Comparing Lattice Enthalpies Flashcards
lattice enthalpy of formation
The enthalpy change when 1 mole of an ionic compound is formed from its constituent gaseous ions
Theoretical lattice enthalpies are…
The enthalpy change when 1 mole of an ionic compound is formed from its constituent gaseous ions assuming a perfect ionic model where the ions are perfectly spherical and the charge is evenly distributed within this sphere
What does the perfect ionic model in theoretical lattice enthalpies assume?
The ions are perfectly spherical
There is even distribution of charge in this sphere
How do experimental lattice enthalpies differ to theoretical ones?
Experimental ones don’t follow perfect ionic model but takes into account the possession of covalent characteristics within the ionic compound: distortion of compound called polarisation
How does distortion occur in an ionic lattice?
When the positive ion distorts the distribution of charge in the negative ion
Polarisation meaning
The distortion within experimental ionic lattices where the positive ion distorts the distribution of charge in the negative ion
What gives the experimental ionic compounds covalent character?
The polarisation in the ionic lattice
More polarisation = more covalent character
What effects the polarisation of the ionic compound?
Size of positive ion/negative ion
Charge of positive ion/negative ion
Size of cation effect on polarisation?
Smaller cations have more polarising power so cause more covalent character compared to big ones
They have high charge density cos the charge is concentrated in a smaller area so pulls electrons in the amino to itself more
Size of anion effect on polarisation
Larger anions are polarised easier than smaller anions
Because the electrons are further away from the nucleus so can be pulled towards cations thus polarised easier
Charge of cation
Larger charge means more charge to pull electrons towards an ion thus causes more polarisation meaning has more covalent character
Charge of anion
Larger charges means more repulsion between electrons to be pulled towards cations thus more polarisation so has more covalent character
Comparing lattice enthalpy values ,,,
Greater difference means the ionic compound is less purely ionic
And contains more covalent character
More polarising power thus more covalent character…
Means stronger thus larger value than if it was 100% ionic
Comparing experimental lattice enthalpies from one ionic compound to another
More exothermic value means stronger ionic compound (so more energy required to break ionic compound)
Because of differences in ionic charge/radius
