Estimate the average values of ΔH and ΔS in this temperature range

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SUMMARY

The discussion focuses on estimating the average values of ΔH and ΔS for the isomerization of 1-butene and trans-2-butene across a specified temperature range. The calculations involve using the Gibbs free energy equation, ΔG = ΔH - TΔS, and require two equations to solve for the unknowns ΔH and ΔS. Participants recommend using a graphical method to plot ΔG against temperature to determine the slope (-ΔS) and y-intercept (ΔH). Tools like Excel are suggested for performing these calculations efficiently.

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Homework Statement



The isomerization of 1-butene and trans-2-butene occurs as shown in the table at selected temperatures. Estimate the average values of ΔH and ΔS in this temperature range.

T/Kelvins....K
883.... 1.10
826.... 1.20
779.... 1.36
733.... 1.49
702.....1.63
686.....1.72

The Attempt at a Solution



ΔG = -RTinK

ΔG = - 8.314x883 x ln1.10 = 699.697 = 6.99697 J/mol

ΔH = ln(1.10 ÷1.72) x (1÷883)-(1/686) = 0.447014111 ,

669.697 = ΔH - TΔS

ΔG = ΔH - TΔS

Thats all I've managed so far, and I am sure its wrong. Can someone help me out please, I've been trying for hours!

Many thanks
 
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You're on the right track.

You have two unknowns, ΔH & ΔS - they should remain constant at all temperatures within the given range (hence the "average" specification).

So you need two equations, at minimum. If you repeat your calculation for another temperature, that will give you (where d = delta):

dG (at T1) = dH - T1*dS
dG (at T2) = dH - T2*dS

2 equations, two unknowns. Solve for dH & dS.

To be more precise, you can do the calculation for all values (use excel or something) and graph dG vs T. The slope of that graph will be -dS, and the y-intercept will be dH. Be sure to keep your units consistent!

Edit: Actually, I think you definitely need to do the graphical method since they specify 'average'
 
Last edited:

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