stonecold's chem questions :)

[fady_22]

How does this question work...  I get that that self ionization of water is endothermic so as temp increases above 25C, pH decreases, and if temp is below 25C, pH increases.

I know it has something to do with it.  STAV Explanation sucked.  From 2008 btw.

Yep, exactly.
So at a lower temperature, the pH of a neutral solution will be greater than 7. Therefore, given the information that the pH is 7, you can deduce that blood is acidic at this temperature, as it has a pH lower than that of a neutral solution.

[kenhung123]

How does this question work...  I get that that self ionization of water is endothermic so as temp increases above 25C, pH decreases, and if temp is below 25C, pH increases.

I know it has something to do with it.  STAV Explanation sucked.  From 2008 btw.

Yea I have no idea what this question is about? Isn't it neutral as [H3O+]=[OH-] at all times when you refer to the self ionisation of water?

[m@tty]

How does this question work...  I get that that self ionization of water is endothermic so as temp increases above 25C, pH decreases, and if temp is below 25C, pH increases.

I know it has something to do with it.  STAV Explanation sucked.  From 2008 btw.

Yea I have no idea what this question is about? Isn't it neutral as [H3O+]=[OH-] at all times when you refer to the self ionisation of water?

I don't really understand your question. But...

Neutrality is dependent on .

However, a pH of 7 is ONLY neutral at 298K (25^oC).
At this temp .
 
And so when then


The self-ionisation of water is endothermic, hence the of pure water fluctuates at different temperatures; it increases at higher temperatures and decreases at lower temperatures.

Since pH is a measure of the concentration of , the pH of neutral water also fluctuates with temperature.

At temperatures above 298K, the neutral pH is less than 7.

At temperatures lower than 298K, the neutral pH is higher than 7.

[kenhung123]

True. What is the question implying then? It says its pH is 7 at a low temp. It is supposed to be acidic...I don't think it only involves the self ionisation of water.

[m@tty]

What do you mean? Please, elaborate. What are you suggesting is involved also?

[stonecold]

I thought it was a rather dumb question.  A, you are left to assume blood has water in it, which you logically assume it would, but still, assumptions are stupid.  B, blood has other stuff in it such as Hydrogen Carbonate, which can both donate/receive protons, which has a whole other bearing on the pH entirely...

[kenhung123]

What do you mean? Please, elaborate. What are you suggesting is involved also?

You suggested water is neutral at all temperatures but pH varies, correct?
The pH of this blood is 7, however if you use your explanation the answer should be neutral regardless of pH.

[fady_22]

I thought it was a rather dumb question.  A, you are left to assume blood has water in it, which you logically assume it would, but still, assumptions are stupid.  B, blood has other stuff in it such as Hydrogen Carbonate, which can both donate/receive protons, which has a whole other bearing on the pH entirely...

You don't really have to assume anything. The fact that the blood has a pH means that it contains water, and you don't have to think about it any further-- it is simply telling you that the pH is 7 (and so the effects of anything in the blood are already accounted for).

[m@tty]

What do you mean? Please, elaborate. What are you suggesting is involved also?

You suggested water is neutral at all temperatures but pH varies, correct?
The pH of this blood is 7, however if you use your explanation the answer should be neutral regardless of pH.

Pure water is always neutral.

The fact that this solution has a pH of 7 at a temperature lower than 25^oC means that it is acidic.

[kenhung123]

Right. This question is very good. I got tricked

[stonecold]

How do you do the half cell equations for this...

Ag2O(s) + Zn(s) + H2O(l) --->  2Ag(s) + Zn(OH)2(s)

I have no idea whether to balance with H2O, OH-, H+, or O2- in these types of questions...

[kyzoo]

How do you do the half cell equations for this...

Ag2O(s) + Zn(s) + H2O(l) --->  2Ag(s) + Zn(OH)2(s)

I have no idea whether to balance with H2O, OH-, H+, or O2- in these types of questions...

4e- + Ag2O + 2H2O --> 2Ag(s) + 4OH-
Zn(s) + 2OH- --> Zn(OH)2 + 2e-

There are acidic redox reactions and basic redox reactions. Acidic involves H+, and basic involves OH-.

[stonecold]

Okay, thanks.  Say they told you a fuel cell was methane oxygen, how do you know what the products of each of the half cell reactions would be so you can balance it?

Particularly for CH4?

[stonecold]

How do you do the half cell equations for this...

Ag2O(s) + Zn(s) + H2O(l) --->  2Ag(s) + Zn(OH)2(s)

I have no idea whether to balance with H2O, OH-, H+, or O2- in these types of questions...

4e- + Ag2O + 2H2O --> 2Ag(s) + 4OH-
Zn(s) + 2OH- --> Zn(OH)2 + 2e-

There are acidic redox reactions and basic redox reactions. Acidic involves H+, and basic involves OH-.

Solutions say this :S

Ag2O + H2O + 2e- --> 2Ag(s) + 2OH-

[iffets12345]

How do you do the half cell equations for this...

Ag2O(s) + Zn(s) + H2O(l) --->  2Ag(s) + Zn(OH)2(s)

I have no idea whether to balance with H2O, OH-, H+, or O2- in these types of questions...

4e- + Ag2O + 2H2O --> 2Ag(s) + 4OH-
Zn(s) + 2OH- --> Zn(OH)2 + 2e-

There are acidic redox reactions and basic redox reactions. Acidic involves H+, and basic involves OH-.

Solutions say this :S

Ag2O + H2O + 2e- --> 2Ag(s) + 2OH-

I agree with stonecold's solutions. Not sure how kyzoo got that, great genius that he is...=/.

Start:

Ag2O   ---> 2Ag

add h20

Ag2O ---> 2Ag + H20

add H+

Ag2O + 2H+   -----> 2Ag +H20

because this is in acidic electrolyte (you can tell by the ZN(OH)2), add OH- for every H+:


Ag2O +2H2O  -----> 2Ag + H20 + 2OH-

cancel out H2O:

Ag2O + H2O  ----> 2Ag + 2OH-