Electrolysis

[CalissaRelics]

Hi, 

Could someone please help me with this question: 

Which statement is correct about the electrolysis of molten PbI₂

1) Chemical Energy is converted into electrical energy

2) Pb²⁺   ions are oxidized at the negative electrode (cathode)

3) I₂   is produced at the positive electrode (anode)

4) Ions are produced at both electrodes

Why is the answer 3) ?????

 

Thanks

 

Edited November 15, 2016 by CalissaRelics

[mack]

The anode is connected to the positive terminal of a power supply. Since electrons from from the negative to the positive terminal, electrons must flow out of the solution via the anode. Since the loss of electrons corresponds to an increase in oxidation state, oxidation must occur to the molten substance at the anode.

The substance consists of Pb²⁺ ions and I^- ions, as it is ionic. Of these two ions, the iodide is oxidised to produce electrons. When the ion gives up an electron, I^- reverts to just iodine, which is presented as the species I₂ (g).

2I^- => I₂ (g) + 2e^-

Thus 3) is correct. 1) is not correct as electrolysis converts electrical into chemical energy. 2) is incorrect as oxidation does not occur at the cathode, and lead ions are not oxidised. 4) Is also incorrect as ionisation does not occur at either electrode: iodine gas is produced at the anode, and lead metal at the cathode.

[CalissaRelics]

Thank you so much!!! I understood where I went wrong now 

[kw0573]

It may be helpful to remember that in both voltaic and electrolytic cells, oxidation occurs at anode and reduction occurs at cathode. And in both cases, electron flow is in direction of negative end to the positive end.

In voltaic cells, there is no external voltage and the end of that generates electrons (anode) is negative; electron consumption (cathode) is positive.

In electrolytic cells, there is external voltage that first generates the positive and negative ends of the cell. Then generated electrons (anode) is at positive end because that's where electrons are attracted to; similarly electron consumption (cathode) is negative, where electrons are supplied. 

[Msj Chem]

Maybe this video will help: