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Course: Chemistry library>Unit 14

Lesson 3: Standard cell potentials

Voltage as an intensive property

Demonstrating that voltage is an intensive property by calculating the voltage and change in Gibbs free energy for a half reaction.

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• But when you calculated the Standard Free Energy change for 2 moles of At you just multiplied it by 2! This is means that you actually considered the Voltage to be the same as before. After that you tried to prove that the Voltage was the same as before! Is that wrong? Because according to mathematics I don't find it to be correct.
• I was confused by this proof too because we considered voltage to be the same as before from the beginning, as you said (ie. multiplying the standard free energy by 2)

The math is right, but here's another way of "proving" voltage isn't affected by the number of mols: Since voltage is a change of potential, and potential is the amount of work done on a single point charge by an electric field, voltage won't change with number of point charges because it represents a single point charge in an electric field. Voltage is not affected by the number of e- moved because it represents only one point charge.

Hope I could help :D
• What if you have 2 moles of 1 substance and 1 mole of another in a voltaic cell and thus twice the number of electrons sent? Would the voltage double?
• Voltage is an intensive property. It doesn't depend on the amount of substance.
Changing the amount of substance doesn't change the voltage. It changes the current.
• How can you connect it with Gibbs free energy, if standard potential is something we decided by convention? It is just comparing with hydrogen electrode has this potential, with another it will be else.
• Yes it would be different with a different reference point, but would still not change with the amount of substance.

As you said, it is the convention to use a hydrogen electrode. It would also change if we altered the conditions (i.e. temp.)
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• Where did ∆G=-nF€° came from? It doesn't seem that I missed any video or article . I'm following these electrochemistry tutorials from chemistry(science).
• The lessons are a bit messed up here. explanation of delta G is given in a latter video but in this topic, it has somehow come out of nowhere i guess.
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• This is making it sound like if you were to put two batteries together, it would still be the same voltage as one battery. Is that correct?
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• here you are saying that voltage is an intensive property but in nernst you considered the effect of concentration on potential. this is confusing .
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• reduction potentials depend on temperature, pressure and concentration. the standard reduction potential of a substance is the reduction potential under the condition 298K, 100Pa and concentration of relevant ions 1mol/L . Is that the case?
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• If voltage is an intensive property then why do we have nernst equation. Which relates the voltage in terms of concentrations?
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• Ok, I have a lot of questions now.
I know that Gibbs free energy is used to calculate if a reaction happens spontaneously, but does it have any physical significance?

What is standard Gibbs free energy or Delta G naught? Is it related to the equilibrium constant in some way?

What is exactly the standard electrode potential? I wonder why is constant, because potential should be affected by position with respect to the charges. Is using a galvanic cell and a voltimeter the only way to measure it?

Is standard electrode potential dependent on electronegativity?

Why is standard Gibbs energy related to standard electric potential as is shown in the formula in this video?