[thuanthuan]

Hi, I have a question on electrolysis ?
Consider an electrochemical cell Zn-Cu. We know that Zn is oxidized to Zn (2+), and aquaeous Cu (2+) is reduced to Cu. But I wonder why it is Cu2+ and not Zn2+ is reduced ? The two kinds of cation are both there, in the solution ?
Why no book mention the reduction of Zn (2+)

[Arkcon]

Electronegativity difference is the reason.  See wikipedia for the series, and come up with an electrode system for Zn|Zn2+

[thuanthuan]

What do you mean ? Can you explain it further ?

I mean that because Cu 2+ and Zn 2+ are both there, there will be two simultaneous reducing reaction. Is that right ?

[Arkcon]

Here's a better page, it even has cute had-drawn drawings:

http://www.tech.plym.ac.uk/sme/strc201/corrosion1.htm

The table has Zn2+, and Cu2+ on it it also has elements with greater electrode potential than zinc, like magnesium or sodium, they can reduce Zn2+ to Zn metal.  And elements like, Ag and Au, which could, theoretically, oxidize Cu to Cu2+.  You don't often hear about people dissolving gold and silver and using them to make Cu2+, however.

The result is pretty intuitive really, which are you more like to see lying around in the crust of the Earth uncombined with other elements?  Gold, silver, yes.  Copper, maybe, for a little while.  Zinc, a little less likely.  Free sodium metal?  Umm.  No, not if there's a convenient lake, and a crazy chemist nearby.

The more easily oxidized elements will reduce the less easily oxidized elements, if the come in contact.  Now, build half cells, connect them with a salt bridge and connect them with wires and the electrons will travel the wires to perform redox reactions, and we can put them to work in the process -- lighting small bulbs for flashlights, powering small motors for toy, running electronics, etc.