The first electron affinity is the enthalpy change when one mole of gaseous atoms attracts one mole of electrons. Values are shown in Table 7 of the IB data booklet.
The lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions
15.2.2 Explain how the relative sizes and the charges of ions affect the lattice enthalpies of different ionic compounds.
The ionic model assumes that the only interaction is due to the electrostatic forces between the ions. The energy needed to separate the ions depends the product of the ionic charges and the sum of the ionic radii
- An increase in the ionic radius of one of the ions decreases the attraction between the ions
- An increase in the ionic charge increases the ionic attraction between the ions.
15.2.3 Construct a Born-Haber cycle for group 1 and 2 oxides and chlorides, and use it to calculate an enthalpy change
A Born-Haber cycle is a larger and more complex version of the Hess's law. It uses two paths to calculate the bond enthalpy of a part of the cycle.
Route 1:
Step 1: Enthalpy of Formation
Route 2:
Step 1: Enthalpy change of atomization
Step 2: First Ionization Energy
Step 3: Half of Average Bond Enthalpy (The energy of one chlorine)
Step 4: Electron Affinity
Step 5: Lattice Enthalpy
15.2.4 Discuss the difference between theoretical and experimental lattice enthalpy values of ionic compounds in terms of their covalent character
The bonding of sodium iodide is stronger than expected from a simple ionic model because the large and "squishy" iodide ion is distorted or polarized by the smaller sodium ion. This gives the compound some covalent character, which provides an additional contribution to the bonding. A covalent bond can be considered an extreme case of distortion, with the negative ion so polarized that we can consider two of the electrons from the iodide ion as shared with the "positive ion".
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