Chapter 1 - Lattice Enthalpy Flashcards
Define “lattice enthalpy”
enthalpy change when one mole of a solid ionic lattice is formed from it’s gaseous ions under standard conditions.
What are two important things to bare in mind with lattice enthalpy?
- lattice enthalpy is EXOTHERMIC so there is a negative sign. Energy is given out as ionic bonds are being formed from gaseous ions.
- lattice enthalpy is a measure of the ionic bond strength. large negative value = strong electrostatic force
Do covalent substances have lattice enthalpy? Why/why not?
No. There are no ions in its structure.
Define “standard enthalpy change of formation”
Hf, when one mole of a compound is formed from its constituent elements in their standard states.
EXOTHERMIC
Define “standard enthalpy change atomisation”
Hat, when one mole of gaseous atoms is formed from its element in its standard state.
ENDOTHERMIC - bonds have to be broken
Define “First ionisation energy”
Hi1, one mole of gaseous 1+ ions is formed from gaseous atoms.
ENDOTHERMIC - electron being lost has to overcome attraction from nucleus
Define “second ionisation energy”
Hi2, one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions.
ENDOTHERMIC
Define “first electron affinity”
EA1, one mole of gaseous 1- ions is formed from gaseous atoms.
EXOTHERMIC -electron is attracted to outer shell atom
Define “second electron affinity”
HE2, one mole of gaseous 2- ions is formed from gaseous 1- ions.
ENDOTHERMIC - electron has to overcome repulsion by 1- ion.
Describe the steps for writing a Born-Haber cycle.
1) draw datum line (elements)
2) atomisation of metal
3) atomisation of non metal
4) 1st ionisation of metal
5) electron affinity of non metal
Define “standard enthalpy change of solution” Hs
One mole of a compound is dissolved in water under standard conditions.
What happens when a solid dissolves?
1) BREAKDOWN OF THE IONIC LATTICE INTO GASEOUS IONS. This process is the exact opposite of the lattice enthalpy - even the sign is +ve not -ve.
2) HYDRATION OF THE IONS. The gaseous ions bond with the H2O molecules. +ve ions are attracted to slightly negative O atoms and the -ve ions are attracted to slightly positive hydrogen atoms.
Define “standard enthalpy change of hydration”
Hhyd, one mole of aq ions are formed from their gaseous ions under standard conditions. EXOTHERMIC
What factors effect the size of lattice enthalpy?
-IONIC SIZE ionic radius increases ions further apart attraction between the ions decreases. The lattice enthalpy becomes LESS negative LESS exothermic.
-IONIC CHARGE ionic charge increases highly charged ion lattice enthalpy becomes MORE negative MORE exothermic
What factors affect the size of hydration enthalpy? How?
The same factors that effect lattice enthalpy (ionic size and ionic charge) effect hydration enthalpy.
Ionic size - hydration depends on how easily ions are able to bond to water molecules, small ions have to exert more attraction so more energy is released.
- Ionic radius increases
- hydration becomes less negative (less exo)
Ionic charge -
ionic charge increases
greater attraction for water molecules
hydration enthalpy is more negative (more exo)
