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Topic: Equivalence point and neutralization point  (Read 325 times)

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Offline Decamethyl

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Equivalence point and neutralization point
« on: May 19, 2019, 04:17:10 PM »
I got stuck somehow when I tried to understand the difference between the equivalence point and neutralization point through a specific example.
Just to get this clear, the theoretical definitions seem clear to me. EP: n (HA) = n (A-) which also means n (H3O+) = n (OH-)
Neutralization point: PH value = 7

But I somehow can't wrap my mind around it when trying to apply it to reaction equation, like this one

CO3HOO- + H3O+ + Na- + OH-  :lequil: CO3HOO- + NA- + 2H20

What I don't understand here is that at the EP n (Na-) = n (CO3HOO-) which also means n (H30+) = n (OH-) which actually means that the balance is all to the right side of the equation.

But why is the PH value not = 7 here anyways ? Is it because CO3HOO- is still in there and could take an H from H20, so that it's PH value is at around 6 at the EP ?

Offline Borek

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Re: Equivalence point and neutralization point
« Reply #1 on: May 19, 2019, 05:22:41 PM »
Never heard abut the neutralization point.

What I know is end point (where we ended the titration) and equivalence point - where we added the stoichiometric amount of titrant.

EP: n (HA) = n (A-) which also means n (H3O+) = n (OH-)

Non sequitur. For a weak acid HA=A- means pH=pKa. Unless you mean nHA=nBOH (number of moles of acid equal to number of moles of base, that's the stoichiometric equivalence). For weak acids it still doesn't mean [H3O+]=[OH-], as pH of a produced salt of a weak acid is never 7.

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Neutralization point: PH value = 7

To be honest - useless idea, unless we are talking about titration of a strong acid/base with a strong base/acid (even then not exactly true).
« Last Edit: May 19, 2019, 06:11:11 PM by Borek »
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Offline pcm81

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Re: Equivalence point and neutralization point
« Reply #2 on: May 26, 2019, 10:24:28 AM »
Never heard abut the neutralization point.

What I know is end point (where we ended the titration) and equivalence point - where we added the stoichiometric amount of titrant.

EP: n (HA) = n (A-) which also means n (H3O+) = n (OH-)

Non sequitur. For a weak acid HA=A- means pH=pKa. Unless you mean nHA=nBOH (number of moles of acid equal to number of moles of base, that's the stoichiometric equivalence). For weak acids it still doesn't mean [H3O+]=[OH-], as pH of a produced salt of a weak acid is never 7.

Quote
Neutralization point: PH value = 7

To be honest - useless idea, unless we are talking about titration of a strong acid/base with a strong base/acid (even then not exactly true).

I think "Neutralization pont" is a term chem instructors came up with trying to sound cool and hipster like talking about spill cleanup etc...

Offline pcm81

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Re: Equivalence point and neutralization point
« Reply #3 on: May 26, 2019, 10:28:01 AM »
I got stuck somehow when I tried to understand the difference between the equivalence point and neutralization point through a specific example.
Just to get this clear, the theoretical definitions seem clear to me. EP: n (HA) = n (A-) which also means n (H3O+) = n (OH-)
Neutralization point: PH value = 7

But I somehow can't wrap my mind around it when trying to apply it to reaction equation, like this one

CO3HOO- + H3O+ + Na- + OH-  :lequil: CO3HOO- + NA- + 2H20

What I don't understand here is that at the EP n (Na-) = n (CO3HOO-) which also means n (H30+) = n (OH-) which actually means that the balance is all to the right side of the equation.

But why is the PH value not = 7 here anyways ? Is it because CO3HOO- is still in there and could take an H from H20, so that it's PH value is at around 6 at the EP ?

CO3HOO- and NA- can take H+ from H2O hence pH is not 7.

Offline Borek

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Re: Equivalence point and neutralization point
« Reply #4 on: May 26, 2019, 03:39:05 PM »
Haven't noticed it earlier, no such thing as Na- (at least not in water solutions).
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Offline pcm81

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Re: Equivalence point and neutralization point
« Reply #5 on: May 26, 2019, 10:51:30 PM »
Haven't noticed it earlier, no such thing as Na- (at least not in water solutions).

NA, not Na. N moles of Acid-, not Sodium.

Offline Borek

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Re: Equivalence point and neutralization point
« Reply #6 on: May 27, 2019, 02:39:26 AM »
That was my initial idea, but it is written and used so inconsistently (n(A-), Na-, NA-, n(Na-)) that I started to doubt that's what OP meant.
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