Trendsin Gibbs Free Energies of Formation, Heat Capacities, Enthalpies ofVaporization, and Viscosities vs. Strait Alkane Length Table 1: Thermodynamic and fluid properties vs.length of straight alkanelength1 2 3 4 Alkane at Boiling Point Methane -50.8 8.6 37.
39 10.3 Ethane -32.9 15.7 52.49 11.0 Propane -23.
4 19.5 73.60 7.4 Butane -15.6 23.4 98.49 7.
0 Pentane -9.2 26.7 120.1 11.7 Hexane -4.2 31.5 196.
5 6.5 Heptane 1.3 32 224.7 6.5 Octane 6.
4 36.3 255.6 7.5 The table above was constructed atfirst through the use of Google. Finding reliable data tended to be easier thesmaller the alkane was. Usually the source used for one property of methanealways had the next five alkanes as well.
Searches about octane viscosity, forexample, either did not have a reliable source, or cost a hefty fee to look at.Most of heptane’s and octane’s values came from Knovel. Knovel always givesreliable, verified information, which ended up saving a lot of time. One sourcethat was particularly astounding to me was a 332 page paper written by James S.Chickos and William E.
Acree, Jr1. It contains tabulated data forthousands of molecules’ enthalpies of vaporizations over many temperatureranges. The references alone are over three pages as well. Figure 1 Alkane Length vs. Gibbs Free Energies of Formation2 Figure 1: Alkane length vs. Gibbs FreeEnergies of Formation 2 The plots above are graphical representationsof the tabulated data.
They were generated using Matlab and aid in the visualdiscovery of trends. The first three plots generally have the same shape. TheGibbs free energy plot has the most logarithmic progression of the three with ahigh slope at the beginning that tapers off. The first six alkanes havenegative Gibbs free energies of formation. This means that under the samepressure and temperature, the constituent elements of that alkane will react andproduce some amount until equilibrium is reached. The logarithmic shape may bedue to how the reaction quotient changes with alkane length as well.
As stoichiometricconstants shift due to the approximate doubling of hydrogen atoms with respectto carbon, the reaction quotient essentially goes to zero. Theresulting change in Gibb’s also decreases according the following5 Enthalpy and Heat Capacity are both relatedin that they deal closely with molecular vibrations and specific energy changesof molecules. As mass of molecule increases, with a constant source of energywould result in overall less internal motion.
A simplified and classicalapproach to energy saysIf the claimis that temperature is the perceived molecular velocities (and other morecomplicated forms of movement), the above equation explains how an increase inmass would result in some decrease in velocity, which is related totemperature. The mass depends on the molecular weights of both hydrogen andcarbon. The resulting velocities end up being related inversely to the squareroots of the mass.
There are much more complicated phenomena occurring deeperbeyond simple translational molecular motion. This simplified model gives arough understanding of why changes in enthalpy or heat capacities are positiveas a function of alkane length. As for viscosity, the data was difficult tofind reliably at one specific temperature, so errors that propagate throughchanges in temperature may be slightly present. The general trend is theydecrease with alkane length, but slightly increase as the change of statehappens at heptane and octane. Intuitively, this makes no sense.
Lowviscosities are associated with less resistance to shear forces which arecharacteristic of gases. The discrepancy may stem from the viscosity dependingon the temperature differently for liquids vs. gases.
Extrapolation is very much possible for theenthalpy and heat capacity gas due to their linearity. It would be possible forthe Gibb’s graph as well but the error would increase greatly as theextrapolated value increased. Extrapolation would be risky for the viscositygraph but still possible.
The points do follow as general pattern of decreasingbut once the state of matter change happens it may not prove effective. Plagiarismand Academic Dishonesty as accordance to Arizona State University My definition of plagiarism is the use of another’sideas or words without giving the company/person/collection of people propercredit. There are many websites today that can assist teachers and students inthe detection of plagiarism. I, personally, have had teachers use thesewebsites throughout all of my high school years whenever I submitted somethingon blackboard. They work by crosschecking the text of a paper verbatim tosources on the internet. Most websites return a percentage match that shows whatpercent your paper was copied from the internet.
The first time I used one ofthese websites I was shocked when a paper turned