Do weak acids and bases neutralize each other?

[sniderwes]

I was under the impression that strong acid+weak base or strong base+weak acid would produce a neutralization, but not weak acid+weak base. However, my TPR book seems to be saying differently, that HF and NH3 will neutralize each other.

"Binary mixtures of equal moles of which of the following acid-base combinations will lead to a complete (99%+) neutralization reaction?
I. HCl and NaOH
II. HF and NH3
III. HNO3 and NaHCO3

Answer is all 3.

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[sniderwes]

bump!

 

[BrawnsNBrain]

Can you post more of the problem? Looks to me you'd get ammonium fluoride.

 

[sniderwes]

Can you post more of the problem? Looks to me you'd get ammonium fluoride.

Sure, the problem is just: "Binary mixtures of equal moles of which of the following acid-base combinations will lead to a complete (99%+) neutralization reaction?
I. HCl and NaOH
II. HF and NH3
III. HNO3 and NaHCO3

The answer is all of the above, but II has weak acid+weak base so I don't think they'd neutralize and certainly not a complete neutralization

 

[BrawnsNBrain]

Good heavens I'm rusty.

The only thing I can come up with which makes sense to me is neutralization is not referring to pH = 7. Only choice I would have a pH of ~7, the other two would not.

 

[BrawnsNBrain]

So after further consideration I think I'm right. Neutralization does indeed not always have to produce water. It only needs to produce a salt that's more favored than the reactants.

Maybe these bumps will bring someone else who remembers this stuff better than I do at the moment.

 

[Paean1]

Correct. A neutralization reaction is one in which an acid and base react to form a salt. Resulting pH does not need to be 7.

A complete (99%+) reaction is just one in which all of at least one reactant is used up.

HF+NH3 can form NH4F, which can break up into NH4+ and F- so it is considered a salt.

 

[sniderwes]

Right, the pH doesn't have to be 7 in a neutralization. But HF is a weak acid and NH3 is a weak base, meaning that only a little bit of HF dissociates and forms H+ and F-, of which the H+ would react with NH3 causing "neutralization." This is as opposed to if one of the reactants were strong, making the reaction go to completion.

So I'm still a little confused as to how a weak acid+weak base would neutralize to completion.

 

[Paean1]

Right, the pH doesn't have to be 7 in a neutralization. But HF is a weak acid and NH3is a weak base, meaning that only a little bit of HF dissociates and forms H+ and F-, of which the H+ would react with NH3 causing "neutralization." This is as opposed to if one of the reactants were strong, making the reaction go to completion.

So I'm still a little confused as to how a weak acid+weak base would neutralize to completion.

At no point is there mention of this being an aqueous mixture. In fact, a "binary mixture" is the mixture of two pure substances. Pure HF liquid is one of the strongest acids because of self-ionization: HF<>H2F + + HF2 - .

 

[Czarcasm]

Right, the pH doesn't have to be 7 in a neutralization. But HF is a weak acid and NH3 is a weak base, meaning that only a little bit of HF dissociates and forms H+ and F-, of which the H+ would react with NH3 causing "neutralization." This is as opposed to if one of the reactants were strong, making the reaction go to completion.

So I'm still a little confused as to how a weak acid+weak base would neutralize to completion.

Bumping this thread for insight. I don't agree with option II also.

 

[Czarcasm]

At no point is there mention of this being an aqueous mixture. In fact, a "binary mixture" is the mixture of two pure substances. Pure HF liquid is one of the strongest acids because of self-ionization: HF<>H2F + + HF2 - .

HF is not a strong acid. Weak acid and bases dissociate only a partial amount, meaning only a few of their conjugates are floating around at any given moment. When these two fuse together, they neutralize each other, but because there's so much still unreacted, I wouldn't consider it a 99% neutralization reaction.

 

[Teleologist]

I was under the impression that strong acid+weak base or strong base+weak acid would produce a neutralization, but not weak acid+weak base. However, my TPR book seems to be saying differently, that HF and NH3 will neutralize each other.

"Binary mixtures of equal moles of which of the following acid-base combinations will lead to a complete (99%+) neutralization reaction?
I. HCl and NaOH
II. HF and NH3
III. HNO3 and NaHCO3

Answer is all 3.

HF has a Ka of 6.6*10^-4; NH3 has a Kb of 1.8*10^-5. There is no way that an equimolar mixture of the two will neutralize each other (unless the volumes are different). The concentration of NH3 would have to be greater than the conc. of HF for there to be complete neutralization.

 

[justadream]

So is the overall conclusion that only strong acids/bases completely neutralize each other? (they both have to be strong)?

 

[Czarcasm]

So is the overall conclusion that only strong acids/bases completely neutralize each other? (they both have to be strong)?

I suppose if you had an equivalent amount of strong acid and weak base, or weak acid and strong base, it would neutralize 100% as well. Afterall, isn't this the point of those particular titrations? Also, to add to the point made above - why we would never consider a weak acid/base titration. At equivalence, moles of analyte = moles of titrant (neutralization). The conjugate (base or acid) of the analyte (if the conjugate of a weak acid/base) can still react further to increase or decrease pH, but at equivalence point, neutralization is 100%.

 

[justadream]

@Czarcasm

Okay that seems logical.

Now if you have a weak base/acid of 1M, the strong acid/base required to neutralize it should be

equal to 1M
or
<1M?

This is all assuming equal volumes.

 

[Czarcasm]

@Czarcasm

Okay that seems logical.

Now if you have a weak base/acid of 1M, the strong acid/base required to neutralize it should be

equal to 1M
or
<1M?

This is all assuming equal volumes.

Yeah, good you mentioned equal volumes because that can easily change things. But yeah, provided we have the same concentration (and equal volumes) of each and provided they are in a 1:1 ratio of H:OH (ie. NaOH and HCN), then that'd be true. You just have to be careful for neutralization reactions, especially when things are not proportionate. TBR had an example of neutralizing CaCO3 with HCl, which tripped me up because I'm use to OH's. But, you had to realize for neutralization to occur, CO32- requires twice as many moles of HCl and to attain that, you'd either need twice the volume or twice the concentration (MV=moles). Be careful how you approach those problems.

 

[justadream]

Yeah, good you mentioned equal volumes because that can easily change things. But yeah, provided we have the same concentration (and equal volumes) of each and provided they are in a 1:1 ratio of H:OH (ie. NaOH and HCN), then that'd be true. You just have to be careful for neutralization reactions, especially when things are not proportionate. TBR had an example of neutralizing CaCO3 with HCl, which tripped me up because I'm use to OH's. But, you had to realize for neutralization to occur, CO32- requires twice as many moles of HCl and to attain that, you'd either need twice the volume or twice the concentration (MV=moles). Be careful how you approach those problems.

Okay this was the answer I was hoping for because now I can address my point of confusion

Why do you need equal molar amount of the strong [acid/base] to neutralize the weak [base/acid]?

For example, let's say you have a 1M weak acid that barely dissociates.

Why does it take 1M NaOH to neutralize that? Isn't that overkill because all of the NaOH dissociates whereas almost none of the weak acid does?

Again, assume equal volumes.

 

[Czarcasm]

Okay this was the answer I was hoping for because now I can address my point of confusion

Why do you need equal molar amount of the strong [acid/base] to neutralize the weak [base/acid]?

For example, let's say you have a 1M weak acid that barely dissociates.

Why does it take 1M NaOH to neutralize that? Isn't that overkill because all of the NaOH dissociates whereas almost none of the weak acid does?

Again, assume equal volumes.

I hate saying equimolar because that's not always true (I prefer to think in terms of moles or equivalents). And the reason being is because strong acids and bases are indiscriminate, I guess is the way to put it. A strong acid or base will dissociate 100%, meaning that their initial concentration will convert fully to either H+ or OH-. A weak base, such as CN- would react in the presence of H+ (it's a reactant), so the more we add H+ to solution, the more CN- will react. This is probably not the best explanation, but I prefer to just skip right over the logic and realize that weak acid or base will react to consume whatever H+ or OH- is present in solution to form it's conjugate.

 

[justadream]

I hate saying equimolar because that's not always true (I prefer to think in terms of moles or equivalents). And the reason being is because strong acids and bases are indiscriminate, I guess is the way to put it. A strong acid or base will dissociate 100%, meaning that their initial concentration will convert fully to either H+ or OH-. A weak base, such as CN- would react in the presence of H+ (it's a reactant), so the more we add H+ to solution, the more CN- will react. This is probably not the best explanation, but I prefer to just skip right over the logic and realize that weak acid or base will react to consume whatever H+ or OH- is present in solution to form it's conjugate.

So is this Le Chatlier's principle?

Basically, as soon as a little bit of the weak base forms, the strong acid consumes it. Thus, a little bit more weak base forms. Then the strong acid consumes it. Repeat.....

 

[Czarcasm]

So is this Le Chatlier's principle?

Basically, as soon as a little bit of the weak base forms, the strong acid consumes it. Thus, a little bit more weak base forms. Then the strong acid consumes it. Repeat.....

Yep, exactly, but rather, it's a little bit of strong acid dissociating and the weak base consumes it.

 

[justadream]

@Czarcasm

Are you saying that the chemistry convention is to always have the acid consume the base?

 

[Czarcasm]

@Czarcasm

Are you saying that the chemistry convention is to always have the acid consume the base?

Well, I suppose it depends on how you're viewing it. If you have a flask of weak base and you add strong acid, then the weak base only reacts with the acid that dissociates. But if you have a flask of strong acid, where dissociate already occurs, then I suppose weak base added to the flask would react to consume the acid already present.