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Offline LHM

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Electrochemistry Questions
« on: August 24, 2010, 08:53:02 PM »
1. When an aqueous solution of potassium fluoride is electrolyzed, which of the following occurs?
A) O2 and H+ are produced at one electrode and H2 and OH- are formed at the other.
B) O2 and OH- are produced at one electrode and H2 and H+ are formed at the other.
C) Metallic K is formed at one electrode and O2 and H+ are formed at the other.
D) Metallic K is produced at one electrode and elemental F2 is produced at the other.

The answer is A, so from that I'm guessing that the half reactions are
O2 + 4H+ + 4e-  :rarrow: 2H2O
4OH-  :rarrow: O2 + 2H2O + 4e-

Is that right? If it is, then where do they come from and how is it related to KF?

2. The overall reaction for the lead storage battery when it discharges is:

Pb(s) + PbO2(s) + 4H+ + 2 SO42-(aq)  :rarrow: 2PbSO4(s) + 2H2O(l)

My only question is, how does the density of the solution decrease? Doesn't the volume stay the same but the mass seems to increase?

3. The standard reduction potential for H+(aq) is 0.00V. What is the reduction potential for a 1*10-3 M HCl solution?
A) 0.355 V
B) 0.178 V
C) -0.178 V
D) -0.355 V

I feel like you're supposed to use the Nernst equation, but I'm slightly confused as to how to figure out what the Q is because I'm not sure what the equation is.
Is it supposed to be HCl (aq)  :rarrow: H+ + Cl-

Thanks!

Offline Borek

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Re: Electrochemistry Questions
« Reply #1 on: August 25, 2010, 03:02:22 AM »
Is that right? If it is, then where do they come from and how is it related to KF?

Where do they come from? Take a look at the reaction equation. What is being decomposed?

Think: is it possible to have metallic potassium in water?

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My only question is, how does the density of the solution decrease? Doesn't the volume stay the same but the mass seems to increase?

Take a look at the reaction equation - something is removed from the solution (moved into the solid phase) and replaced by something else.

Quote
I feel like you're supposed to use the Nernst equation, but I'm slightly confused as to how to figure out what the Q is because I'm not sure what the equation is.
Is it supposed to be HCl (aq)  :rarrow: H+ + Cl-

This is dissociation, not H+ reduction.
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Offline LHM

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Re: Electrochemistry Questions
« Reply #2 on: August 26, 2010, 11:04:00 PM »

Where do they come from? Take a look at the reaction equation. What is being decomposed?

Think: is it possible to have metallic potassium in water?


Oh wait, so is it not possible to have metallic potassium in water because it would be potassium ions instead? I feel like this isn't what you were looking for when you asked what is being decomposed, but the only thing I can think of is that the 4H+ and 4 OH- become water? What happened to the F then?


Take a look at the reaction equation - something is removed from the solution (moved into the solid phase) and replaced by something else.

Um, well the sulfate ion reacts with the lead to form lead sulfate? So does the sulfate ion being removed from the solution decrease the density of the solution?

Ohh and for the last one, was I supposed to use O2+4H++4e- :rarrow: 2 H2O instead then?

Offline Borek

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Re: Electrochemistry Questions
« Reply #3 on: August 27, 2010, 04:07:19 AM »

Where do they come from? Take a look at the reaction equation. What is being decomposed?

Think: is it possible to have metallic potassium in water?


Oh wait, so is it not possible to have metallic potassium in water because it would be potassium ions instead?

That's not the answer you are looking for. What happens when you put a piece of metallic potassium into water?

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I feel like this isn't what you were looking for when you asked what is being decomposed, but the only thing I can think of is that the 4H+ and 4 OH- become water?

And thats correct.

Quote
What happened to the F then?

We will deal with F- once you will get what happens to potassium.


Take a look at the reaction equation - something is removed from the solution (moved into the solid phase) and replaced by something else.

Um, well the sulfate ion reacts with the lead to form lead sulfate? So does the sulfate ion being removed from the solution decrease the density of the solution?[/quote]

You are on the right track, but you have ignored "replaced by something else" part.

Quote
Ohh and for the last one, was I supposed to use O2+4H++4e- :rarrow: 2 H2O instead then?

You don't need oxygen to reduce H+. Start with just H+ and think what it can be reduced to.
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Offline LHM

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Re: Electrochemistry Questions
« Reply #4 on: August 30, 2010, 11:12:05 PM »
1. Oh wait, since potassium is in the first group, it's really reactive and reacts with the water right? So that's why you can't have metallic potassium in water? Something like K+H2O :rarrow: KOH + H2 ? And that's where the OH- comes from?

2. Is it the sulfate ion being replaced by the lead sulfate that decreases the density of the solution then?

3. Do you mean 2H+ + 2 e-  :rarrow: H2 ?

Offline Borek

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Re: Electrochemistry Questions
« Reply #5 on: August 31, 2010, 02:43:18 AM »
1. Oh wait, since potassium is in the first group, it's really reactive and reacts with the water right? So that's why you can't have metallic potassium in water? Something like K+H2O :rarrow: KOH + H2 ? And that's where the OH- comes from?

Right.

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2. Is it the sulfate ion being replaced by the lead sulfate that decreases the density of the solution then?

No - lead sulfate is solid and is not part of the solution. Look at liquid products.

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3. Do you mean 2H+ + 2 e-  :rarrow: H2 ?

Yes.
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Offline LHM

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Re: Electrochemistry Questions
« Reply #6 on: September 02, 2010, 11:02:25 PM »
1. Would the other equation be 4F- + 2H2:rarrow: 4HF + O2 then?

2. So you mean the sulfate ion being replaced by water?

And thank you very very much for the *delete me*

Offline Borek

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Re: Electrochemistry Questions
« Reply #7 on: September 03, 2010, 03:40:35 AM »
1. Would the other equation be 4F- + 2H2:rarrow: 4HF + O2 then?

Yes, although I would just left the F- out of the reaction - while as a relatively strong base it does react with a H+, it doesn't matter much. So I would treat it just as a spectator.

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2. So you mean the sulfate ion being replaced by water?

Not just sulfate ion, sulfuric acid. That's effectively diluting the acid.
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