Electrochemical cells

For an electrochemical cell, when does it stop releasing electrons?
And why?

And also:

If we increase conc of Cu2+ ions, does the reaction this way > proceed faster and electrons accepted faster therefore the electrode potential increases?

Thanks!

(edited 9 years ago)

Original post by Zenarthra

For an electrochemical cell, when does it stop releasing electrons?
And why?

And also:

If we increase conc of Cu2+ ions, does the reaction this way > proceed faster and electrons accepted faster therefore the electrode potential increases?

Thanks!

If you think of a galvanic cell e.g Copper - Zinc then the flow of charge between the electrodes will stop when all of the Reducing agent(zinc) is reacted- meaning you will have an abundance of Zn2+ ions and an abundance of Copper metal produced.

Increasing the conc. of Cu2+ ions will shift the equilibrium - Cu2+ + 2e- ------><-------- Cu
to the right as le chatelier's principle will be applied and the equilibrium shifts in the direction that opposes the change.

Reply 2

Zenarthra

OP

3

Original post by cuckoo99

If you think of a galvanic cell e.g Copper - Zinc then the flow of charge between the electrodes will stop when all of the Reducing agent(zinc) is reacted- meaning you will have an abundance of Zn2+ ions and an abundance of Copper metal produced.

Increasing the conc. of Cu2+ ions will shift the equilibrium - Cu2+ + 2e- ------><-------- Cu
to the right as le chatelier's principle will be applied and the equilibrium shifts in the direction that opposes the change.

Ahh ok, in my book its quite vague and talks about the concentrations of ions being the same at which the electrode potential is then equal.
In the situation you have referred to, what would be the ions in which the concs are the same?

And if equilibirum shifts to the right, how does this affect electrode potential?

Thanks!

Reply 3

cuckoo99

13

Original post by Zenarthra

Ahh ok, in my book its quite vague and talks about the concentrations of ions being the same at which the electrode potential is then equal.
In the situation you have referred to, what would be the ions in which the concs are the same?

And if equilibirum shifts to the right, how does this affect electrode potential?

Thanks!

Well usually you would measure the electrode potentials under standard conditions of 1.00mol dm^-3 . 100kPa, 293 k... So im not really sure in what way deviating from the standard conditions would do for the electrode potentials.

Reply 4

ChemistryBud

Original post by Zenarthra

Ahh ok, in my book its quite vague and talks about the concentrations of ions being the same at which the electrode potential is then equal.
In the situation you have referred to, what would be the ions in which the concs are the same?

And if equilibirum shifts to the right, how does this affect electrode potential?

Thanks!

This is a great question. Your book is vague because the answer is a little complicated (maybe too much for A-level). I'll try to walk you through it.

So it is important to note that the cell potential (E_cell) will vary as the concentrations change. To demonstrate this we use the Nernst equation:

E_cell = E^o_cell - (RT/nF) InQ

Where E^o_cell is the cell potential under standard conditions
and Q is the reaction quotent ([products]/[reactants])

n= the number of electrons transferred, 2 in the example i show below. Also, F is Faradays constant. As a result, (RT/nF) can essentially be ignored as it is constant for a given temperature.

In the case of a galvanic cell:
Zn_(s) +Cu²⁺_(aq) ---><--- Zn²⁺_(aq) + Cu(s)

we therefore say Q = [Zn2+]/[Cu2+]

This answers the question "what would be the ions?"

Now as the reaction proceeds in the forwards direction; the concentration of reactants [Cu2+] will decrease and the concentration of products [Zn2+] will increase. This causes Ecell to gradually decrease to zero. At this point, Q is also the same as Keq (the equilibrium rate constant). Important, when Ecell= 0 it means that the concentrations of products and reactants are no longer changing, NOT that all of the reactants have been used up (a common misconception).

We can also relate the Nernst equation to Le Chateliers principle.
Le Chateliers principle: if we increase the concentration of reactants [Cu2+], the rate increases in the forwards direction and Ecell increases.
Using the Nernst equation: when we increase the concentration of reactants [Cu2+], InQ will decrease and Ecell increases (As expected based on Le Chateliers principle).

Again, this is fairly complicated stuff. I've found a link that goes through the derivation of the equation, Le Chateliers principle, the situation where Ecell =0 and the galvanic cell example. That is, the perfect link:

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Nernst_Equation

Let me know if you need any more help.
Apologies for any spelling mistakes :biggrin:

, its pretty late and there have been a few tricky questions today.

(edited 9 years ago)

Electrochemical cells

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