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Topic: A question about metals  (Read 5147 times)

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Offline duck

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A question about metals
« on: March 27, 2008, 09:45:17 AM »
A copper plate and a zinc plate are placed in a beaker containing dilute sulphuric acid. The plates of
copper and zinc are touching each others.

Which statement is correct about the experiment?

A. Both zinc and copper react with acid.
B. The zinc becomes coated with copper.
C. Electrons flow form zinc to copper.
D. Gas bubbles are formed on the surface of the zinc plate.

The correct answer is C.
I understand statement A and B but I don't understand D.
Can anyone help me?
« Last Edit: March 27, 2008, 11:25:27 PM by duck »

Offline flightman233

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Re: A question about metals
« Reply #1 on: March 27, 2008, 10:54:40 AM »
I'll give it a shot, but I don't know how much it will help, and there is a partial guess on my part.

If the zinc was just placed in the acid, a redox (oxidation/reduction) reaction would occur between the acid and the metal, specifically the hydronium ions resulting in the ions becoming reduced from the electrons of the zinc metal and joining with other hydroium ions to form hydrogen gas.

Offline agrobert

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Re: A question about metals
« Reply #2 on: March 27, 2008, 12:46:35 PM »
Right.  Competing reactions with Zn and H2 and Zn and Cu.

Duck,

Can you right the appropriate half-cell equations for Zn, Cu and hydrogen with potentials?

Make sure they are in the right direction.

If C, is right, then electrons flowing from the Zn to the CU means that Zn is being (reduced or oxidized)?
In the realm of scientific observation, luck is only granted to those who are prepared. -Louis Pasteur

Offline duck

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Re: A question about metals
« Reply #3 on: March 27, 2008, 11:24:44 PM »
I think I'd better show you the set up.
I have uploaded the picture. Please take a look=]

Offline duck

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Re: A question about metals
« Reply #4 on: March 27, 2008, 11:29:52 PM »
Right.  Competing reactions with Zn and H2 and Zn and Cu.

Duck,

Can you right the appropriate half-cell equations for Zn, Cu and hydrogen with potentials?

Make sure they are in the right direction.

If C, is right, then electrons flowing from the Zn to the CU means that Zn is being (reduced or oxidized)?
Zn is being reduced?

Offline agrobert

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Re: A question about metals
« Reply #5 on: March 28, 2008, 02:59:46 AM »
Potentials?
In the realm of scientific observation, luck is only granted to those who are prepared. -Louis Pasteur

Offline flightman233

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Re: A question about metals
« Reply #6 on: April 01, 2008, 07:57:43 AM »
Ok let's take a look at a standard reduction potential table

Cu+2 + 2e- --> Cu(s)     E = 0.337  (stronger oxidizing agent)

Zn+2 + 2e- --> Zn(s)    E= -0.763    (stronger reducing agent)

Cu is a stronger oxidizing agent which means that in the process of the chemical reaction it will become reduced by gaining electrons from an outside source (in this case the Zn plate.

Similarly, Zn is a stronger reducing agent and in the process of a chemical reaction it will become oxidized (loose electrons) to Cu.

Honestly I have a bit of difficulty in explaining away the answer to D.
It would seem to me from an intuitive standpoint that the Zn would also react with the Sulfuric Acid in solution, but since it is touching the Cu, plate directly then maybe this prevents that rxn.

Zn(s) + H2SO4 -->ZnSO4(aq) + H2(g) (these are the bubbles you would see if the
plates were not touching I guess)

Ah, this would not occur because Cu is the stronger oxidizing agent than H+ therefore, the electrons would go towards the Cu plate to reduce the copper.

Hope that helps.

weiguxp

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Re: A question about metals
« Reply #7 on: April 01, 2008, 12:18:45 PM »
the Cu is in the form of Cu(s) Oxidation state zero.

Seeing how there is no Cu 2+ in the solution, where is the Cu 2+ to reduce?

Offline flightman233

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Re: A question about metals
« Reply #8 on: April 01, 2008, 10:22:47 PM »

Great comment, I wondered about that myself when I was reading the original post and I just made an assumption that there was a small amount of Cu+2 present in solution.  This is not necessarily a mistake on my part, because in reality there would be a slight amount of Cu+2 present in solution. 

However I will admit that this would not be my first choice of answers, as I would lean towards D being the correct answer.

You would have to have some copper ions present in solution in order to reduce them and force them on to the copper plate.

I say we all agree that this is just a poorly written question.

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