HELP!! Does anyone have an explanation for this?! My calculations are completely wrong? (1 Viewer)

[radioheadfan42]

Hello! Bloody hell im a bit confused.

You know how, erm, the more carbons there is in an alcohol the more negative the enthalpy of combustion, yes?

In a practical i did on the combustion of alcohols of increasing sizes, the opposite seems to apply. 1-Propanol has the highest enthalpy of combustion, and 1-Pentanol the lowest. Whats up with that?

Of course, it was most likely some flaws with the apparatus. Luckily we arent marked on how accurate our values are, rather how we go about explaining it all in the depth study.

Which is what I'm attempting to do now. I have a few explanations for why my results were off: it was an incomplete combustion (soot was on the can, got all over me fingers too) and my 1-Pentanol was a bit, erm, suspicious. Top seal was a bit broken, flame was dramatically huge. And it makes sense in my calculations, ish...It has a more negative enthalpy of combustion than 1-Butanol. Assuming the seal wasn't broken, though, it very well could have been lower than 1-butanol. Which is confusing me.

Heres what i got for each alcohol (rounded to 3 sigfig):

1-Propanol= -75.3 kJmol-1
1-Butanol= -24.3 kJmol-1
1-Pentanol= -42.9 kJmol-1

So, i need help! How on earf do i explain this? Maybe its my calculations? I can provide those if need be

Thank you so much!!

 

[wizzkids]

Don't be too discouraged by the inaccurate results. This is quite normal for student first-hand investigations of enthalpy of combustion. The errors are huge. You are right about the incomplete combustion causing an error; it is much more difficult to achieve complete combustion for pentan-1-ol because each 1 mole of C₅H₁₂O consumes 7.5 moles of O₂ and this presents quite a transport problem in the flame, to deliver all that oxygen to the reaction in a short time, so you will see quite a long, luminous yellow flame. That luminosity comes from the incandescent carbon particles in the flame.
You are quite right about the marking rubric - you are not going to be marked on the accuracy of the ΔH_c values. Your teacher will be looking for your understanding of the method, your processing of the data, and the quality of your discussion.