OCR A2 Chemistry: Lattice Enthalpy

The small section on lattice enthalpy made into handy little revision notes for the impending exam tomorrow :) x

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Emily Summers
Lattice Enthalpy
The standard lattice enthalpy is the enthalpy change when one mole of a solid ionic
compound is formed from it's gaseous ions from it's constituent elements under standard
It is a measure of ionic bond strength.
Na+(g) + Cl-(g) NaCl(s)
Figure 1 from chemguide
Enthalpy change of hydration: The enthalpy change when one mole of isolated gaseous
ions is dissolved in water to form one mole of aqueous ions under standard conditions
Na+ (g) + aq Na+ (aq)
Enthalpy change of solution: The enthalpy change when one mole of compound is
completely dissolved in water under standard conditions
NaCl(s) + aq NaCl (aq)

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Emily Summers
It can be positive or negative!
Calculating Lattice Enthalpy from Enthalpy Changes of Solution and Hydration
Route One: Lattice enthalpy and enthalpy change of solution
K+ (g) + Cl- (g) KCl(s)
KCl(s) + aq K+ (aq) + Cl- (aq) +26 KJ Mol-1
Route Two: Hydration of K+ (g) and Cl- (g) ions
K+(g) + aq K+ (aq) -322 KJ Mol-1
Cl-(g) + aq Cl- (aq) -363 KJ Mol-1
Put them together:
-322 + (-363) = Lattice enthalpy + 26
Lattice enthalpy = -322…read more

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Emily Summers
Factors Affecting Lattice Enthalpies and Enthalpy of Hydration
Lattice Enthalpy:
Ionic Size
Smaller ions pack more closely together so attract each other more strongly in a
lattice, whereas larger ions are further apart in the lattice and are attracted less
As ionic radius increases:
Attraction between ions decreases
Lattice enthalpy is less exothermic
Ionic Charge:
The stronger the charge, the more exothermic the lattice enthalpy.
So small, highly charged ions in a lattice have the MOST exothermic lattice enthalpies.…read more


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