5.2.1 Lattice Enthalpy Flashcards
why are solid ionic compounds very stable
they contain very strong ionic bonds
- electrostatic attractions between oppositely charged ions in the ionic lattice structure
- provides a substantial barrier that must be overcome to break down
how can you measure the strength of ionic bonds in a lattice
lattice enthalpy
what is lattice enthalpy
- the enthalpy change
- that accompanies the formation of one mole of
- an ionic compound from
- its gaseous ions
- under standard conditions
what would be an equation of lattice enthalpy
gaseous ions ==> solid ionic compound
- e.g. K+(g) + Cl-(g) ===> KCl(s)
what is the sign for lattice enthalpy
normal symbol with LE in middle
is lattice enthalpy always endo or exo
- always exothermic
- as you are forming ionic bonds
- so always negative
how can you indirectly measure lattice enthalpy
Born-Haber cycle
- as you cannot measure lattice enthalpy any other way
what are the two routes of a lattice enthalpy cycle
- always start with elements in gaseous state
1) lattice formation
2) form gaseous atoms, then ions and then lattice enthalpy
what is the order of lattice enthalpy, longer route
1) standard enthalpy change of atomisation
2) first ionisation energy
3) first electron affinity
4) LATTICE ENTHALPY
what is standard enthalpy change of formation
enthalpy change that takes place when 1 moles of a compound is formed from its element under standard conditions, with all reactants and products in their standard conditions
- sign = f
what would be the equation of standard enthalpy change of formation
elements in their standard state ===> one mole of ionic compound in solid lattice
- e.g. Na(g) + 1/2 Cl2(g) ===> NaCl(s)
what is the standard enthalpy change of atomisation
enthalpy change that takes place for the formation of one mole of gaseous atoms from the elements in their standard states and conditions
- at
what is the equation for atomisation
1/2Cl2(g) ===> Cl(g)
- related to moles, so if we have 1/2 moles, then 1/2 the bond enthalpy
is atomisation endo or exo
always ENDO as bonds are being broken to form gaseous atoms
- POSITIVE
what is first ionisation energy
the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to one mole of gaseous 1+ ions
-IE
what is the equation of first ionisation energy
gaseous atom ===> gaseous 1+ ion
- e.g. Na(g) ===> Na+(g) + e-
- can have multiple following dependant on the ion needed
is ionisation energy always endo or exo
ENDO
- energy is required to overcome the attraction between the negative electron and the positive nucleus
- POSITIVE
what is first electron affinity
the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
- EA
what is the equation of electron affinity
Cl(g) + e- ===> Cl- (g)
is electron affinity ENDO or EXO
FIRST: exothermic as electron is being added is attracted to the nucleus, so negative
AFTER: endothermic as you are adding a negative electron into a negative ion, which repels, so energy must be put into system for them to react
what is always important to remember with a Born-Haber cycle
ALWAYS TAKE INTO ACCOUNT THE MOLES
enthalpy change of solution
enthalpy change that takes place when one mole of solute dissolves into a solvent
- overall energy energy change associated with the dissolving process
what is the equation for enthalpy of solution
solid ionic lattice + aq ===> aqueous ions
- e.g. NaCl(s) + aq ===> Na+(aq) +Cl-(aq)
- aq represents excess of water molecules
is enthalpy change of solution endo or exo
could be either
