Born/Haber Help

Hiya,
Here's a link (idk how to put the pic here) - https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FPhysical-II%2F1.08-Thermodynamics%2FSet-F%2FBorn-Haber%20Cycles%205%20QP.pdf
but for question 1aii I am getting the answer -347.5, which is the perfect inverse of the correct answer. The method I'm trying to use is starting at the end of the desired arrow and moving around the cycle. so -2(122) -965-503-180+-859--2056 and then divide by 2

Reply 1

UtterlyUseless69

Original post by mitostudent

The formation enthalpy is the sum of all the other enthalpy changes involved (scaling things like atomisation enthalpies as appropriate), so we have

-859 = 180 + 2(122) + 503 + 965 + 2x - 2056

This simplifies to

-859 = 2x - 164

And this does indeed lead to x = -347.5 kJ/mol

Strictly this is the electron gain enthalpy, which you would expect to be negative as there is attraction between the +ve nucleus of the chlorine atom and the -ve electron. The electron affinity is the negative of the electron gain enthalpy and so would have a value of +347.5 kJ/mol. I honestly thought knowledge of this wasn’t expected at A level and that the distinction between electron gain enthalpy and electron affinity was an undergraduate concept.

EDIT: I have now found the MS for the sheet where you found this question and the answer does require a -ve sign - the answer is written weirdly in the solution and it does stipulate that you lose a mark for assigning a +ve sign. You do not need to know the distinction between electron gain enthalpy and electron affinity.