Physical chemistry question.

[dazed1980]

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What is the key to breaking down titration problems.
I know the definitions of what a half equivalent and equivalent point is but it does not help me answer the questions.

For instance, i thought equivalence point is where the OH- AND H+ concentrations are equal. which implies pH at equivalence should always be 7. But i know that not the case.

Can someone break it down for me?
Thanks!

 

[Ahmed786]

I dont know if this helps but the pH of water (H+ and OH-) is only 7 at 25 degrees celscius whereas it can vary slightly at different temperatures.

 

[tncekm]

There are three important points (two half equivalence and one equivalence) for any given titration curve and one important equation (Henderson Hasselbach).

When you're at the half equivalence points you've got a concentration of acid/base that is half the concentration of the base/acid being titrated. That means that HA = :A, which turns the Henderson Hasselbach equation into simply pH = pKa or pH = pKb (depending on how you set it up).

When you're at the equivalence point the moles of titrant added are equal to the moles of product being titrated; note: unless you have a strong acid and strong base, not all products will react so pH is not 7.

I.e. NaOH titrated into HCN.

-OH is a strong base, right? And, HCN is a weak acid right? Well, we know that weak acids make strong bases, so if you were to "theoretically" react equal amounts of -OH and HCN and you "instantaneously" got -CN + H2O, the -CN would act as a base and start abstracting protons. The net result is you'd end up with excess -OH and the equivalence point is >7. Same, but opposite, goes for a strong acid titrating a weak base.

In all honesty, to really "get it" it just takes practice. Find examples, and do them.

GL

 

[Food]

The above poster's response is beautiful and has everything you need to know. I'd just add that for any acid base reaction (for the most part), keep in mind what's forming:

water + salt (again, if it's a lewis acid/base reaction not in aqueous solutions, then this doesn't apply)

The pH of the equivalence point will depend entirely on the nature of the salt formed. You're right that the pH should be neutral (since water is formed) but don't forget that the salt can also be acidic or basic. So with strong acid/base titrations like HCl + NaOH, you only form NaCl which is a neutral salt, whereas NH3 + HCl would give you NH4+ Cl- which is not a neutral salt, so the equivalence point will not be at a pH of 7.

And I wholeheartedly agree that you need to do a bunch of practice problems. You can't be shaky like you are right now, on the real test, and the only way to fix this is to do lots of questions.

 

[dazed1980]

Thanks guys.

 

[masterMood]

EK did a poor job explaining this you're not alone lol.

 

[Vihsadas]

Moving to Study Forum Q&A 🙂

 

[dazed1980]

There are three important points (two half equivalence and one equivalence) for any given titration curve and one important equation (Henderson Hasselbach).

When you're at the half equivalence points you've got a concentration of acid/base that is half the concentration of the base/acid being titrated. That means that HA = :A, which turns the Henderson Hasselbach equation into simply pH = pKa or pH = pKb (depending on how you set it up).

When you're at the equivalence point the moles of titrant added are equal to the moles of product being titrated; note: unless you have a strong acid and strong base, not all products will react so pH is not 7.

I.e. NaOH titrated into HCN.

-OH is a strong base, right? And, HCN is a weak acid right? Well, we know that weak acids make strong bases, so if you were to "theoretically" react equal amounts of -OH and HCN and you "instantaneously" got -CN + H2O, the -CN would act as a base and start abstracting protons. The net result is you'd end up with excess -OH and the equivalence point is >7. Same, but opposite, goes for a strong acid titrating a weak base.

In all honesty, to really "get it" it just takes practice. Find examples, and do them.

GL

How do you decide to set it up as pH=pKa vs pH=pKb?

 

[tncekm]

How do you decide to set it up as pH=pKa vs pH=pKb?

Oh! Its pOH = pKb!!!! My bad!

http://users.stlcc.edu/gkrishnan/hend-hassl.htm

 

[dazed1980]

Oh! Its pOH = pKb!!!! My bad!

http://users.stlcc.edu/gkrishnan/hend-hassl.htm

so, if you were titrating an acid into a base, at half equivalence, pOH = pkb?

 

[tncekm]

so, if you were titrating an acid into a base, at half equivalence, pOH = pkb?

Yeah, the first half equivalence point would be pOH = pKb I believe.