A-Level Chemistry Help??

Using a Hess' triangle I've calculated an enthalpy change as +87.9KJ/mol, yet the given standard enthalpy change is +95KJ/ mol. I'm being asked reasons for this and how the experiment could be improved to cater for these. Could anyone please help??

Reply 1

eastereggfun

I wouldn’t be entirely sure without seeing the actual question, but I take A level chemistry, and it sounds like it could be because there is slight covalent character. The theoretical values assume that ionic compounds are purely ionic, but in actual fact most ionic compounds have slight covalent character, changing the enthalpy. Another thing to consider is mean bond enthalpy is different from experimental bond enthalpy. Not sure how to improve the experiment though. Out of interest, where was this question from?

Reply 2

Timtim1020

Original post by eastereggfun

Hi, thanks for your answer! Our teachers give us these practical booklet things which have extension questions at the end of each CPAC. I presume they just come up with the questions themselves. I can send a pic of the full question and the practical if you need??

Reply 3

eastereggfun

Sure if you are able to. Thinking about it more, it could be that in the practical heat is lost to the surroundings. You could improve by using a polystyrene cup instead of a glass beaker, or if they already mention that, then you could put a lid on it. Did you get the answer in the end?