CHEM1201 Lecture Notes - Lecture 6: Ionization Energy, Lattice Energy, Electron Affinity
7 Jun 2018
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Ionic compounds:
- Can e alled salts
- Consist of oppositely charged ions
- Consist of:
o A cation, usually derived from a metal. E.g. Na+, Ca2+, NH4+
o An anion, usually derived from a non-metal (or group of non-metals). E.g. Cl-,
OH-, CO3-, O2-
- E.g. Ca(OH)2, Na2CO3, Al2(SO4)3
Energetics and forming ionic compounds:
- Ionisation energy must be provided
- Electron capture follows with the release of electron affinity energy
- Lattice energy is released when newly formed cations and anions come together
o Lattice energy + electron affinity energy > ionisation energy
o I.e. total energy released > total energy used
Summary:
- Ionic compounds consist of cations and anions, so the SUM OF POSITIVE CHARGES =
SUM OF NEGATIVE CHARGES
- Electrons are removed by another chemical reagent rather than just lost
o Results in formation of ionic compounds
- Lattice energy is the driving force which makes the formation of ionic compounds
energetically favourable
o Excess energy is primarily given off as heat
Ionic compounds in water:
- Generally dissociate into ions in water to form a SOLUTION
o NaCl(s) → Na+(aq) + Cl-(aq)
o Solute → ions in solution
- Solute – substance that dissolves
- Solvent – the medium in which something dissolves
- (aq) – aquated (hydrated or solvated or ion surrounded by water molecules)
solvate
The dissolution Process:
- Dissolving ionic compounds requires sufficient energy to break the lattice
- Where does this energy come from?
o The aquation (hydration, solvation) process
o i.e. when the compound dissolves, weak bonds form between the solvent
and the ions – usually 6 water molecules
o known as solvation (or hydration) energy
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Document Summary
Consist of: a cation, usually derived from a metal. Na+, ca2+, nh4: an anion, usually derived from a non-metal (or group of non-metals). Electron capture follows with the release of electron affinity energy. Lattice energy is released when newly formed cations and anions come together: lattice energy + electron affinity energy > ionisation energy. I. e. total energy released > total energy used. Ionic compounds consist of cations and anions, so the sum of positive charges = Electrons are removed by another chemical reagent rather than just lost: results in formation of ionic compounds. Lattice energy is the driving force which makes the formation of ionic compounds energetically favourable: excess energy is primarily given off as heat. Generally dissociate into ions in water to form a solution (aq) (aq) + cl: nacl(s) na, solute ions in solution. Solvent the medium in which something dissolves (aq) aquated (hydrated or solvated or ion surrounded by water molecules) solvate.
