Ask A Scientist Chemistry Archive

  
name         K. H.
status       educator
age          40s

Question -   I am doing a lab of calculation of the heat of fusion for
"parowax"( a brand name of paraffin wax. Could you tell me the formula
for paraffin ( I have found several answers with carbon values of 22-26)
and where could I find the accepted value for the heat of fusion of
paraffin. It would greatly appreciated if the value was in kJ/mol.
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There is a fundamental problem with this experiment if you require accurate
results. "Paraffin wax" is not a single well defined chemical substance.
Rather it is a mixture of hydrocarbons. That is why you are finding
different carbon values. The product(s) you find on the store shelf is
likely formulated to give a range of softening points depending upon the
intended use. Consequently it does not have a sharp "melting point" like a
pure chemical substance. You can find soft waxes, like Vaseline, to rather
hard brittle waxes that would be used in a furniture finish and all
varieties in between. In addition, long chain hydrocarbons, even pure ones,
frequently do not have a simple melting temperature. They have a number of
phase transitions at temperatures less than the conventional melting
temperature, so that they tend to become less crystalline and more glassy
as they are heated.

Vince Calder
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Unfortunately, "paraffin" is a somewhat general term meaning:
"approximately saturated hydrocarbons of unspecified molecular weight".

My CRC  has an old table of heats of fusion.  They vary, from 30.7 cal/gm 
for n-tricosane (C23H48) to
39.1 cal/gm for n-pentacosane (C25H52), to 59.1 cal/gm for n-eicosane 
(C20H42) (so radically different, could that number be wrong?).
The crystallization properties of alkanes are generally very inconstant, 
seemingly unpredictable, from one molecular weight to the next.
I think this is due to geometric stacking problems or strengths,
which reasonably can depend on the exact number of carbons in the zig-zag 
chains.
Properties would vary with branching too, but there is less specific data 
around for those species.

If there was a constant heat-of-fusion value for wax,
it would represent  Van-der-Waals attraction between the sides of long 
(CH2)x chains,  constant per unit length,
and so it would be nearly constant only if expressed in kJ/gm.
When expressed in kJ/mol, it would instead be proportional to molecular 
weight.
If you want to convert all those varying cal/gm figures to kJ/mol figures, 
I recommend you use a computer spreadsheet.
I think they still will not show a simple, consistent trend.

Meanwhile, your Parowax is probably a mix of species, defined by the 
distillation process it was
(quite economically) manufactured with.
Wax is sold in the grocery stores primarily for sealing the top surface of 
the food in glass jars, in home food "canning".
In this use, a moderate melting range is not objectionable.
Possibly it's even beneficial, because it implies a bit of yield and creep 
and slow self-healing of any cracks on the solid wax.

Waxes of one large molecular weight can seem like very dry, densely 
frostly putty.
And they transition from entirely solid to entirely liquid over a narrow 
(~1 degree C) range of temperature.
The addition of lower weights to this makes the wax feel more "oily", look 
a little clearer,
and melt over a wider range of temperature.
Going farther, C35 wax dissolved in C14 oil might be a wet but thick 
grease, a little less sticky than petroleum jelly.
So, find out from doing your experiment if your parowax has a narrow 
melting range.
Will your heat-of-fusion measurement be impaired if the melting range is 5 
or 10 degrees C?

I have a small jar of pellets of 99% pure C30 or C34, fairly expensive, 
from Alfa-Aesar.
Even-numbered lower weights are less expensive:  C24 is  $60/100gm;  C18 
is $20/100gm.
But C18-wax almost melts at room temperature.
97% Octadecanol (H-(CH2)18-OH) or 99% hexdecanol are a little nicer, 
50&60degC, $14/500gm.
My CRC has 33.8 cal/gm for cetyl alcohol (hexdecanol).
Multiplying by mw 242.45 gm/mol, and 4.184  J/cal, that would be 34.3 kJ/mol.
(Cetyl alcohol is a major component in most hair-conditioners and 
hand-lotions.)
I'd heat these to 100C to drive out water, at the beginning of an 
experiment day.

I might take the average carbon-number for your parawax to be 24, and the 
average heat of fusion to be 37 cal/gm.
37 cal/gm x 4.184  J/cal =  155 J/gm.
(CH2):=(12+1+1) gm/mol;  (14  gm/mol x 24 )=  336 gm/mol.
155 J/gm * 336 gm/mol =  ~52 kJ/mol.
But I wouldn't give that estimate much credence.

I am wondering if pure beeswax is a good material to try, with respect to 
price and melting range.
Bags of granulated wax for candle-making, in art stores, might have a 
narrower melting range than parowax.
Utility candles from the grocery store are not pure hydrocarbon.  There is 
a white powder that forms
when one tries to dissolve the wax in heptane;  I think it might be 
stearic acid.

cordially,

JS
====================================================
KH,

On this website (http://www.hotwatt.com/paraffin.htm), I find the heat of
fusion for paraffin listed as 63 BTU/lb.  On this web site
(http://www.iscienceproject.com/labs/finished_labs/6338_heatcombustion.html)
I find the chemical formula for paraffin to be C36H74.

So the gram formula weight for paraffin is 506 g/mol.  Converting the units,
I get the heat of fusion for paraffin to be 74 kJ/mol (Please check my
math!)

Todd Clark, Office of Science
U.S. Department of Energy
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