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### Topic: Electrolysis  (Read 9704 times)

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#### 777888

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##### Electrolysis
« on: November 10, 2004, 09:29:14 PM »
1)What reaction will take place at the cathode and the anode when each of the following is electrolyzed?
1.0 M H2O2 solution containing 1.0 M H2SO4

2)Mg is produced in an electrolytic cell containing molten MgCl2. A current of 2.00000x105A passed through the cell for 18.0h.
===I know how to calculate this but how many significant digits should I use in the final answer? I have to convert 18.0h to s 64800 seconds to calculate. Then will the time have 3 or 5 significant digits?

Can anyone teach me how to do this question?

Thanks!
« Last Edit: November 10, 2004, 09:36:05 PM by 777888 »

#### jdurg

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##### Re:Electrolysis
« Reply #1 on: November 10, 2004, 10:09:56 PM »
For the significant figures question, your answer can only have three significant figures in it because the time specified contains only three sig-figs.  Your answer can only have as many sig-figs as your least precise value, which is the time.  Also, your time only goes out to one decimal place, so your answer can only go out to one decimal place.  If your answer winds up being something like 0.15694, then the answer you would have to provide is 0.2.  While the answer doesn't have three sig-figs in it, it's limited to only one place after the decimal which has precedence.  This is why scientific notation is used.  It helps get rid of the decimal place problem.  For 0.15694, you could write that in as 15.7x10-2 and it would be just fine.
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#### 777888

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##### Re:Electrolysis
« Reply #2 on: November 11, 2004, 03:21:12 PM »
Thanks!

When a question says Hydrochloric acid, does that mean it is a solution? (ie H2O are present?) Acids are always solutions, right?
For example, copper is dropped into hydrochloric acid. Is the hydrochloric acid an aqueous solution?
« Last Edit: November 11, 2004, 03:39:14 PM by 777888 »

#### jdurg

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##### Re:Electrolysis
« Reply #3 on: November 11, 2004, 05:33:13 PM »
Yes.  When it says hydrochloric acid, it means it's a solution of hydrogen chloride.  Anhydrous hydrogen chloride is a gas.

Now, not all acids have to be in a solution.  Sulfuric, phosphoric, acetic, and many other acids can and do exist in an anhydrous form.
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#### 777888

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##### Re:Electrolysis
« Reply #4 on: November 11, 2004, 06:17:46 PM »
Oh, I see!!
Predict the reactions at the cathode and anode for the electrolysis of Lime(CaO)molten compounds.

How can I do this? I can't find O(2-) in the redox table, then how can I compare the reduction potentials and identify the cathode?

#### jdurg

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##### Re:Electrolysis
« Reply #5 on: November 11, 2004, 09:13:47 PM »
Well, which one is the negative ion in CaO?  Which one's the positive ion?  The positive ion will be attracted to the negative electrode, and the negative ion will be attracted to the positive electrode.
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#### 777888

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##### Re:Electrolysis
« Reply #6 on: November 11, 2004, 09:54:31 PM »
Well, which one is the negative ion in CaO?  Which one's the positive ion?  The positive ion will be attracted to the negative electrode, and the negative ion will be attracted to the positive electrode.
Oh I see! So the reaction at the anode is 2O(2-) -> O2+4e

A question says one teachnological process for refining zinc metal involves the electrolysis of a zinc sulfate solution. Predict the minimum potential difference for the ELECTROLYSIS of a zinc sulfate solution.

I thought it was 0.00V because Zn should be the electrode, right? So Zn is the strongest reducing agent and Zn(2+) is the strongest oxidizing agent.
But the answer is 1.99V. I don't know why...(seems like they did not consider the electrode Zn..)

#### jdurg

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##### Re:Electrolysis
« Reply #7 on: November 12, 2004, 10:59:06 AM »
Oh I see! So the reaction at the anode is 2O(2-) -> O2+4e

A question says one teachnological process for refining zinc metal involves the electrolysis of a zinc sulfate solution. Predict the minimum potential difference for the ELECTROLYSIS of a zinc sulfate solution.

I thought it was 0.00V because Zn should be the electrode, right? So Zn is the strongest reducing agent and Zn(2+) is the strongest oxidizing agent.
But the answer is 1.99V. I don't know why...(seems like they did not consider the electrode Zn..)

Zinc is not the electrode.  In fact, it is HIGHLY unlikely that zinc would be the electrode.  When calculating the potential difference, you ignore what the electrodes are.  You just determine what half reaction is occuring at the cathode and what half reaction is occuring at the anode.  You then calculate the difference in the voltages between those two.
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