Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: gavindor on February 22, 2024, 09:53:03 PM
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Looking at this reaction
H2O + Na2O --> 2NaOH
Is that a Bronsted acid and Bronsted base.. (a bronsted acid since water is an amphoteric molecule and is an acid here). And Na2O is a base.
But two interesting things I notice about this.
It seems to me that
- It's not a neutralisation reaction.. Because NaOH has a pH not equal to 7. NaOH's pH is not neutral.
- There's no conjugate acid and conjugate base.
The product is one product 2NaOH, which could be seen as NaOH + NaOH
But to be a conjugate acid and conjugate base, there'd have to be two products, the products would be different, one positive one negative , and those products would have to be Na2OH+ + OH-
So am I correct that it's a bronsted acid base reaction, but not a neutralisation reaction, and with no conjugate acids/base pairs?
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You have to look to the oxide anion O2-. This is an Proton acceptor, means a base.
H+ + O 2- => OH-
Water is a Proton donator it is the acid.
H2O => H+ + OH-
In sum
O2- + H2O => OH- + OH-
The hydoxid two times on product side works also one as the Proton acceptor what will form water again the other one is the Proton donator to form the oxide ion.
O2- ( base I) + H2O (acid I) => OH- ( base II) + OH- ( acid II)
The same we have with equilibrium of water
2 H2O => H3O+ + OH-
One water is acid, the other one base.
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I know H+ goes from the H2O to the Na2O
H2O + Na2O --> 2NaOH
so OK it's a bronsted lowry acid base reaction.
I suppose H2O + Na2O --> 2NaOH
is the same as
H2O + Na2O --> NaOH + NaOH
So one NaOH is the conjugate acid, and one NaOH is the conjugate base?
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Correct
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thanks, and would you call it a neutralisation reaction (even) if the salt is not neutral?
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So one NaOH is the conjugate acid, and one NaOH is the conjugate base?
Correct
But doesn't the conjugate acid have to have a positive charge, and the conjugate base a negative charge?
NaOH whether you look at it as Na+(OH)- or whether you look at it as H+ NaO it's neutral. 'cos H is +1, Na is +1, O is -2.
So it seems to me that NaOH can't be a conjugate acid or a conjugate base.
Even though it works from the donation of H+ and acceptance of H+ perspective
H2O + Na2O
becomes OH- + Na2OH
= OH- + Na + + NaOH = NaOH + NaOH = 2NaOH
So the transfer of H and acceptance of H works.
But I can't see how NaOH is a conjugate acid and/or conjugate base.
I can see how OH- and Na2OH+ are conjugate base and conjugate acid respectively.
And then simplify to 2NaOH
But it seems to me.. The Conjugate base is OH- , and the Conjugate acid is Na2OH+
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But doesn't the conjugate acid have to have a positive charge, and the conjugate base a negative charge?
No. They have to differ by one charge as they interconvert by accepting/donating proton, but the definition doesn't say anything about the initial charge value.
I would not call NaOH neither Bronsted acid nor base. Na+ is just a spectator, it is OH- that is reacting.
In this case I would say O2- is a Bronsted base (accepts a proton), H2O is a Bronsted acid (donates a proton):
O2- + H2O :rarrow: OH- + OH-
O2-/OH- is one conjugate acid/base pair, H2O/OH- is another. Yes, OH- can be thought as both Bronsted acid and Bronsted base in two different reactions. A bit stretching, but technically correct.
Na2OH+
I doubt something like that exists, making things up is not a way forward. Na2O is highly ionic.
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But doesn't the conjugate acid have to have a positive charge, and the conjugate base a negative charge?
No. They have to differ by one charge as they interconvert by accepting/donating proton...Na+ is just a spectator...
I guess you don't mean to say that the conjugate acid and conjugate base should differ by one..
You mean to say that the (non-conjugate)acid and conjugate base, should differ by 1. And that the (non-conjugate) base, and conjugate acid, should differ by 1.
It seems to me that the conjugate acid and conjugate base could have the same charge and do in this example.
e,g. So in Na2O + H2O --> 2NaOH
Given that Na is just a spectator ion.. (or taking Na as a spectator ion)..
Our "non-conjugate acid" is H2O
Our "non-conjugate base" is O
Our conjugate base is OH- (H2O minus H)
Our conjugate acid is OH- (O plus H)
Our "non-conjugate base" and conjugate acid differ by 1
Our "non-conjugate acid" and conjugate base, differ by 1
And that makes sense because it's a transfer of H+ to make a difference of a charge of 1
I notice that here we have conjugate base of OH- and conjugate acid of OH-
That sounds similar to saying NaOH is the conjugate base and NaOH is the conjugate acid.. just excluding the spectator ion. But perhaps more technically correct to exclude the spectator ion, as you do.
Our conjugate acid and conjugate base have the same charge. Not differing by one.
Often they can differ by two, which they would if the conjugate acid is + and the conjugate base is - like HCl + NH3 <--> NH4+ + Cl-
But you make an interesting point that the conjugate acid and base don't have to be + and - respectively. And in the Na2O + H2O example they're the same. OH- so same charge. I can see that if the charges of the non-conjugate acid and non-conjugate base are neutral, then you get + and - with the conjugate acid and conjugate base. And that supports what you show about not counting the Na+.
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I guess you don't mean to say that the conjugate acid and conjugate base should differ by one.
That's exactly what I am saying.
Looks to me like you are confused about what conjugate acid and base are - these are things that react directly by transferring a proton between them. They always come as conjugate pairs. We do have two pairs of conjugate acid/base here, OH-/O2- and H2O/OH-, in each pair their charges differ by one. None of these substances is "non-conjugate".
It is perfectly possible for something to be both conjugate acid and conjugate base in different pairs. In such cases there is no way to tell whether something is a conjugate base or conjugate acid without stating the context (typically just mentioning the other conjugate of the pair is enough). That's what OH- does here.
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I guess you don't mean to say that the conjugate acid and conjugate base should differ by one.
That's exactly what I am saying.
Looks to me like you are confused about what conjugate acid and base are - these are things that react directly by transferring a proton between them. They always come as conjugate pairs. We do have two pairs of conjugate acid/base here, OH-/O2- and H2O/OH-, in each pair their charges differ by one. None of these substances is "non-conjugate".
It is perfectly possible for something to be both conjugate acid and conjugate base in different pairs. In such cases there is no way to tell whether something is a conjugate base or conjugate acid without stating the context (typically just mentioning the other conjugate of the pair is enough). That's what OH- does here.
Here is how i've seen the terms used.
(https://i.stack.imgur.com/jXNSe.png)
So certainly a conjugate pair would differ by one
But a conjugate pair involves a (non-conjugate) acid and conjugate base, or a (non-conjugate) base and conjugate acid
A conjugate acid and conjugate base, wouldn't be a conjugate pair.
A conjugate pair is one from the left and one from the right. (an acid from the left and corresponding base from the right). Or a base from the left and corresponding acid from the right).
If you say "conjugate acid and conjugate base should differ by one."
Then I think you are using the terms differently than the examples of usages of terms in that image.
'cos a pair that should always differ by one, wouldn't be composed of two on the right.
You seem to be considering O2- , which is on the left, to be a conjugate base, and then the OH- on the right, a conjugate acid. So then you can say that the conjugate acid and conjugate base differ by one. But it doesn't seem to be standard usage looking at that image, to refer to a formula unit or ion or atom on the left to be a conjugate acid or conjugate base. Based on the image i've shown, those terms seem to be reserved for that which is on the right of the equation.
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Just because sometimes one "conjugate" is omitted doesn't mean acid and base of a pair are not both conjugate. There is a full symmetry. What you suggest is that sometimes one of the pair (selected at random) is not "conjugate" for no apparent reason.
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Just because sometimes one "conjugate" is omitted doesn't mean acid and base of a pair are not both conjugate. There is a full symmetry. What you suggest is that sometimes one of the pair (selected at random) is not "conjugate" for no apparent reason.
It's not for no reason, I think they're saying the one on the right is derived.
This one for example doesn't just not use the word conjugate for those on the left, but it marks the ones on the left as the parent.
https://courses.lumenlearning.com/suny-mcc-organicchemistry/chapter/bronsted-lowry-acids-and-bases/
(https://i.stack.imgur.com/BV1vS.png)
Looking on google images, most examples don't have the word conjugate on the left. I think because the word conjugate can suggest it's a derivation..
But you could look at the ones on the left as conjugates.. in the sense of being alike.
And I did find two images on google images that use your approach in labelling.. plus an entry on marriam webster. https://www.merriam-webster.com/dictionary/conjugate "The acid NH4+ and the base NH3 are conjugate to each other."
https://www.chemistrystudent.com/acidsandbases.html
(https://i.imgur.com/Z78qotK.png)
https://www.researchgate.net/figure/Examples-of-acid-base-conjugate-pairs_fig1_352039113
(https://i.imgur.com/flYpN31.png)
From what I understand there might be some technicality with arrows in a reaction, whether the arrows are making a statement about the equilibrium or the reversibility or even perhaps both.. If they're making a statement about reversibility then I suppose you could say that both the LHS and RHS are derived.
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It's not for no reason, I think they're saying the one on the right is derived.
And how do you select which one is "original" and which one is "derived"?
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And how do you select which one is "original" and which one is "derived"?
well it's a bit flawed 'cos the reaction is reversible so while they want to say the products are derived, one could also say the reactants are derived.
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So for Na2O + H2O --> 2NaOH, we have the conjugate acid-base pairs.
OH-/O2- and H2O/OH-
So we have the conjugate base on the left of the equation O2-, gaining an H+ and becoming the conjugate acid OH-
And we have the conjugate acid on the left of the equation H2O, losing an H+, and becoming the conjugate base OH-
Does that apply to all metal oxides, that the metal oxide is not a bronsted lowry base, only the oxide anion is. And the metal is a spectator ion?
So likewise for CaO + 2HCl --> CaCl₂ + H₂O?
Would you say the conjugate pairs are O²⁻ / H2O. And HCl/Cl
One possible issue I have with those pairs, is that O²⁻ receives -two- Hydrogen cations to become H2O. But I suppose that's ok?
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Does that apply to all metal oxides, that the metal oxide is not a bronsted lowry base, only the oxide anion is. And the metal is a spectator ion?
No, metals are not always spectators.
Fe3+ + H2O :requil: FeOH2+ + H+
Would you say the conjugate pairs are O²⁻ / H2O
No, this is a two step reaction, so if anything there are two separate pairs of conjugate acid/base.
But in general: don't try to overgeneralize. There are several theories of acids and bases, each one trying to be more general (check the wikipedia page on acid-base reactions). None of them is universal enough to cover everything, at the same time sometimes trying to squeeze every reaction into the same scheme is fruitless and doesn't produce any new insight, just confuses things.
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In reference to NaOH on the RHS of the reaction
Na2O + H2O --> 2NaOH
You say
NaOH neither Bronsted acid nor base. Na+ is just a spectator, it is OH- that is reacting.
Would that apply also to Na2O in Na2O + H2O, since we don't get Na2OH?
And to CaO + H2O since we don't get CaOH we get Ca(OH)2 ?
I have heard that all arrhenius acids and bases, are bronsted lowry ones, but if we look at an example like
HCl + KOH --> H2O + KCl
We don't get KOH2 So does that make KOH not a bronsted lowry base? (And K+ a specatator ion?) So the base is just the OH- and not the KOH?
I can see that for NH3 + HCl --> NH4+ Cl- There are no spectator ions. And you have easy to see conjugate pairs. Clear bronsted acids and bronsted bases.
Thanks
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So does that make KOH not a bronsted lowry base?
Think in terms of net ionic reactions. All the time you ignore fact most of the substances involved are dissociated and you try to classify as bases or acids whole molecules. It doesn't work this way.
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Think in terms of net ionic reactions. All the time you ignore fact most of the substances involved are dissociated and you try to classify as bases or acids whole molecules. It doesn't work this way.
Would you say it does work in this case with no spectator ions? NH3 + HCl --> NH4+ Cl-
And based on what you've said about not considering whole molecules, would you agree that HCl + KOH --> H2O + KCl has spectator ion K+ ? I think even there, we would consider HCl as a whole, as the conjugate pair is HCl/Cl- And we'd consider H2O as a whole because the conjugate pair there is OH-/H2O. But not KOH as a whole because K+ is a spectator ion. Would you agree?
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HCl
You do know HCl is 100% dissociated in water solutions?
(Technically there exist some theoretical concentration of HCl, but it is completely negligible in the context of the reaction)
Sounds to me like you have learned earlier that KOH is a base and HCl is an acid, and you can't move ahead and understand it is only a simplification, one that at this moment (when learning about Bronsted acids/bases) doesn't matter any longer.
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HCl
You do know HCl is 100% dissociated in water solutions?
(Technically there exist some theoretical concentration of HCl, but it is completely negligible in the context of the reaction)
Sounds to me like you have learned earlier that KOH is a base and HCl is an acid, and you can't move ahead and understand it is only a simplification, one that at this moment (when learning about Bronsted acids/bases) doesn't matter any longer.
I do know that HCl dissociates and then reacts immediately. If it were HCl + H2O then we get H+(aq) + Cl-(aq) and the H+ is generally considered to be H3O+ and the Cl- just interacts with the H2O.
And of course in the examples, the HCl dissociates hence there's no HCl on the other side.
When you look at the conjugate pairs for NH3 + HCl --> NH4+ Cl- what do you say they are - I'd have thought they are HCl/Cl- and NH3/NH4+ ?
I don't know if you're trying to get at the idea that the HCl dissociates therefore HCl isn't among the conjugate pairs.
I don't understand what you are getting at by pointing to the fact that HCl dissociates. I'm aware that it does
I'm fine with you saying HCl isn't a bronsted acid, and KOH isn't a bronsted base but i'm asking what the acid and base conjugate pairs are then?
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I don't understand what you are getting at by pointing to the fact that HCl dissociates. I'm aware that it does
So start writing reaction equations as net ionic.
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I don't understand what you are getting at by pointing to the fact that HCl dissociates. I'm aware that it does
So start writing reaction equations as net ionic.
My issue there is with the conjugate pairs I end up with..
HCl + KOH --> H2O + KCl
I suppose perhaps you don't like writing that HCl on the left because that's HCl(aq) and it's not explicitly stating that it's dissociated H+(aq) + Cl-(aq). And might suggest that it's not dissociated? For me HCl(aq) means H+(aq) + Cl-(aq). Just as KOH(aq) means K+(aq) + OH-(aq).
OH- <-- fine that's not a spectator ion
H+ <-- that's not a spectator ion
K+ <-- that's definitely a spectator ion
Cl- <-- i'm not sure but I think it is a spectator ion..
H+ <-- not a spectator ion
--
H+ + OH- --> H2O
H+/BLANK, OH-/H2O
If I take Cl- as a spectator ion then I get a blank. in one of my conjugate pairs. And that seems problematic?
When you write "Sounds to me like you have learned earlier that KOH is a base and HCl is an acid, and you can't move ahead and understand it is only a simplification, " I am not sure if you are suggesting that K+ and Cl- are spectator ions. But if you are then, treating Cl- as such, I get that issue of H+/BLANK, OH-/H2O
I am unsure how to identify spectator ions in bronsted lowry acid base reactions.
Normally with net ionic equations , as I understand it, one identifies spectator ions by them being those that don't change in oxidation state.
But in these bronsted lowry acid base reactions, at least in examples i've seen, none of the atoms change in oxidation state. e.g.
H+ + OH- --> H2O <-- H is +1. O is 2-. No change in oxidation state in any of the atoms there. But none of those are spectator ions. So it opens the question of how to identify spectator ions in bronsted lowry acid base reactions.
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Think in terms of H3O+.
In water there are no acids stronger than H3O+ nor bases stronger than OH-, it is called a levelling effect.
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Normally with net ionic equations , as I understand it, one identifies spectator ions by them being those that don't change in oxidation state.
But in these bronsted lowry acid base reactions, at least in examples i've seen, none of the atoms change in oxidation state. e.g.
You are freely mixing ions and atoms, as if they were the same thing. Don't.
Spectator is something that doesn't change and is written the same way before and after the reaction.
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okay i've looked into spectator ions again, and for ionic equations so granted, no neutral atoms..
first using this example AgNO3 + HCl --> HNO3 + AgCl(s) and getting a net ionic equation from it.
Ag+(aq) + NO3-(aq) + H+(aq) Cl-(aq) --> H+(aq) + NO3-(aq) + AgCl(s)
I see oxidation states aren't relevant.. And there are no changes in oxidation states in this anyway..
The aqueous ions cancel. And we are left with this net ionic equation
Ag+(aq) + Cl-(aq) --> AgCl(s)
Ag and Cl are involved in a phase change, so aren't spectators.
The others are just aqueous ions on both the left hand side and right hand side, no phase change.. So are considered to be spectator ions and can be cancelled out.
The thing is though if I apply that to
Na2O + H2O --> 2NaCl
That's
Na2O(s) + H2O --> 2NaCl(aq)
So the Na+ is involved in a phase change, so it doesn't seem like a spectator ion. It's going from solid form, to solvated ions.
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Na2O(s) + H2O --> 2NaCl(aq)
NaCl?
Besides, the name IONIC implies you should write the product as dissociated on the RHS.
So the Na+ is involved in a phase change, so it doesn't seem like a spectator ion. It's going from solid form, to solvated ions.
Yes.
Please don't hijack old threads to discuss a new subject, start a new thread to do so.
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well, when I wrote "Na2O(s) + H2O --> 2NaCl(aq)" that was a typo at the end I meant Na2O(s) + H2O --> 2NaOH(aq)
and spectator ions there I don't think is a new subject 'cos eg you mentioned that Na+ is a spectator there in post #6..and it has been but I take your point that you see it as a new subject and an old thread, so I will start a new thread re the spectator ions.
Thanks