ps1265A
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My book states the following:
Lattice enthalpy is a measure of the strength of ionic bonding - the more negative the lattice formation enthalpy the more energy is required to break the structure apart, so the stronger the ionic bonding.

Now what I am confused about is whether lattice enthalpy refers to lattice dissociation enthalpy or lattice formation enthalpy.

It makes sense that it is lattice dissociation enthalpy as you are litterally splitting up the molecule whereas formation is the opposite. So why does the definition say "lattice formation enthalpy"?

Thanks in adv!
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Pigster
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Strictly, the term lattice enthalpy should always refer to dissociation and should always be +ve.

At A-level it usually pops up in Born Haber calcs and usually refers to lattice formation, so we lazily call it formation, rather than trouble ourselves with -ve signs.
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username1445490
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(Original post by Pigster)
Strictly, the term lattice enthalpy should always refer to dissociation and should always be +ve.

At A-level it usually pops up in Born Haber calcs and usually refers to lattice formation, so we lazily call it formation, rather than trouble ourselves with -ve signs.
It depends whether you are talking about lattice enthalpy of formation (the enthalpy change when 1 mole of an ionic compound is formed from its constituent gaseous ions) or the lattice dissociation enthalpy (the enthalpy change when 1 mole of ionic compound is disassociated into its constituent gaseous ions). The size of the enthalpy change for each of these is the same but the signs are different. Formation is exothermic (negative delta H) and dissociation is endothermic (positive delta H).
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Pigster
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The OP asked what lattice enthalpy is.

If exactly that is the Q, then my version is correct.

If you introduce the terms formation and dissociation, then you're asking a different Q. And we will agree as to what those definitions are.
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username1445490
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Some A level courses (OCR in particular) only deal with lattice formation enthalpy and so delta H latt is always negative in their literature. The quote relates lattice enthalpy to ionic bond strength and so the quote is saying the more negative (ie the larger the value) of the lattice enthalpy the stronger the ionic bonds and so the more energy needed to separate the ions.
It is a strange way of phrasing it and like many text books not a very clear way of expressing a concept.

The term lattice enthalpy on its own does not refer to either dissociation or formation as it refers to the numerical value (which is the same for each) rather than the sign (+ or -).

eg Lattice enthalpy for MgO = 3791 kJ/mol
Lattice dissociation enthalpy for MgO = +3791 kJ/mol
Lattice formation enthalpy for MgO = -3791 kJ/mol
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ps1265A
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(Original post by Madasahatter)
Some A level courses (OCR in particular) only deal with lattice formation enthalpy and so delta H latt is always negative in their literature. The quote relates lattice enthalpy to ionic bond strength and so the quote is saying the more negative (ie the larger the value) of the lattice enthalpy the stronger the ionic bonds and so the more energy needed to separate the ions.
It is a strange way of phrasing it and like many text books not a very clear way of expressing a concept.

The term lattice enthalpy on its own does not refer to either dissociation or formation as it refers to the numerical value (which is the same for each) rather than the sign (+ or -).

eg Lattice enthalpy for MgO = 3791 kJ/mol
Lattice dissociation enthalpy for MgO = +3791 kJ/mol
Lattice formation enthalpy for MgO = -3791 kJ/mol
Thanks Madasahatter, I understand it now!

And thanks for the buffer calculations you PM'd me about!
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