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Dispersion formula mystery...

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electrogeek:
Hi everyone,

I have been given the question that I've attached below about the use of the London formula to calculate the energy of interaction. I don't understand why the answer is positive however because I got a value of -8.74 kJ per mole...

Because all the numbers are positive which are going into the formula, the answer has to be negative like what I have calculated? Any help would be great because I've had another question on this formula and the same thing happens there where the answer is given to be positive but I calculate a negative value.

Cheers

Borek:
Perhaps it is a matter of convention - are you asked about energy of interactions, or energy of formation?

mjc123:
Since the same symbol V is used in question and answer, it ought to refer to the same thing, and therefore be negative.

electrogeek:
Thanks for the replies. I will have to ask my lecturer because on some revision problems which are about the same idea the values for V are negative in his mark scheme.

Thanks for checking though - just wanted to make sure there wasn't something that I was missing.

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