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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: La-Lu on November 21, 2019, 08:42:38 AM

Title: Electrochemistry, calculating current
Post by: La-Lu on November 21, 2019, 08:42:38 AM
Hello all! I am doing some echem exercises and came across this:

- What is the current for 5 x 10^6 molecules undergoing a 2-electron (n=2) reaction in one second at an electrode?
- What is the current for 1 molecule undergoing 5 x 10^6 reactions with n = 2 in one second at an electrode?

So I have number of molecules, number of reactions, number of electrons and time and I have to find the current.

What do you think I should start with? Any hints to which equation I should look at to solve it?

Thank you in advance!!!

L
Title: Re: Electrochemistry, calculating current
Post by: chenbeier on November 21, 2019, 11:04:05 AM
Check the Development of Faradays Law.
Title: Re: Electrochemistry, calculating current
Post by: Borek on November 21, 2019, 11:53:18 AM
Either Faraday's law, or direct application of the (old) ampere definition - one coulomb (how many electrons is that?) per second.

Actually Faraday's law can be derived using the second approach, F (faraday's constant) is jut Avogadro's number divided by the number of electrons in a coulomb.
Title: Re: Electrochemistry, calculating current
Post by: Enthalpy on November 22, 2019, 12:03:35 PM
I wonder about the figures in this exercise.

5×106 molecules/s make a current badly difficult to measure. Electrical engineers meditate for long the precautions they need before trying such an experiment, and among the very first condition are to superdry all parts and circuits, build guard rings everywhere, add varnish one the conductor tracks. Now the exercise tells "electrolyte". My intuition shouts "unrealistic", by 4 to 6 magnitudes.

And then, under what conditions shall we isolate one molecule? In what kind of cell shall the molecule arrive 5 million times per second at an electrode? Or is there some overclever trick that I missed in the setup?

Take a proton: mobility 3.62×10−3cm2V−1s−1. 5V need a cell 0.6µm long for 0.2µs transit time, neglecting the velocity saturation. Is this microelectronics?
Title: Re: Electrochemistry, calculating current
Post by: Borek on November 22, 2019, 04:52:54 PM
Quote from: Enthalpy on November 22, 2019, 12:03:35 PM
I wonder about the figures in this exercise.

Yes, they are a bit abstract, but it is about understanding where the micro and macro worlds meet.

In the late eighties we tried to record voltammetric curves with fA currents, but we never managed to. pA was more or less OK.