Enthalpy Change: Reaction between Calcium Carbonate and Hydrochloric Acid

[masochist]

I'm doing a lab on the reaction between calcium carbonate and HCl.

My independant variable is the mass of calcium carbonate.

So basically, I add in different masses of CaCO3 to show that more CaCO3 = higher temperature therefore higher enthalpy change until CaCO3 becomes limiting reagant.

However, I still need to calculate the enthalpy change for stuff. And I'm supposed to do it like this:

(e.g. from a previous lab using different chemicals.) by plotting the temperature and extrapolating the initial temperature to use in enthalpy change calculations.

But for some reason something WEIRD happens with Calcium Carbonate and HCl. Though the higher masses of CaCO3 (near and post being in excess/being the limiting reagant) follow the normal situation of adding in CaCO3 and then temperature getting really high and then gradually dropping as the calorimeter fails to retain all the heat, the lower masses do not. For some reason, the smaller masses - once their tmperature shoots up it gradually CONTINUES going up!

That REALLY confuses me! I have a theory (entirely based on my own guesses) that the calcium carbonate really CLUMPS so unless its so majorly in excess that its everywhere then it takes a while for it to disperse despite my attempts to shake around and evenly disperse the reactants while conducting the experiment.

Do you think that is an accurate guess/assumption? And if so, how should I write up my lab? Is there any way of doing it legitimately? Cause if I use these extrapolations to do my calculations all my final answers will be really messed up.. Should I just screw around with my data and like switch the temperatures? Ahh I don't know what to do .

Thanks so much for reading all that!

[Sandwich]

I never did this particular area of chemistry myself, so my apologies if this is wrong -- but could it be: the larger masses reach a peak of reaction, the CaCO3 becomes the limiting factor and therefore the reaction effectively stops. No heat is being given out, and heat is being lost to the environment, so temperature decreases. With smaller masses the reaction continues (this assumes it's a pretty slow reaction) and the temperature continues to rise as the reaction is still occurring -- whereas with the other one it stopped rising pretty quickly as the reaction stopped pretty quickly?

That may be totally flawed I was never that great at chemistry explanations.

All I would say is that if this happened consistently and several times, chances are it was meant to happen. SO if there IS a proper scientific reason (just you have no idea what it is), the person marking it will look at any edited data you give and be like HRMM well that's certainly not how it goes. There's a danger that by correcting for the unknown, you accidentally make a bigger mistake!

[masochist]

Thanks so much for your help.

I spent many days mulling over this but if the reaction were a slow reaction causing the temperature to continue increasing after the initial spike, that should still apply to the one with the CaCO3 in excess. If anything, the temperature should discontinue increasing at an earlier time, because there are LESS reactants. Do I make sense?

An idea I had was that: perhaps you are right in that this is a slow reaction and continues to increase post temperature spike, BUT the remaining CaCO3 (i.e. if it is in excess once everything reacts there will be leftovers) serves some sort of purpose in absorbing that heat and that would be why the temperature would drop post the spike.

Anyway thanks so much for your help it really got me thinking and hopefully I can still get a good mark on this lab haha.