Essay Example on Trends in Gibbs Free Energies of Formation








Trends in Gibbs Free Energies of Formation Heat Capacities Enthalpies of Vaporization and Viscosities vs Strait Alkane Length Table 1 Thermodynamic and fluid properties vs length of straight alkane length1 2 3 4 Alkane Δ_f G at 298 K kJ mol ΔH_vap at Boiling Point kJ mol C_p at 298 K J mol K µ at 298 K µPa s Methane 50 8 8 6 37 39 10 3 Ethane 32 9 15 7 52 49 11 Propane 23 4 19 5 73 60 7 4 Butane 15 6 23 4 98 49 7 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 at first through the use of Google Finding reliable data tended to be easier the smaller the alkane was Usually the source used for one property of methane always had the next five alkanes as well Searches about octane viscosity for example 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 gives reliable verified information which ended up saving a lot of time One source that was particularly astounding to me was a 332 page paper written by James S Chickos and William E Acree Jr1 It contains tabulated data for thousands of molecules enthalpies of vaporizations over many temperature ranges The references alone are over three pages as well Figure 1 Alkane length vs Gibbs Free Energies of Formation 2

The plots above are graphical representations of the tabulated data They were generated using Matlab and aid in the visual discovery of trends The first three plots generally have the same shape The Gibbs free energy plot has the most logarithmic progression of the three with a high slope at the beginning that tapers off The first six alkanes have negative Gibbs free energies of formation This means that under the same pressure and temperature the constituent elements of that alkane will react and produce some amount until equilibrium is reached The logarithmic shape may be due to how the reaction quotient changes with alkane length as well As stoichiometric constants shift due to the approximate doubling of hydrogen atoms with respect to carbon the reaction quotient essentially goes to zero The resulting change in Gibb s also decreases according the following5 ΔG_f ΔG ΔG RTln Q Enthalpy and Heat Capacity are both related in that they deal closely with molecular vibrations and specific energy changes of molecules As mass of molecule increases with a constant source of energy would result in overall less internal motion A simplified and classical approach to energy says KE 1 2 mv 2 If the claim is that temperature is the perceived molecular velocities and other more complicated forms of movement the above equation explains how an increase in mass would result in some decrease in velocity which is related to temperature 

The mass depends on the molecular weights of both hydrogen and carbon The resulting velocities end up being related inversely to the square roots of the mass v 2KE m There are much more complicated phenomena occurring deeper beyond simple translational molecular motion This simplified model gives a rough understanding of why changes in enthalpy or heat capacities are positive as a function of alkane length As for viscosity the data was difficult to find reliably at one specific temperature so errors that propagate through changes in temperature may be slightly present The general trend is they decrease with alkane length but slightly increase as the change of state happens at heptane and octane Intuitively this makes no sense Low viscosities are associated with less resistance to shear forces which are characteristic of gases The discrepancy may stem from the viscosity depending on the temperature differently for liquids vs gases Extrapolation is very much possible for the enthalpy and heat capacity gas due to their linearity It would be possible for the Gibb s graph as well but the error would increase greatly as the extrapolated value increased

Extrapolation would be risky for the viscosity graph but still possible The points do follow as general pattern of decreasing but once the state of matter change happens it may not prove effective Plagiarism and Academic Dishonesty as accordance to Arizona State University My definition of plagiarism is the use of another s ideas or words without giving the company person collection of people proper credit There are many websites today that can assist teachers and students in the detection of plagiarism I personally have had teachers use these websites throughout all of my high school years whenever I submitted something on blackboard They work by crosschecking the text of a paper verbatim to sources on the internet Most websites return a percentage match that shows what percent your paper was copied from the internet At the time of writing this a free website plagiarism test says this paper is 100 unique 7 A second website

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