In Gibbs Free Energy, why multiply -T?

[gracy]

If it reads "... potential/voltage input minus the cell potential must be > 0 ..." it makes a little more sense to me.

How?I mean why it makes more sense?Can you please elaborate?

 

[Bystander]

Is there any cell potential in electrolytic cell?I am not talking about voltage given by external source,i am referring to it's own potential i.e because of it's electrodes.

Yes. You can do electrolysis of water by running a current through it at a voltage sufficient to decompose it (~ 1 volt at standard conditions if memory serves), or you can allow the oxygen and hydrogen to react at the two electrodes to form water and generate electricity in a fuel cell.

 

[gracy]

But in all my textbook examples of electrolytic cell ,both electrodes i.e cathode as well as anode are of same element,so how can there be cell potential?As in one of my threads 'why cathode is positively charged' we have discussed that potential depends on element/material of electrodes.

 

[Bystander]

How?I mean why it makes more sense?Can you please elaborate?

Consider a "dead" automotive battery. To charge it, a power/voltage source is connected positive terminal to positive terminal of the battery, and negative terminal to negative terminal of the battery. The difference between the voltage of the source and the voltage in the battery is what drives the charging process. Connecting in series, as is implied by "... input + the cell potential must be greater than zero ..." is what starts car fires, battery explosions, and all sorts of other mayhem by short-circuiting two power sources through each other. Looking at the difference (parallel connection) we have one source (that with the larger magnitude) charging the other.

electrodes i.e cathode as well as anode are of same element,so how can there be cell potential?

A fuel cell uses Pt for both electrodes with a supply of oxygen at one and a supply of hydrogen at the other. The Pt catalyzes dissociation of hydrogen and of oxygen, and of the electron transfers necessary for formation of water. Same electrode materials, but different processes occurring at each electrode.

Electrolytic refining of copper occurs by dissolving copper at the anode, immersed in a sulfuric acid-copper sulfate solution, transport of the copper ions through the solution to the cathode, and reduction of the copper ions at the copper cathode to yield pure copper. In principle, there is a zero potential between the electrodes; in practice, the composition of the impure copper is sufficiently different from that of pure copper that it is more easily oxidized, but that potential is more than offset by the resistance to transport of the copper ions between the electrodes.

Bottom line: cell potentials are meaningful only for ZERO current flow, and only under special circumstances of cell design. You don't have to learn all of this at one time. There will be upper level courses that cover it in more detail.

 

[gracy]

Consider a "dead" automotive battery

Is that galvanic cell?As far as I know all batteries are galvanic cell.Anyways ,I am asking about electrolytic cell in my 73rd post.

 

[fernandofortes]

I haven't heard that term used with Gibbs free energy as I really don't know much about this. But I've learned that when ΔG is positive, a favorable (spontaneous) reaction will occur and non-spontaneous reactions when its negative.

opposite you said: negative favorable or spontaneous; positive unfavorable or non-spontaneous. You could use also negative possible; positive impossible. Or negative allowed; positive prohibited

 

[fernandofortes]

Sorry I meant favorable reactions at negative Gibbs Energy. Also are you saying that reactions only occur at negative Gibbs energy?

Better say negative (or positive) DELTA Gibbs energy, since we deal with changes.

 

[fernandofortes]

A positive standard Gibbs free energy change does not mean that the reaction will not occur at all. It simply means that the conversion at which the reactants will be at equilibrium with the products will be small.

Chet

Or we could say that at standard P, T, and, most important, 1 M all participants, it will not occur at all, and worst, you will see the opposite: reactants increase above 1 M and products decrease bellow 1M.

 

[gracy]

Or we could say that at standard P, T, and, most important, 1 M all participants, it will not occur at all, and worst, you will see the opposite: reactants increase above 1 M and products decrease bellow 1M.

Is it right?

 

[fernandofortes]

Chet's just pointing out that Gibb's free energy has nothing to do with whether a reaction "goes" or does "not go." It limits the degree of "completion" of a reaction, allowing quantitative calculation of quantities/concentrations of products.

I think that since you write (substance A) arrow (substance B) you refer tot A as reactant and B as product, so A must decrease and B increase IF your reaction is possible. DeltaG negative will say that you correctly assumed reactants and products, and your reaction is allowed to "go".

 

[fernandofortes]

You seem to be ascribing some special significance to the situation where the equilibrium constant is unity. This value of K is no more special than any other value of K. Remember also that the standard free energy change refers to the change in free energy starting out with the pure reactants in stoichiometric proportions at 1 atm., and ending up with the pure products in corresponding proportions at 1 atm. In the equilibrium constant K, you are talking about the reactants and products not necessarily in stoichiometric proportions nor at 1 atm pressure, and the partial pressures of the reactants and products even don't have to be equal; in fact, even, if there are only single moles involved, the only requirement is that the product of the reactant partial pressures must match the product of the partial pressures of the products. If the final number of moles is different from the initial number of moles, not even this is required.

Chet

And remember that say "Why system has no available energy to do work if it's reactants and products have same concentration?" is wrong if you forget that you are talking about equilibrium constant equal 1. I think there is no example of reactants and products at same concentration and the system at equilibrium.

 

[AndrewBworth]

Thermodynamics only determines whether or not a reaction can occur and what the equilibrium will be at completion. The rate at which a reaction occurs is determined by kinetics. So oxygen and hydrogen don't spontaneously react due to reasons of kinetics -but the thermodynamics is correct in that it is a downhill reaction.