Topic 13: Energetics II
Topic 13A: Lattice energy
1. be able to define lattice energy as the energy change when one mole of an
ionic solid is formed from its gaseous ions
4. know that lattice energy provides a measure of ionic bond strength
Lattice energy
Lattice energy, ΔLEH: the energy change when one mole of an ionic solid is formed from its
gaseous ions under standard conditions
Ionic compounds form giant ionic lattices, where +ve and -ve ions are held
together by electrostatic forces of attraction.
The stronger the ionic bonding, the more exothermic the lattice energy as it is a measure of ionic
bond strength. Bonds are made so it is exothermic (bendy mexican).
Na+(g) + Cl-(g) NaCl(s) ΔLEH˚ = -787kJ/mol
Mg2+(g) + O2-(g) MgO(s) ΔLEH˚ = -3,791kJ/mol
Factors affecting lattice energy:
Ionic charge – the higher the charge on the ions, the more energy
released when a lattice forms as stronger forces form.
Size – the smaller the ionic radii, the more exothermic the lattice
energy as small ions have a higher charge density so stronger bonds
form
2. be able to define the terms:
Enthalpy change of atomisation, ΔatH˚: the energy change needed to produce one mole of
gaseous atoms of the element from the element in its standard state
For sodium this is: Na(s) Na(g) ΔatH˚ = +107 kJ/mol
For chlorine this is: ½Cl2(g) Cl(g) ΔatH˚ = +122 kJ/mol
Trend
Enthalpy of atomisation becomes less endothermic down the group. E.g. strontium’s is less
endothermic than calcium’s because strontium is larger so there is a weaker attraction between
the delocalised electrons and the nuclei (weaker metallic bonds) so less energy required to
overcome the attraction.
Electron affinity: the energy change when each atom in one mole of gaseous atoms gains one
electron to form one mole of gaseous ions with a single negative charge (opposite of ionisation
energy)
First electron affinity of oxygen: O(g) + e- O-(g) 1st EA = -141 kJ/mol
Second electron affinity of oxygen: O-(g) + e- O2-(g) 1st EA = +798 kJ/mol
Gaining the first electron is exothermic because the electron forms an attraction with the
positively charged nucleus. The second electron is added to a negative ion, so energy is required
to overcome the repulsion, hence endothermic.
Trend
First electron affinity becomes less exothermic as you go down the group. E.g. the first electron
affinity of bromine is less exothermic than chlorine because bromine is larger, so the attraction
formed between the positive nucleus and the electron is weaker (less energy released).
Topic 13A: Lattice energy
1. be able to define lattice energy as the energy change when one mole of an
ionic solid is formed from its gaseous ions
4. know that lattice energy provides a measure of ionic bond strength
Lattice energy
Lattice energy, ΔLEH: the energy change when one mole of an ionic solid is formed from its
gaseous ions under standard conditions
Ionic compounds form giant ionic lattices, where +ve and -ve ions are held
together by electrostatic forces of attraction.
The stronger the ionic bonding, the more exothermic the lattice energy as it is a measure of ionic
bond strength. Bonds are made so it is exothermic (bendy mexican).
Na+(g) + Cl-(g) NaCl(s) ΔLEH˚ = -787kJ/mol
Mg2+(g) + O2-(g) MgO(s) ΔLEH˚ = -3,791kJ/mol
Factors affecting lattice energy:
Ionic charge – the higher the charge on the ions, the more energy
released when a lattice forms as stronger forces form.
Size – the smaller the ionic radii, the more exothermic the lattice
energy as small ions have a higher charge density so stronger bonds
form
2. be able to define the terms:
Enthalpy change of atomisation, ΔatH˚: the energy change needed to produce one mole of
gaseous atoms of the element from the element in its standard state
For sodium this is: Na(s) Na(g) ΔatH˚ = +107 kJ/mol
For chlorine this is: ½Cl2(g) Cl(g) ΔatH˚ = +122 kJ/mol
Trend
Enthalpy of atomisation becomes less endothermic down the group. E.g. strontium’s is less
endothermic than calcium’s because strontium is larger so there is a weaker attraction between
the delocalised electrons and the nuclei (weaker metallic bonds) so less energy required to
overcome the attraction.
Electron affinity: the energy change when each atom in one mole of gaseous atoms gains one
electron to form one mole of gaseous ions with a single negative charge (opposite of ionisation
energy)
First electron affinity of oxygen: O(g) + e- O-(g) 1st EA = -141 kJ/mol
Second electron affinity of oxygen: O-(g) + e- O2-(g) 1st EA = +798 kJ/mol
Gaining the first electron is exothermic because the electron forms an attraction with the
positively charged nucleus. The second electron is added to a negative ion, so energy is required
to overcome the repulsion, hence endothermic.
Trend
First electron affinity becomes less exothermic as you go down the group. E.g. the first electron
affinity of bromine is less exothermic than chlorine because bromine is larger, so the attraction
formed between the positive nucleus and the electron is weaker (less energy released).
