[Rutherford]

In a glass filled with sulfuric acid, Cu and Zn plates are immersed. On what plate are appearing bubbles if the plates:
a)are touching each other;
b)aren't touching each other?

a)Zn can be easier oxidized than Cu, so it is Zn.
b)here is help needed, what is happening here?

[Arkcon]

Something different definitely does happen when a less reactive metal is in contact with a more reactive metal. http://en.wikipedia.org/wiki/Sacrificial_anode

[Rutherford]

I am being redirected from the link you gave.

[Arkcon]

I know.  I wanted to use the more apt jargon "sacrificial anode", but the wikipedians want to consolidate the similar terms under one heading.  Anyway, that is the generalized term, covering the topic at hand.

[Rutherford]

I think that this is it:
"Cathodic protection works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.
For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit"

Why is this caused by a bigger anode surface? How does Cu have a bigger surface than Zn?
When they are connected, electrons can flow from Zn to Cu, and Cu can be reduced so this doesn't make any sense  .

[confusedstud]

A) Zinc is more reactive than copper, so electrons flow from zinc to copper. Zinc is negative while copper is positive (electrons are attracted to the positive copper). So the reaction at the zinc plate is Zn -->Zn2+ +2e, as a result the zinc plate slowly becomes smaller. At the copper plate, hydrogen gas is evolved as the H+ ions are lower down on the reactivity series than Zn2+. So the reaction at the copper plate is 2H+ +2e--> H2.

This is like an electric cell and as Arkcon said, zinc acts sacrificial anode as zinc is corroded instead of copper. But hydrogen bubbles still form on the copper plate. Both of them have the same principles but they are used for different purposes.

B) since there is no contact, zinc will react to form hydrogen gas and zinc sulfate. Copper is unreactive so it doesn't react with the dilute acid.

Hope this helps.

[Rutherford]

Thanks for the answer.
A)I suppose that by positive copper you mean the positive electrode potential. I will think about this.

[Rutherford]

Thought for a while, the second part is understandable. Now the first part: Zn has a lower electrode potential than Cu so it gets easier oxidized, while Cu²⁺ should be reduced, but there are no Cu²⁺ in the solution so H⁺ gets reduced instead and bubbles are made. Is this correct?

[confusedstud]

Hi, imagine two electrodes that are connected via wires (as good as touching each other) with a sulphuric acid electrolyte. So applying basic electrochemistry, electrons from the more reactive metal would flow to the less reactive metal. Hence, the zinc is negatively charged while the copper is positively charged. When this happens, zinc metal will be oxidised to form zinc ions. At the same time, since the  copper electrodes has the electrons it would reduced the H+ ions to hydrogen gas. At the first instance, the only ions present are H+, OH- and SO4 2-. So the only thing that's able to get reduced is H+ and even if zinc forms ions in the future, they are too far apart to cause zinc to form.

This works like a simple cell. Hope this helps

[Rutherford]

Thanks very much, I think I understood. When separate Zn, reduces H⁺ by reacting with H₂SO₄, but when connected the electrons flow from Zn to Cu so the H⁺ can only be reduced on the Cu plate.

[confusedstud]

Yup that's the right answer! :-)