Homework Help: chemistry

Posted by Gemma on Tuesday, February 8, 2011 at 12:05pm.

standard enthalpies of formation

H20(l) = -286 LiOH(s) = -487

standard enthalpies of solution for LiOH(s) to split into it's ions is 21

all values are in KJmol-1

Use the data given to calculate a value for the enthalpy change for the reaction of lithium with water.

I get how to do the cycle and I've seen the mark scheme and I don't understand why the 21 becomes negative.

I can draw my cycles. I use the equation in hess's law:

AH1 = AH2+AH3
= 286 - 487
= -201 and that is the wrong answer, it has -21 aswell so the answer is -222. I would have thought that if the solution was being formed then it would have a positive value as you are going in the right direction, is the mark scheme wrong or is there something I don't understand.

Thank you for any help you can provide.

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