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# Hess's Law - enthalpy change question

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1. So I know it all adds up +344 ----> +350.

But how is -3 the answer? Why is it overall divided by 3?

Thanks
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2. (Original post by _Lizabeth_)
So I know it all adds up +344 ----> +350.

But how is -3 the answer? Why is it overall divided by 3?

Thanks
The question can't be answered without the enthalpy of vaporisation of both bromine and iodine ...
3. (Original post by charco)
The question can't be answered without the enthalpy of vaporisation of both bromine and iodine ...
This is the answer, I just want to know why it's /2
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4. (Original post by _Lizabeth_)
This is the answer, I just want to know why it's /2
The enthalpy of formation only requires 1/2 mole of each of the halogens....
5. (Original post by _Lizabeth_)
So I know it all adds up +344 ----> +350.

But how is -3 the answer? Why is it overall divided by 3?

Thanks
0.5 I2 + 0.5 Br2 --> 1 IBr
Formation infers 1 mole (enthalpy of formation)

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