic with some covalent character
Perfect ionic model The ions are not perfect spheres and there is some covalent bonding between the ions The type of bonding in silver halides is ionic with covalent character The perfect ionic model does not take into account any covalent bonding present between the ions Ionic Ionic with some covalent character

12 Perfect ionic model The type of bonding in silver halides is ionic with some covalent character More energy would be required to separate the ionic solid into gaseous ions as the bonding is stronger This explains the higher values of ΔHϴL (diss) from the Born-Haber cycle compared to the perfect ionic model Silver halides ΔHϴL Born-Haber ΔHϴL Perfect ionic AgCl 915 864 AgBr 904 830 AgI 889 808

13 Silver halides The electron cloud of the negative ion can be (distorted) polarised by the positive ion Negative ions are easily polarised if they are large Iodide ions are large and are easily polarised and therefore the ionic bond has the greatest covalent character This explains why there is the greatest difference in ΔHϴL (diss) values between the B-H cycle and the perfect ionic model for silver iodides

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