perfect ionic model
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what 2 factors affect lattice enthalpy
ionic charge (more highly charged ions form stronger electrostatic forces) ionic radius (smaller ions have a greater charge density. They form stronger electrostatic forces)
compare the lattice enthalpy of NaCl with MgCl2
Mg2+ is more highly charged with a smaller ionic radius than Na+. Mg2+ has greater charge density and forms a stronger ionic bond/electrostatic force with Cl- ions. MgCl2 has a more exothermic lattice formation enthalpy
compare the lattice enthalpy of NaCl with NaBr
Cl- has a smaller ionic radius than Br- therefore a greater charge density. Cl- forms a stronger ionic bond/electrostatic force with Na+ ions and therefore has a more exothermic lattice formation enthalpy
give the 2 characteristics of a perfect ionic compound
perfect spheres (point charges) no covalent interaction/bonding
why are some ionic compounds said to have some covalent character
some ionic compounds have distorted ions that are no longer spheres and some covalent interaction (bonding) the electron clouds of the ions overlap and form a weak covalent bond the covalent bond is stronger than the ionic bond and the lattice enthalpy will be more exothermic
how is theoretical lattice enthalpy calculated
using a physics equation (purely mathematical)
what does the theoretical lattice enthalpy assume about a compound
assumes the ions are point charges and calculates the energy of interaction between them it assumes the ionic compound has the perfect ionic model
how is experimental lattice enthalpy calculated
using a Born Haber cycle (the enthalpy changes in the cycle have ultimately been found by doing experiments)
if the theoretical and experimental lattice enthalpy are very similar what does this mean about the compound?
the compound only has ionic bonding and obeys the perfect ionic model the ions are perfect spheres and there is no covalent character
if the experimental lattice enthalpy value is greater than the theoretical ls lattice enthalpy value what does this mean about the compound?
the actual bonding is stronger (more exothermic) the compound has some covalent character and the ions have been distorted (no longer spheres)
why do some ionic compounds have covalent character
if the positive (+) ion is polarising it pulls the electron cloud of the negative ion towards itself this distorts the ion and creates some overlap of the clouds creating a weak covalent bond
what are the 2 factors that affect the polarising ability of the positive ion and explain how
ionic radius - smaller ions have a greater charge density and are more polarising ionic charge - more highly charged ion have greater charge density and are more polarising
what 2 factors affect the polarisability of the negative ion and explain how
ionic radius - larger ions are easier to distort ionic charge - lower charged ions are easier to distort