3.1.8 Thermodynamics - BHC Flashcards
3.1.8.1 Born Haber Cycles
Define ‘enthalpy change’
heat energy change at a constant pressure
Define ‘standard enthalpy change’
enthalpy change measured under standard conditions
Define ‘enthalpy of formation’.
Enthalpy change when 1 mole of compound is formed from its elements (under standard condtions - 298k and 100 kpa) with all reactants and products in their standard states
Define ‘enthalpy of atomisation’.
Enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.
Define ‘1st ionisation enthalpy’.
The enthalpy change when 1 mole of gaseous 1+ ions is formed from its gaseous atoms.
Define ‘2nd ionisation enthalpy’.
The enthalpy change when 1 mole of gaseous 2+ ions are formed from gaseous 1+ ions.
Define ‘1st electron affinity’
The enthalpy change when 1 mole of gaseous 1- ions are formed from gaseous atoms.
Define ‘2nd electron affinity’.
The enthalpy change when 1 mole of gaseous 2- ions is formed from gaseous 1- ions.
Define ‘bond dissociation enthalpy’.
The enthalpy change when 1 mole of covalent bonds is completely separated to form gaseous atoms.
Define ‘lattice enthalpy of formation’.
The enthalpy change when 1 mole of solid ionic compound is formed from gaseous ions.
Define ‘lattice enthalpy of dissociation’.
The enthalpy change when 1 mole of solid ionic compound dissociates into gaseous ions.
How can we use the Born-Haber Cycle to work out lattice enthalpies?
Elements converted to gaseous atoms
Electrons lost/gained to form ions
Ions attract to form a lattice
Write start/finish then go around to find enthalpy change
ON GOOGLE DOC
What is the perfect ionic model?
Ions are perfect spheres
with evenly distributed charge
State why some ionic compounds display covalent character.
+ve ions attract the outer electrons of the -ve ions
Electron clouds are polarised / charge cloud distorted
In what scenario is covalent character more likely for an ionic compound?
Cation has high charge density
Anion has low charge density
How does covalent character change the lattice enthalpy?
larger than expected
as it requires more energy to break / form
What two factors does the strength of lattice enthalpy depend on and how?
Size of ions (bigger size, charges are further apart, weaker attraction)
Charge on the ion (bigger charge, stronger attraction)
What happens when NaCl dissolves?
Water molecules are polar
Na+ attracted to δ- O atoms
Cl- ions are attracted to δ+ H atoms
Define ‘enthalpy of solution’ - ΔHsolnӨ:
The enthalpy change when 1 mole of ionic compound dissolves in water.
Define ‘enthalpy of hydration’ ΔHhydӨ:
The enthalpy change when one mole of aqueous ions is formed from gaseous ions.
Using enthalpy cycles, qualitiavely describe how we can work out enthalpy of solution.
Ionic lattice broken down into gaseous ions
Gaseous ions hydrated by polar water molecules
ON GOOGLE DOC
What are the two factors affecting solubility of an ionic compound?
Higher charge - greater attraction:
ΔHLӨ increases
ΔHhydӨ increases
Larger ion - less charge density
ΔHLӨ decreases
ΔHhydӨ decreases
Recall the equations for enthalpy of solution when given enthalpy of hydration and either lattice enthalpy. Use enthalpy cycles! (think of LiCl as an example)
ON GOOGLE DOC
