I don't understand what causes redox reactions to occur.

[emr564]

For example, on this website http://www.chemguide.co.uk/physical/redoxeqia/combinations.html in the second diagram from the top, it shows that *if a high resistance voltmeter is placed between two pieces of metal, zinc and copper, the zinc metal will accumulate a build up of electrons.* Then when the resistance is relieved, the electrons will necessarily flow to the copper metal, which I understand.

What I'm confused about is: why do electrons build up on the zinc metal in the first place, and where do the electrons come from?

Thank ya.

 

[Borek]

why do electrons build up on the zinc metal

This is exactly the same question you can ask for every chemical reaction - why does it go the way it does. Short answer is - because it is thermodynamically favorable. In most cases that means that products have lower energy than reactants (although this is oversimplification).

where do the electrons come from?

From the half reaction - Zn "prefers" to be in the ionic form, and some of the atoms "jump" into solution leaving their electrons in the bulk of the metal.