Lattice energy and Born-Haber cycles Flashcards
Define enthalpy change of atomization
The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
The ΔHatꝋ is always endothermic as energy is always required to break any bonds between the atoms in the element
What are the standard conditions
temperature of 298 K and a pressure of 101 kPa
Define Lattice Enthalpy mentioning whether it’s exothermic or endothermic
the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)
The ΔHlattꝋ is always exothermic, as when ions are combined to form an ionic solid lattice, there is an extremely large release of energy
Define First Electron Affinity
the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms, to form 1 mole of gaseous ions each with a single negative charge under standard conditions
X(g) + e- → X-(g)
EA1 is usually exothermic, as energy is released
Why are EA2 and EA3 usually endothermic
This is because the incoming electron is added to an already negative ion, energy is required to overcome the repulsive forces between the incoming electron and negative ion
Factors affecting EA
Nuclear charge: the greater the nuclear charge, the stronger the attractive forces between an incoming electron and the nucleus
Distance: the greater the distance between the nucleus and the outermost shell/orbital where the electron is added, the weaker the force of attraction
Shielding: the greater the number of shells, the greater the shielding effect and the weaker the force of attraction
What’s the trend of electron affinities for non-metals across a period?
become more exothermic across a period, with a maximum at Group 17 but the pattern
is not always clear
The pattern of electron affinities down Group 16 and 17
The electron affinities generally become less exothermic for each successive element going down both Groups, apart from the first member of each Group (oxygen and fluorine respectively)
The reason for decreasing electron affinities down Group 16 and 17
The outermost electrons are held less tightly to the nucleus as they are further away
The number of electron shells increases causing an increased shielding of the outermost electrons
It gets more difficult to add an electron to the outer shell
Less energy is released upon adding an electron to the outer shell
So generally, the EA1 becomes less exothermic
Use an example to explain the increase of electron affinities across a period
An atom of chlorine has a greater nuclear charge than an atom of sulphur
Chlorine will therefore have a greater attractive force between its nucleus and its outer electrons
More energy is released upon adding an electron to chlorine, so the EA1 of Cl is more exothermic than for S
Why does a Fluorine have a smaller EA1 than Chlorine
Fluorine has a very small atomic radius
This means that the electron density of fluorine is high
There is more repulsion between the incoming electron and the electrons that are already present in fluorine
These repulsive forces reduce the attractive forces between the incoming electron and nucleus
As a result, the EA1 of fluorine is less exothermic than expected
Why is Born-Haber cycle constructed
To calculate the lattice energy (ΔHlattꝋ) of an ionic compound
Hess’s law
the enthalpy change in a chemical reaction is the same regardless of the route taken, as long as the final and initial conditions and states of reactants and products are the same for each route
ΔHfꝋ=
ΔHatꝋ + ΔHatꝋ + IE + EA + ΔHlattꝋ
or
ΔH1ꝋ + ΔHlattꝋ
(ΔH1ꝋ = the sum of all of the various enthalpy changes necessary to convert the elements in their standard states to gaseous ions)
ΔHlattꝋ =
ΔHfꝋ - ΔH1ꝋ
