Explanation to Acid Base

[kenhung123]

1. So if acid base reactions are exothermic, then wouldn't H2O + H2O => H3O+ + OH- be classified as an exothermic reaction since water is amphiprotic?

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+]?

[Mao]

1. No. That reaction is endothermic. Most neutralization reactions are exothermic.

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.

[kenhung123]

@ 3. Does it have anything to do with the H2O formed?

[Mao]

No. It's the equilibrium , if the amount of hydroxide decreases, the system shifts to the left and more H+ must be produced.

[kenhung123]

Ah ok. Thanks a lot Mao!

[kenhung123]

I came into this interesting question that relates to this. So if a reaction produces OH-, would you say the pH increases as OH- is produced or increases because from the ionisation of pure water, the reaction would consume H+?

[Mao]

Depends.

If the conditions are already acidic, the latter.

If the conditions are already basic, the prior.

[kenhung123]

Hmm how does that work Mao?

[Martoman]

D, H's are being used up.

[Mao]

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

[kenhung123]

So pH can be affected by changing [OH-] and not affecting [H+] at all?

[toshibaj]

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.

[Blakhitman]

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.

[jasoN-]

But A and C are reactions occuring at the anode (as they indicate oxidation reactions)
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

[Blakhitman]

Yea we're saying B for q 14 but A for q 15

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.