What is wrong with my calculation of combustion of ethanol?

Question

I'm doing a lab on combustion of ethanol and something is wrong with my calculations, but I can't tell what.

 

Molar mass of ethanol = 46,1g mol-1
Mass of ethanol used = 2g

n=2g/46,1g mol-1 = 0,04338 mol

Delta T = 31,7 - 24,9 = 6,8 degrees celsius

Energy = 4,184 J K-1g-1 x 20g x 6,8 degrees celsius = 569,024J = 0,569024kJ

0,04338 mol of ethanol produced 0,569024kJ of heat

Heat produced by 1 mol of ethanol = 0,569024kJ / 0,04338 mol = 13,12kJ mol-1

Answer 1

Hi Emilia,

Looks like you're heating 20 g of water by 6.8C, right? In a styrofoam cup calorimeter?

Your calculations look fine to me. If your value is far away from theory or values of your labmates, check to see what about your data is way off. Is it your temperature rise? If so, consider possible sources of error: 1) failure to capture all combustion heat 2) loss of some sample before combustion 3) incorrect value of calorimeter water mass 4) failure to stir calorimeter water to obtain true representative temperature

There may be other possibilities. Sometimes analysing sources of possible errors is the hardest part of an experiment. But it's the part that will teach you the most, in the long run. Scientists make errors all the time, just like other people (well, maybe not quite as often!). Don't get depressed if that happens -- just say, "OK, that didn't work so well. Let me do it better this time." Or talk it over with your teacher, TA, etc. if you need to pick some additional brains to consider the possibilities. Look at it this way: if everything went according to plan, nobody would ever make interesting discoveries. Experiments that "fail" might show you something about your technique, or .... maybe the world just got a whole lot more interesting. 

If it's any guide, I was a klutz in college chem labs -- then a happy analytical chemist as a career for 20 years.

Cheers