Energy changes and bonding
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Please help me understand the trends in lattice enthalpy.
How does the value (enthalpy change of lattice) of MgO compare with the lattice enthalpy of NaCl? Why is MgO lattice enthalpy change so large?
Ulex says
The magnitude of the lattice energy of an ionic substance depends on two factors:
• the charges on the ions and
• the sizes of the ions and therefore the inter-ionic distance.
Magnesium ions have two positive charges each, compared with only one for sodium ions. Oxide ions have two negative charges, whereas chloride ions have only one.
The electrostatic attraction in magnesium oxide would therefore be about four times as great as that in sodium chloride, even if the interionic distance were identical. But the interionic distance is considerably smaller in magnesium oxide than in sodium chloride. This means that the lattice energy of magnesium oxide is more than four times as great as that of sodium chloride.
You can apply the same sort of reasoning to other lattice energy comparisons.
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updated: 30 September 2004
