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Topic: Electrochemistry Problem  (Read 3065 times)

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Offline cas23

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Electrochemistry Problem
« on: May 29, 2018, 04:20:43 AM »
The overall cell reaction occurring in a direct methanol fuel cell is:
1/6𝐶𝐻3𝑂𝐻+1/4𝑂2→1/3𝐻2𝑂+1/6𝐶𝑂2
with the following thermodynamic values:
ΔG0 = -117 kJ mol-1
ΔH0 = -121 kJ mol-1

(i)
Write the balanced half-reactions for the processes occurring at the anode and cathode of the fuel cell.
(ii)
Derive an expression describing how the cell potential varies with temperature.
(iii)
Assuming that the above thermodynamic quantities are independent of temperature, calculate the cell potential for the fuel cell at 150C if it is 0.830 V at 75C.
(F = 9.6485 x 104 C mol-1; R = 8.314 J K−1 mol−1)

So I have had a go at this question, however I'm not really too sure if this is correct. My methodology was I derived the following expression:

E1-E2 = ΔS/nF *(T1-T2)

I calculated ΔS = 1033 J/molK, plugged in the numbers to the above expression and got E2=0.96V. Now, I'm not sure if my derived expression is correct or my ΔS. I'm slightly confused where it says 'assume that the above thermodynamic quantities are independent of temperature' - does that apply for only ΔG0 and ΔH0 or does it also apply for ΔS.

Any help would be much appreciated.

Offline mjc123

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Re: Electrochemistry Problem
« Reply #1 on: May 29, 2018, 04:49:30 AM »
This is a bad question. If ΔG° is independent of temperature, then E° is independent of temperature, from ΔG° = -nFE°.

Offline cas23

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Re: Electrochemistry Problem
« Reply #2 on: May 29, 2018, 05:03:33 AM »
This is a bad question. If ΔG° is independent of temperature, then E° is independent of temperature, from ΔG° = -nFE°.

Thanks for the reply.

So, are you saying it can't actually be solved then?

To be honest, this question has 2 versions; the original version's part iii) was:

iii) Assuming that ΔS° is independent of temperature, calculate the cell potential for the fuel cell at 150C

I've had a go at both versions, but I strongly suspected something was wrong, especially when I saw that the newer version had the extra data of 75C and a given cell potential.

Offline mjc123

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Re: Electrochemistry Problem
« Reply #3 on: May 29, 2018, 07:18:18 AM »
Values of ΔH° and ΔS° are often approximately constant over a not-too-large temperature range. ΔG° generally is not, because of the T-dependence in ΔG = ΔH - TΔS.
Try it assuming ΔH° and ΔS° are independent of temperature.

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