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I've learnt that the lattice enthalpy (defined as the energy change from a solid ionic lattice to separate gaseous ions) is always positive, obviously. However, I've seen it explained as the opposite other places, so it's negative.

What is correct?

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up vote 2 down vote accepted

When it is explained as opposite, it is usually the heat RELEASED when a crystal is formed, and since the word "released" already makes up for the sign of heat(i.e. if heat is -10 that means the heat released or evolved is +10) , it is taken as positive. Nonetheless, some books also present a negative value to it but it doesn't matter unless you are careful about it in calculations.

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