Post by: kenhung123 on September 27, 2010, 02:12:03 pm
2. Why is it that the pH change is low for weak acids when diluted when compared to the dilution of strong acids?
3. For this reaction: Cl2+2OH-<-----> Cl- +ClO- +H2O adding Cl2 would decrease OH-, pH is the measure of [H+] why would the pH decrease when there is no change in [H+]?
Post by: Mao on September 27, 2010, 07:52:34 pm
2. Dilution increases %ionisation for weak acids, thus pH drop is smaller
3. [H+]*[OH-] = 10^-14 (at 25 degrees). If OH- is decreased, then H+ must increase, thus pH drop.
Post by: kenhung123 on September 27, 2010, 08:02:38 pm
Post by: Mao on September 27, 2010, 09:03:38 pm
Post by: kenhung123 on September 27, 2010, 09:05:49 pm
Post by: kenhung123 on October 08, 2010, 03:45:21 pm
Post by: Mao on October 08, 2010, 08:37:59 pm
If the conditions are already acidic, the latter.
If the conditions are already basic, the prior.
Post by: kenhung123 on October 08, 2010, 11:22:02 pm
Post by: Martoman on October 09, 2010, 12:12:23 am
Post by: Mao on October 09, 2010, 02:25:38 am
D, H's are being used up.Unfortunately that's not correct.
The fuel cell uses an alkaline electrolyte. At the cathode, even more hydroxides are bring produced. The pH rises due to this accumulation.
B is correct.
I clarify my statement: if you add hydroxides to acid, the pH rises because you are removing acid by a reaction. If you add hydroxides to base, there isn't much H+ yo react with, and the pH rise is due to accumulation of hydroxide
Post by: kenhung123 on October 09, 2010, 09:58:58 am
Post by: toshibaj on October 09, 2010, 12:51:55 pm
In that case, as hydroxides are being produced wouldn't pH increase, making the answer A?
I'm not really sure. I always get confused in regards to acid/base questions.
Post by: Blakhitman on October 09, 2010, 01:33:10 pm
Since its an alkaline electrolyte, does that make the cathode reaction O2 + 2H2O + 4e- => OH- ?
In that case, as hydroxides are being produced wouldn't pH increase, making the answer A?
I'm not really sure. I always get confused in regards to acid/base questions.
That's what I thought.
Post by: jasoN- on October 09, 2010, 03:00:04 pm
The answer must be B or D, but as people above said, H+ indicates an acidic environment, whereas OH- indicates an alkaline environment.
OH- is being produced to maintain the alkaline environment (OH- used up during anode reaction <- A)
hence B
Post by: Blakhitman on October 09, 2010, 03:28:14 pm
D, H's are being used up.Unfortunately that's not correct.
The fuel cell uses an alkaline electrolyte. At the cathode, even more hydroxides are bring produced. The pH rises due to this accumulation.
B is correct.
I clarify my statement: if you add hydroxides to acid, the pH rises because you are removing acid by a reaction. If you add hydroxides to base, there isn't much H+ yo react with, and the pH rise is due to accumulation of hydroxide
Mao must've gotten confused between the two questions cause he said "The pH rises due to this accumulation [of OH-]" then said B (pH decreases as OH is used up) is correct.
Post by: matt123 on October 09, 2010, 03:32:10 pm
Yea we're saying B for q 14 but A for q 15yeh agreedD, H's are being used up.Unfortunately that's not correct.
The fuel cell uses an alkaline electrolyte. At the cathode, even more hydroxides are bring produced. The pH rises due to this accumulation.
B is correct.
I clarify my statement: if you add hydroxides to acid, the pH rises because you are removing acid by a reaction. If you add hydroxides to base, there isn't much H+ yo react with, and the pH rise is due to accumulation of hydroxide
Mao must've gotten confused between the two questions cause he said "The pH rises due to this accumulation [of OH-]" then said B (pH decreases as OH is used up) is correct.
Post by: kenhung123 on October 09, 2010, 04:22:57 pm
Post by: kenhung123 on October 10, 2010, 09:02:29 pm
Post by: Whatlol on October 10, 2010, 09:08:33 pm
Why does diluting strong acid increase pH when the reaction is HCl + H2O => H3O+ + Cl-?
Concentration of H30+ will fall when you dilute...
Post by: matt123 on October 10, 2010, 09:28:30 pm
Why does diluting strong acid increase pH when the reaction is HCl + H2O => H3O+ + Cl-?
Concentration of H30+ will fall when you dilute...
NO UR WRONG. its because CHEMISTRY IS MAGICAL!..
nah hes right
i mean .. think of it.
if you have a strong concentrated acid .. and u put water in it .. guess what happens .. it becomes LESS concentrated
its the same thing here ... add water .. = less concentration of H30+ .. meaning B00M HIGHER PH.
Post by: Blakhitman on October 10, 2010, 09:35:22 pm
No magic here ;D
Post by: kenhung123 on October 10, 2010, 09:40:10 pm
Post by: matt123 on October 10, 2010, 09:40:35 pm
Lol I was thinking in terms of equilibrium and shifting concentrations. Thanks for the clarification
No , thank you ;)
Post by: 98.40_for_sure on October 10, 2010, 09:43:28 pm
Why does diluting strong acid increase pH when the reaction is HCl + H2O => H3O+ + Cl-?
I think the question he is trying to ask here has to do with physical change vs. chemical change. The point he's making is that adding water leads to a forward reaction as 1 particle vs. 2 particles, it will shift to the side with more dissolved particles to increase the overall concentration, thus increases the conc. of H+, which in turn decreases pH. However, the initial diluting cannot be fully compensated for through the system returning to equilibrium, thus overall conc of H+ decreases, and pH will increases.
Post by: Mao on October 11, 2010, 11:24:56 pm
Why does diluting strong acid increase pH when the reaction is HCl + H2O => H3O+ + Cl-?
Remember LCP?
Dilution will decrease concentration of all species. Even though the subsequent change is to produce more H+, this will only be partial, and [H+] invariably drops.
Another point is strong acids do not behave as if they are in equilibrium, because they are so damn strong you get 99.9999999999999999999999999999999999% conversion, the equilibrium lies all the way to the right by about twenty million billion miles. Thus when you dilute, there is no subsequent change, you just get diluted.
Weak acids tend to bounce back a little, but LCP, the pH still rises.
Also, apologies for the previous confusion. You are right in that I was confused, A is the correct answer.
Post by: kenhung123 on October 13, 2010, 10:58:10 am
Post by: kenhung123 on October 13, 2010, 11:01:59 am
If I add water, the equilibrium position shifts forward and more OH- produced but still [OH-] is lower. Would you say the pH decrease because [OH-] is lower or because [H+] is higher?
I am not sure if this is considered an alkaline environment or neutral.