Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Zero_1846 on January 07, 2023, 08:15:28 PM
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My boss asked me to tabulate thermodynamic data (Delta H, S and G) of different organic compounds (ethanol, ethylene, butadiene etc.) in the rage from 473K to 673K, but I have absolutely no clue how to obtain this data… Can anyone of you help me? Thanks!
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The CRC Handbook and NIST Chemistry Webbook are good places to start. But they may have limited data for wide ranges of temperatures. You can use the assumption that enthalpy and entropy changes are invariant to temperature over small ranges. But for large ranges you may need to seek out dedicated studies in the peer reviewed literature.
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I found NIST but how can I obtain the data from this page?
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You type in the compound or formula you want, then scroll down to where the thermodynamic data is.
https://webbook.nist.gov/chemistry/
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And how I’m applying this data to the temperature? My boss wants me to do it in 20 degree steps.
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Your boss may be asking for something that doesn't exist. Either you need to search the peer reviewed literature for whatever compound you need and hope you get lucky. Or make some assumptions and estimate.
Sounds like you need to talk to your boss for more guialdance.
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Ok, thank you very much for your *delete me* I’ll talk to him at Monday.
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The normal practice is to measure such data at one or very few meaningful temperatures, where equilibria exist, then publish data extrapolated to room temperature, from which the user will extrapolate it again to the desired temperature. Formulas exist for the operations, they involve mainly the heat capacity, or better its integral.
The operation isn't completely trivial. It raises interrogations about its reliability, since errors occur frequently (mind the aggregation state!), and its accuracy because equilibria depend on exponentials of the data, while the proper data for extrapolation isn't always available. A heat capacity at flame temperature differs much from room temperature and is measured for few compounds - you're lucky with ethanol and ethylene. One must also use data that is meaningless, like the heat capacity of gaseous alumina or atomic hydrogen at room temperature, and amounts that may underflow or overflow a calculator.
So when you can use proven software to compute equilibria, it's safer, and much faster.
But sometimes, hand computations work.
Especially for the common compounds you mention, good data may well exist in the Janaf tables, which are meant initially to compute equilibria in flames, more specifically in rocket engines. They are accurate, but use data formats less convenient for hand computations, with polynomials everywhere.
1000 pages:
https://www.govinfo.gov/app/details/GOVPUB-C13-bfd606acc2525ccef2762b19002a6d4f
https://janaf.nist.gov/
I'm not fully convinced that an equilibrium computation makes sense for butadiene at +400°C. I expect it to polymerize. Ethylene at low pressure maybe.
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I'm not sure what your background is. In order to calculate the hentalpies and entropies at certain temperatures, you need:
[tex]\Delta H = \int^{T_{2}}_{T_{1}} C_{p} dT[/tex]
[tex]\Delta S = \int^{T_{2}}_{T_{1}} \frac {C_{p}}{T} dT[/tex]
[tex]\Delta G = \Delta H - T\Delta S[/tex]
Over the range of temperature you listed, you can actually use the approximation:
[tex]C_{p} = a + bT + \frac {c}{T^2}[/tex]
a, b and c are tabulated values that only depend from the substance.
So, in practice, for each compound:
- Look up ΔH°, ΔS° and ΔG° values at 273 K;
- Look up the characteristic parameters a, b and c and substitute the numeric values into the Cp equation;
- Plug in the equation for Cp in the integrals above;
- Solve two integrals with the above parameters starting at T1 = 273 K and T2 = 473 K: you will find the variations ΔH and ΔS going from 273 K to 473 K;
- Find H = ΔH° + ΔH and S = ΔS° + ΔS for each substance at 473 K;
- Find G = H - 473 × S;
- Now you've completed your first task. You've got H, S and G at 473 K. Notice that this quantities are usually named ΔH, ΔS and ΔG but I used the H, S and G notation so you won't risk confusing them with their variations from 273 to 473 K, which I named with the preceding Δ.
- Solve the Cp integrals for T1 starting at 473 and T2 = T1 +20 and ending at T1 = 653 and T2 = 673, proceding in steps of 20;
- Tabulate the results for ΔH and ΔS at each step;
- Calculate H(T2) as your H(T1) + ΔH, and S(T2) as S(T1) + ΔS;
- For each pair H and S calculate the corresponding G as G(T2) = H(T2) - T2 × S(T2);
- Write down each triplet H(T2), S(T2), G(T2) and premise the letter "Δ" before each H, S and G (again, I didn't do it to avoid confusion with my notation for the increments). That's it. That's the result you need to show your boss.
Program this into a computer and you'll get it done in no time.
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That’s exactly the solution I searched for thanks! Where do I get these heat capacity constants for various compounds?
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All well and good, but before you go blindly calculating things, you should know what assumptions you are making in the calculation and what tolerance for error you have. Calculated thermodynamic values are not the same as experimental data. Are calculated values acceptable? If so, you need to know whether the method of calculation will provide reasonable (however you define that) estimates of the true values under the conditions (temperature, concentration, pressure, whatever) in which they will be used. You should be able to answer those questions before you go putting numbers in a calculator.
Please don't take this the wrong way, but based on your questions here, it seems like you need to have a better understanding of what you're doing and why you're doing it. This is evident even in your first post: "thermodynamic data (Delta S, H, and G)" is completely ambiguous. Do you mean "of formation"? Even beyond that, you've given no indication of what these values will be used for, so I for one would be hesitant to provide any specific recommendations for how to calculate them, since the tool may ultimately not be appropriate for the job.
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The data is available for ethylene and ethanol, and a few others but not butadiene, in the
CRC Handbook of Thermophysical and Thermochemical Data
chapter: Thermochemical Properties of Pure Substances
table: Thermochemical Properties at High Temperatures
tabulated in 100K steps, so interpolate adequately. By far the safest way.
I hope you're not considering rocket fuels. Several requests have popped up, seemingly from one new company, about such totally unfit compounds.