posted by Gemma on .
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.