Energaetics 2 Born Harber Flashcards
What two processes can lattice enthalpy be
Formation or dissociation process
What is a formation process and endo or eco
The enthalpy change when one mole of an ionic compound is formed from its gaseous ions
Therefore is exothermic as when ions are combined to form a ionic lattice , a large amount of energy is released
What does the large negative value of enthalpy of lattice suggest.
The compound is very stable as there are strong electrostatic forces of attraction between the oppositely charged ions.
The more exothermic the value , the more stable the lattice
Why are Born harder cycles needed
The enthalpy lattice cannot be found direction by one single experiment
So multiple experimental values are used
How to draw a born harber cycle
Draw the elements with their state symbols about a third way up the diagram draw horizontal line across.
Begin to atomize one of the elements to make it a gas. Enthalpy goes up then arrow up. Don’t forget to multiple if there is many of the atom
Essentially repeat for both atoms until they are gaseous IONS,
Then big arrow down for lattice enth
How to calculate lattice enthalpy in born harber cycle
Enthalpy f = all others + lattice enth
what d
o theoretical lattice enthalpys assume
a perfectly ionic model
spherical ions
no covelant charecter
purely electrostatic attractions
why is experimental lattice enthalpy often larger than theoretical
in reality ions have covelant charecter meaning that the cation is polarising which distorts the electron cloud of the anion.
which cations will have the largest polarising power
cations with the largest charge density
how to calculate which ion will have the largest charge density
charge of the cation / surface area (ionic radius) of cation.
what happens to anions in lattice
they are polarised
what does the polarisability of the anion depend on
its ionic radius
the larger an anion, the more easily distorted it is.
what is the standard enthalpy change of solution
the enthalpy change wjen 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution.
is the standard enthalpy change of solution exo or endo
can be either
(aq shows if a substance is dissolved in sufficient water)
what is the standard enthalpy change of hydration
the enthalpy change when one mol of a specific gaseous ion dissolves in sufficient water to form an infinitely dilute solution.
why are hydration enthalpies always exothermic
it is the measure of the energy RELEASED when there is an attraction formed between ions and water molecules
what happens when an ionic solid dissolves in water
positive and negative ions are formed
water is polar so ion-dipole interactions are formed
oxygen atom attracted to the positive ions and the H to the anions
what is the hydration enthalpy the sum of
all the hydration enthalpies of each ion
(if there is more than one ion eg. MgCl2 - you must multiply by the appropriate number for that ion)
relationship between enthalpy of solution and hydration enthalpy
enthalpy of solution = reverse lattice enthalpy + hydration enthalpy.
what are the 2 routes to go from gaseous ions to ions in solution (aq)
gas ions –> solid ionic —> ions aq
gas ions —-> ions in aq
hess law ethalpy change for solutes of hydration and solution
hydration enthalpy = lattice enthalpy + solution enthalpy
what are the two factors that affect lattice enthalpy
charge and radius of the ions in the lattice
how does ionic radius result in a more exothermic lattice enthalpy
the charge on the ions is more spread out
ions are also further apart in the lattice
therefore electrostatic forces of attraction between the oppositely charged ions are weaker
how does a greater ionic charge result in a more exothermic reaction
there is ahigher charge density
stronger electrostatic attraction between the oppositely charged ions in lattice
so more exothermic
