The pH relation Vs. acid and base strength

[capostat]

I would like to clear something up with my understanding about acid or base strength and the pH.


For an acid:
Strength refers to Ka (pKa)
pH refers to concentration of H+ ions

A LOW pH means that there are FREE H+ ions in a solution. So this should mean that BOUND H+ ions RAISE the pH. You get the bound H+ions when there is a base present, right?

So you can be a strong acid (high Ka), but if there is a strong base mopping all that H+ up, then the pH will be higher that what it would be if there was a weaker base instead.

So it's not the act of giving your proton away that makes you a good pH lowerer, but you must factor in the quality of base in the environment too.


IF this is all true, H3O+ is BOUND hydrogen ion to water, but we take this as the FREE H+ ion. Why is that?

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[ksmi117]

Yes, pH depends on all the species present. If you have equal concentrations of a strong acid and a strong base in solution the pH will be 7 because they end up neutralizing each other. Saying that a strong acid gives a low pH is only correct when there are no other species in solution with it. It may still be low in other cases, but you'd have to check.

You are also correct in saying that if you have a strong acid/strong base solution and a strong acid/weak base solution (assuming equal concentrations), that the strong acid/strong base solution would have a higher pH.

As far as the H+ vs. H3O+ thing, it's just convention. Technically H+ ions do not exist in solution, but it's just easier to write that way sometimes. H3O+ is the correct way of talking about those ions, but in practice you can just think of them as the same thing.

I hope this makes sense.

 

[capostat]

Yes, pH depends on all the species present. If you have equal concentrations of a strong acid and a strong base in solution the pH will be 7 because they end up neutralizing each other. Saying that a strong acid gives a low pH is only correct when there are no other species in solution with it. It may still be low in other cases, but you'd have to check.

You are also correct in saying that if you have a strong acid/strong base solution and a strong acid/weak base solution (assuming equal concentrations), that the strong acid/strong base solution would have a higher pH.

As far as the H+ vs. H3O+ thing, it's just convention. Technically H+ ions do not exist in solution, but it's just easier to write that way sometimes. H3O+ is the correct way of talking about those ions, but in practice you can just think of them as the same thing.

I hope this makes sense.

big thank you. it's a big relief.

something is still confusing me though....



HI and put it with a weak base ClO-,

HI + ClO- --> HClO + I- The equation doesn't show the free H+ ions right? We're only seeing them bound in the form of HClO.

 

[ksmi117]

big thank you. it's a big relief.

something is still confusing me though....



HI and put it with a weak base ClO-,

HI + ClO- --> HClO + I- The equation doesn't show the free H+ ions right? We're only seeing them bound in the form of HClO.

Right, doesnt mean that they aren't there though. If you have an excess of HI, then the left over amount will dissociate into I- and H+

 

[capostat]

Right, doesnt mean that they aren't there though. If you have an excess of HI, then the left over amount will dissociate into I- and H+

Ok thanks.

I feel I understand this better now.

 

[capostat]

I was also making a critical error I finally realized...


The Ka is not the reaction equation between acid A and base B.

It's the reaction between acid A and water (the base).


This is why I was still confused about the equilibrium expression.

If we titrate a weak acid HCN, with a strong base, NaOH.

The reaction is HCN + NaOH ---> H2O + NaCN.

But the Ka expression is NOT Ka = [H2O][CN-]/[HCN]

it's always Ka= [H3O+][CN-]/[HCN] from HCN + H2O --> H30+ + CN-


Wow, I was spinning my wheels trying to comprehend how WATER, H2O, was our H+ ion.

HCN + NaOH makes water indeed, but because the free H+ ions are taken up by the OH- it gives the effect of raising pH.


I was scratching my head why we were using the BOUND H+ ions in place of the FREE H+ions in the Ka expression. I was using the wrong reaction equation. Ka always concerns dissociation in water.

 

[ksmi117]

I was also making a critical error I finally realized...


The Ka is not the reaction equation between acid A and base B.

It's the reaction between acid A and water (the base).


This is why I was still confused about the equilibrium expression.

If we titrate a weak acid HCN, with a strong base, NaOH.

The reaction is HCN + NaOH ---> H2O + NaCN.

But the Ka expression is NOT Ka = [H2O][CN-]/[HCN]

it's always Ka= [H3O+][CN-]/[HCN] from HCN + H2O --> H30+ + CN-


Wow, I was spinning my wheels trying to comprehend how WATER, H2O, was our H+ ion.

HCN + NaOH makes water indeed, but because the free H+ ions are taken up by the OH- it gives the effect of raising pH.


I was scratching my head why we were using the BOUND H+ ions in place of the FREE H+ions in the Ka expression. I was using the wrong reaction equation. Ka always concerns dissociation in water.

Yes! A Ka only refers to an acid dissociating in water without anything else there.

 

[capostat]

Yes! A Ka only refers to an acid dissociating in water without anything else there.

I glad this was cleared up now. I should have just laid out everything I was making associations with, and wasnt able to reconcile.

thanks ksmi.