lecture 7🅱️ Flashcards
enthalpy change
the amount of heat evolved or absorbed for a reaction
under constant pressure
H
standard enthalpy change refers to what
enthalpy change when all reactants are in standard states and done in standard conditions
standard temp
25*C
standard pressure
1 bar
standard conc
1 mol dm-3
enthalpy change of formation
enthalpy change for a reation where 1 mol of product is formed from its reactants in their standard states
at standard conditions
enthalpy of atomisation
enthalpy change that occurs from going from a solid to a gas
ionisationenthalpy
enthalpy needed to remove electron from element
electron affinity
enthalpy change when an electron is added to an element
bond dissociation enthalpy
enthalpy change when bonds dissociate
eg: Cl2g -> 2Cl g
bde value can also be
2 x atomisation enthalpy
lattice enthalpy
enthalpy change when 1 ol of solid is converted to one mol of gas ions
endothermic (needs energy) to overcome strong electrostatic forces of attraction
- lattice enthalpy issss
exothermicccc
refers to the enthalpy of 1 mol of gaseous ions forming 1 mol of solid
lattice enthalpy +
solid to gas
lattice enthalpy -
gas to solid
hess cycle steps for NaCl
- write equation: Na(s) + 1/2Cl2g -> NaCl(s)
- atomisation from S -> G
- bde for Cl2 -> 2Cl
- ionise Na
- electron affinity for Cl
- gas to solid ( lattice enthalpy - )
hess law definition
total enthalpy change is independednt of route taken
total enthalpy is the sum of the enthalpy for individual steps
if the states remain the same for pro and rea
born haber cycle can be written in 2 ways — its a diagram of individual step reactions
hess law cycle : the new thing
enthalpy level diagram: alevel one
bde is going from a diatomic to 2 monatomics so what do we need to do to the dbe value if we only need 1 monatomic (eg: Cl in NaCl)
u need to divide the dbe by 2
or multiply bde by 1/2
experimental lattice enthalpy
use hess cycle or haber born cycle
delta H formation =
atom + bde + ie + ea - lattice
lattice =
atom + bde + ie + ea - formatioon
lattice is normally
solid to gassss
endothermic
solid to gas
endothermic
+ lattice enthalpy
gas to solid
exothermic
- lattice enthalpy
experimental lattice enthalpies
born harber cycle
hess cycle
theoretical lattice enthalpy
use of ionic model
- ions = hard sphere character
- defined ionic radius
- ions are sep by internuclear distance ‘r’
- consider total electrostatic attractions to find lattice energy
- difference in potential energy between free ions and ions bound together in solid.
madelung constant
accounts for repulsive and attractive forces between ions in a solid, A
depends on structure type
if we use madelung constant we also need
avo constant
6.022 x 10^23
what else is wrong
ions arent point charges,, their ion charge can overlap
we use born exponent , n. to account for this.
n for 2 elements of the same exponent group
n = just that singular number
n for 2 elements from different exponent groups
add both then divide by 2
basically just get an average of the 2.
born lande equation
gives theoretical lattice enthalpy
z+. z-
multiply the ion charges
what is r
interionic distance
distance from nuc to nuc
radius + radius
difference between theoretical (BL) and experimental (HB HC)
BL is based on ionic model,, this is flawed.
- no compound is 100% ionic
- e- cloud can be polarised if large and negative (low charge)
- small and high charge ion (+ ions) can polarise the e- cloud.
- this gives higher covalent character
what can born lande be used for
assessing how ionic a compound is
if theoretical ( BL) and experimental (HB HC) are similar = ionic character
big difference = some covalent character present..
describe this quantitatively!! find the %difference.
born mayer equation
better born lande one
takes ion ion repulsions into account
p is constant but in pm,, so interionic distance must also be converted into pm
kapustinskii equation
no knowledge of crystal structure needed,, A or r
lattice energy in kj mol-1
must be in pm
