I’ve got a new chemistry assignment, which I’m a bit stuck with. I think my teacher makes some of the questions a little too hard, but they’re challenging, and that’s why I like them. However, if I’m getting stuck, I dread to think how some of the other students are getting on.
Anyhow, it’s supposedly organic chemistry, but the thermochemistry that I’m stuck with.
The data necessary for the question is here:
Alkane ¦No. Of C atoms ¦Relative Molecular Mass ¦Enthalpy Change of combusions/kJ per mol
Methane ¦ 1 ¦16 ¦-890
Ethane ¦ 2 ¦ 30 ¦-1560
Propane ¦ 3 ¦ 44 ¦-2220
Butane ¦ 4 ¦ 58 ¦-2877
Pentane ¦ 5 ¦ 72 ¦-3509
Hexane ¦ 6 ¦ 86 ¦-4195
I’ve been told to draw a graph of the enthalpy change against no. of carbon atoms, and that I’ve done. I’ve also been told to work out the intercept on the deltaH axis which is about -238kj per mol.
What is the physical significance of this value? This is the question that I’ve been stuck on. The only thing I’ve worked out so far is that it’s roughly one deltaHc of the CH2 structural group less than methane, as you would expect, since it’s a straight line graph, well near enough (y = mx + c, and C is obviously this value). I can’t work out what significance this value has, but it’s probably kickmyself obvious, and I’m going to look really silly when someone explains it to me.
If anyone can please help, that’d be great, it’s a shame there are so many people who can help with physics on SDMB, but far fewer chemists!