[Shah]

It is written in my chemistry book that PKa(H3O+/H2O)=0 and PKa(H2O/HO-) =14?
However, all sites and lectures say that PKa(H3O+/H2O)=-1.7 and PKa(H2O/HO-) =15.74 i.e Who is right??? In fact, in my book it is written that H2O + H2O --> H3O+ + OH- and [H2O]^2=constant so Ka(H2O/HO-)= Kw = [H+][OH-]. However, other sources say that Ka(H2O/HO-)= [H+][OH-] )/[H2O] then PKa(H2O/HO-)=15.74. Is it wrong to consider PKa(H3O+/H2O)=0 and PKa(H2O/HO-) =14? Please help and thanks in advance.

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product

[Shah]

the matter is that in my book they are omitting [H2O]^2 (Ka(H2O/HO-)= Kw = [H+][OH-]), while others are omitting only [H2O] (Ka(H2O/HO-)= [H+][OH-] )/[H2O]). So why is there such difference? Can we consider both as right? Please reply.

[Borek]

Concentration of water is assumed to be constant. Sometimes it is not, hence the discrepancies. As long as concentration of water is close enough to 55.55M assumption holds and all approaches give the same results.

[Shah]

Can [H2O] be involved in calculations although it is constant as it is written Ka(H2O/HO-)= [H+][OH-] )/[H2O]??

[Borek]

Please elaborate, it is not clear what you are asking about. Give an example for the type of calculations you are thinking about.

[Shah]

In brief, why some sources consider PKa(H3O+/H2O)=0 and PKa(H2O/HO-) =14, while others
consider PKa(H3O+/H2O)=-1.7 and PKa(H2O/HO-) =15.74? Are both right?


By the way, i will be thankful if anybody could explain to me the leveling effect of water in details.

Thanks

[rabolisk]

http://en.wikipedia.org/wiki/Leveling_effect_%28chemistry%29

[Shah]

regarding leveling effect of water, it is written that any acid that is stronger than H3O+ reacts with H2O to form H3O+; therefore, no acid can be stronger than H3O+ in H2O.

Why can't an acid stronger than H3O+ be in equilibrium with water i.e HA + H2O -> A- + H3O+
suppose that pka(HA/A-)=-1.8, than the constant of equilibrium of this reaction will be 10^1.8, hence it is in equilibrium and not complete reaction, so HA still exists in water although it is stronger than H3O+??

   

[Borek]

Compare concentrations of HA and H₃O⁺. You are right that HA still exists, but its amount is usually negligible.

[Shah]

you are right, its amount is very negligible compared to that of H3O+.

What about PKa(H3O+/H2O) and PKa(H2O/HO-), is it right to consider PKa(H3O+/H2O)=0 and PKa(H2O/HO-) =14 i.e Ka(H2O/HO-)= Kw = [H+][OH-]

Also, i would like to ask about complete reactions. what is the definition of complete? when to consider a reaction as complete? i.e for what equilibrium constant we consider a reaction is complete and not in equilibrium? i have read in my book that an acid-base reaction is considered as complete(quantitative) when its constant K is greater than 10^4. Is that right???

[Shah]

Acid-Base Reaction:    A1+B2<--->B1+A2   where K=[B1][A2]/[A1][B2]

It is written that in such a case K greater than 1, this means that at equilibrium the amount of base B1 and* acid A2 formed would be greater than that of base B2 and* acid A2 remaining.

The problem is that do they mean by the word and* "multiplied by". since it is possible for this equilibrium to have amount of [A1] greater than [B1] when we add a lot of A2 to the equilibrium.??

Thanks a lot

[rabolisk]

Acid-Base Reaction:    A1+B2<--->B1+A2   where K=[B1][A2]/[A1][B2]

It is written that in such a case K greater than 1, this means that at equilibrium the amount of base B1 and* acid A2 formed would be greater than that of base B2 and* acid A2 remaining.

The problem is that do they mean by the word and* "multiplied by". since it is possible for this equilibrium to have amount of [A1] greater than [B1] when we add a lot of A2 to the equilibrium.??

Thanks a lot

Yes, that is what they mean.

[Shah]

it is written in this site: http://nptel.iitm.ac.in/courses/IIT-MADRAS/Engineering_Chemistry_III/pdf/6_Chemistry_of_Carbanions.pdf
that the pKa of the acid H2O is 14 or 15.7 depending on source slight variation in numbers possible.

Why is there such difference?

[Borek]

I have posted a link to page that explains the reason, have you read it?