3.1.8.1 - born-haber cycles Flashcards
if starting from LiCI (s) how do you create born haber cycles going up
then going down
enthalpy of formation (exo) -> Li(S) + 1/2CL2(g)
enthalpy of atomisation of chlorine Li(s) + cl(g)
enthalpy atomisation of lithium Li(g) + cl (g)
1st ionisation energy of lithium li+(g) + cl(g) + e-
going down..
1st electron affinity of chlorine (exo)
Li+(g) + Cl-(g)
lattice enthalpy of formation of lithium chloride
LiCI (s)
steps going up __ ____ ____ as the steps going down
steps going up is the same as the steps going down
when calculating born haber cycles
when you go w the arrow you …
when you go agianst the arrow you…
when you go with the arrow, keep the sign the same
when you go against the arrow change the sign
down arrows are ..
exo
up arrows are ..
endo
if calc enthalpy for 1/2 cl2 and values given is for cl2 what do you do
half the enthalpy
is lattice enthalpy exo or endo
exo
is enthalpy of formation exo or endo
exo
is enthalpy of atomisation endo or exo
endo
is ionisation energies endo or exo
endo
is first e- affinity endo or exo
exo
every other e- affinity apart from 1st
endo
we represent atomisation steps …
first w metal
second non metal
The greater the value of the lattice enthalpy, the ….
stronger the ionic bonds
Smaller ions and higher charge ions give stronger ionic bonding
because
Smaller ions are more closely packed in the lattice, so are more
attracted to each other, resulting in high lattice enthalpy.
* Ions with high charge have stronger electrostatic forces of attraction
between the ions which results in high lattice enthalpy
assumptions of perfect ionic model
✴The bonding in the compound is completely ionic.
✴The ions are regarded as perfect spheres + are not distorted (i.e. have
their charge evenly distributed around them).
if expirmental value is diff from theortical value it indicates..
compound doesnt follow perfect ionic model - there is some covalent character to the compound
when is their covalent character in ionic compounf
a small and highly charged cation can distort a large anion so
that there is covalent character to the ‘ionic compound’. This means that
ions are distorted and not spherical (That IS assumed by the perfect
ionic model)
positive ion polarises negative ion
the more polarisation we get…
the more covalent character there will be
covalent character to a compound leads to …
This leads to a difference between the values of the experimental
lattice enthalpy and the theoretical lattice enthalpy. The greater the
difference in the values, the greater the covalent character in the
compound.
define enthalpy change of solution
the enthalpy change when one mol of an ionic sub is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution
2 things that need to occur for a substance to dissolve
substance bonds must break (endo)
new bonds formed between solvent and substance (exo)
explain how a substance is dissolved
ionic lattice in solid form
substance bonds broken to create to create free moving
steps of dissolving a solid ionic lattice
1) ionic lattice in solid form
+ water
2) substance binds broken to create free moving ions
3) bonds formed between ions and water - ions are hydrated
most ionic compounds dissolve in _____ solvents
most ionic compounds dissolve in polar solvents
how do ionic compounds dissolve in polar solvents
the delta positive H is attracted to negative ions and the delta neg oxygen is attracted to positive ions, the struc starts to break down
the water molecules surrond the ions in a process called …
hydration
what must happen for hydration to occur
new bonds formed must be the same strength or greater than those broken
if not then the sub is very unlikely to dissolve. soluble sub tend to have exothermic enthalpies of solution bc of this
enthalpy of hydration
1 mole of aqueous ions are formed from gaseous ions
Na+ (g) + aq –> Na+(aq)
Cl-(g) + aq –> Cl-(aq)
extra steps needed for born haber cycle of group elements
Second ionisation energy of Mg needs to be included because Mg forms 2+
ions.
* Atomisation of Cl is multiplied by 2 as there are 2 moles of Cl for
every 1 mole of Mg
* First electron affinity of Cl is multiplied by 2 as there are 2 moles
of Cl- formed in the ionic lattice.
the bigger the difference in lattice enthalpy
the more polarisation you have
the bigger the lattice enthalpy
