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How do temperature and concentration affect Ecell of a half cell?
Q. Hi, it's me again.
I carried out some experiments where I varied the concentration of a copper half cell and compared the potential difference against a silver half cell. I obtained some results that appeared to agree with the Nernst equation.
However, when I varied the temperature I could not see a way of using the Nernst equation to find out what the correct values should be, since I was using a concentration of 1 mol dm^-3 so any change caused by the temperature would be canceled out using the Nernst equation by the fact that it was being multiplied by log(1), which is 0. Please tell me if I am using the equation wrong, or if not does anyone know how changing the temperature should in theory affect the potential difference of a half cell?
Thanks in advance for any replies, if you need any more information please ask.
- Birmingham, England
2003
A. I do not know what version of the Nernst Equation you are using. The simplest form is:
E(1) = E(0)+(0.058/n).logC(1).
However, the 0.058/n part of the equation is a simplification of 2.303RT/nF. So, at 18 °C, 2.303RT/F = 0.058 volts; if the temperature is increased to 25 °C, the value goes up to 0.0591 volts. I hope this helps.
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2003
Q. The version I am using is the same as you have stated, but I was just referring to the last part. The last part of your equation states logC(1). But the log of 1 is 0. Therefore if I was using a concentration of 1M, then I would be multiplying the value obtained by the 2.3RT/nF by 0. This would mean even if I varied the temperature of a 1M solution, the effect would be ignored since it is being multiplied by 0. That is why I think I am using the equation wrong, or there is another equation linking temperature and electrode potential.
Any further help would be greatly appreciated ASAP.
Thank you,
- Birmingham, England
2003
A. Surely when C = 1, log C = 0, so E(1) = E(0). You will then have to go back further into the kinetic and electrochemical equations to determine how the electrode potential varies with temperature. E(0) by definition is at 25° C, so to take account of a difference in temperature, you will have to go back to the Third Law of Thermodynamics and recalculate E(0) at different temperatures. Remember that for the purpose of calculations, T is in degrees Kelvin. One option may be to review your results and look at
- 1) the effect of concentration at constant temperature and;
- 2) the effects of temperature at constant concentration.
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2003
Q. I have done many results for constant temperature at different concentrations. These results were all confirmed by the Nernst equation. I only had time to carry out 2 sets of temperature readings, using a 0.5M solution of copper sulphate [on eBay or Amazon]and 1M solution of CuSO4. The problem was I could not check the 1 molar solution because I only knew the Nernst equation. I will have a look into the third law of thermodynamics and hope this helps. Thank you very much for your help. If you think of anything else that may be of use please post.
Robert Warwood [returning]- Birmingham, England
2003
2003
Q. I have looked at the third law of thermodynamics, and found some equations that appear to be of use, but I cannot use them because I don't know what the symbols stand for:
DG = - nFE
DG = Delta"G" = Do not know what this stands for
n = number of electrons transferred, I think
F = Do not know what this stands for
E = Electrode potential (EMF)
E° = .0592 / n log Keq (eq is subscript)
E° = Standard electrode potential
n = number of electrons transferred
Keq = Do not know what this stands for.
If you could clear any of this up for me I would greatly appreciate it, and any other help on how to work out E° at given temperatures would be very useful.
Thanks again.
Rob
- Birmingham, England
A. The formula makes no reference to log(1). It does however make reference to logC(1), that is, the log of a quantity C with subscript 1. You have not read what is there, nor have you recognised the absurdity of your interpretation :-)
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2003
Q. It refers to the log of the concentration of the solution. The solution was of concentration 1M so therefore the log was 0. Therefore when this was multiplied by whatever the temperature was it made no difference. It doesn't matter now, everything has been sorted.
Thanks for all your help. :D
- Birmingham, England
2003
A. Temperature does not affect the e-cell when the concentrations of the two half cells are the same.
Ross Maspero- Hampshire
January 16, 2008
A. Sorry to interrupt but your conversation before regarding log1 was incorrect.
The quotient Q in the Nernst equation deals with the total concentrations of each ion in the solutions, not the concentration of the solutions. The quotient is equal to the ratio of ions (since solids are not included) so when you are calculating the log of 1, you are looking at when the cell has reached equilibrium, and yes, there will be no voltage.
This can be explained by the fact that the voltage can be seen to decrease while the cell is connected, and it will theoretically continue to decrease until the entire cell is in equilibrium and the voltage will be 0.
- Brisbane, Australia
July 24, 2011
A. The interplay between temperature and voltage is not a matter of thermodynamics and those applications are not relevant. The voltage does increase as temperature decreases. This is a matter of kinetics not thermodynamics.
I believe the mechanism may be due to the delayed redox reactions, so the concentrations of reactions may decrease more slowly, or at a different rate than otherwise, but I am not sure; regardless think of how your battery on your phone gets scrambled if overheated. You can place it in the refrigerator and you'll gain a percentage back.
It may have to do with the salt bridge... It's difficult to find the exact mechanics but, just letting you know, you're not at all wrong with your findings.
- NYC, New York, USA
May 2, 2017
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