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    I am studying A2 Edexcel board Chemistry. I cannot understand which definition I have to learn for lattice enthalpy because I have seen both being used in the blue A2 book by Ann Fullick etc. and the black Hodder textbook.

    These are the two definitions:

    1) The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive.

    2) The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. Lattice formation enthalpies are always negative.

    Which one do edexcel want?
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    (Original post by Lili94)
    I am studying A2 Edexcel board Chemistry. I cannot understand which definition I have to learn for lattice enthalpy because I have seen both being used in the blue A2 book by Ann Fullick etc. and the black Hodder textbook.

    These are the two definitions:

    1) The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive.

    2) The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. Lattice formation enthalpies are always negative.

    Which one do edexcel want?
    In the AS paper 1 June 2012 one of the questions quoted the lattice enthalpies as positive values. This suggests that they require the endothermic definition...
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    (Original post by Lili94)
    I am studying A2 Edexcel board Chemistry. I cannot understand which definition I have to learn for lattice enthalpy because I have seen both being used in the blue A2 book by Ann Fullick etc. and the black Hodder textbook.

    These are the two definitions:

    1) The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive.

    2) The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. Lattice formation enthalpies are always negative.

    Which one do edexcel want?
    Well the correct answer is 1. That doesn't mean the exam board will want that though, but you'd hope they would know which one was correct.
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    Obviously you should really know both, as they virtually the same (one is just reversed), but to the question "when Edexcel say lattice energy, what do they mean" in my experience (Edexcel AS student) it's lattice formation enthalpy (i.e. exothermic).

    (Original post by JMaydom)
    Well the correct answer is 1. That doesn't mean the exam board will want that though, but you'd hope they would know which one was correct.
    They're different enthalpies, the reaction of one and associated enthalpy is the reverse (negative) of the other. Why should either be wrong or right? They refer to different things ...
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    One topic may have two definitions and for here, they are defined in different context in both. You can write any of them.

    click on this
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    I've found chemguide really useful for things like this. The guy who made the website also writes the calculations book for chemistry as/a level. Anyway here's the link http://www.chemguide.co.uk/physical/...s/lattice.html
    On this page he states that they're basically the same but it depends on the question, the born haber cycle that you're looking at and what you're trying to find.

    i suggest learning both so that when you're faced with the question all you need to do is check the sign of the enthalpy change, or the direction of the arrows.
 
 
 
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