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Offline dirt

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Water as an acid
« on: October 18, 2007, 04:08:30 AM »
2Na + 2H20 = 2NaOH + H2
is water acting as an acid in this equation? i see it as a typical  'acid + reactive metal reaction' but my chemistry teacher says that if behaved as an acid, then one of of the products would be H+. i think this wrong. anyone want to shed some light?

Offline Borek

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Re: Water as an acid
« Reply #1 on: October 18, 2007, 06:04:17 AM »
In a way water is acid and base at the same time, as it dissociates (slightly! see water ion product) giving both H+ and OH-:

H2O = H+ + OH-

I would not say it is a 'typical acid + reactive metal reaction', although it can be treated as one. Just drop that 'typical' part.
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Offline constant thinker

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Re: Water as an acid
« Reply #2 on: October 18, 2007, 08:03:55 PM »
Water is amphoteric (good word to know).
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Offline dirt

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Re: Water as an acid
« Reply #3 on: October 19, 2007, 01:00:18 AM »
i dont see how it isnt acting as an acid. hydroxide is always the product when water acts as an acid, it donates a proton to form a hydroxide ion. hydrogen ions are only present when water acts as a base.

Offline enahs

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Re: Water as an acid
« Reply #4 on: October 20, 2007, 10:44:27 AM »
It depends on what type of acid your are talking about.
http://en.wikipedia.org/wiki/Acid-base_reaction_theories

Some of those types of acids are pretty broad and the vast majority of chemical reactions could be considered acid-base reactions.

If you limited it to the Lewis acid-base (acid produces H+) then no, it is not an "acid".

Offline dirt

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Re: Water as an acid
« Reply #5 on: October 21, 2007, 04:30:01 AM »
that definition does not apply to this situation because acids only produce hydronium ions in the presence of water, but this is 'water in water', so to speak. cant you see that hydrogen ions are only produced when water acts as a base in the presence of another acid? and that hyroxide ions are always produced when water acts as an acid?
Water as a base: HCl + H2O = Cl(-) + H3O(+)
Water as an acid: NH3 + H2O = NH4(+) + OH(-)    (note: no H(+) or H3O(+) present)
« Last Edit: October 21, 2007, 04:41:03 AM by dirt »

Offline Mitch

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Re: Water as an acid
« Reply #6 on: October 21, 2007, 04:53:03 AM »
that definition does not apply to this situation because acids only produce hydronium ions in the presence of water, but this is 'water in water', so to speak. cant you see that hydrogen ions are only produced when water acts as a base in the presence of another acid? and that hyroxide ions are always produced when water acts as an acid?
Water as a base: HCl + H2O = Cl(-) + H3O(+)
Water as an acid: NH3 + H2O = NH4(+) + OH(-)    (note: no H(+) or H3O(+) present)

You have faulty logic. The problem is, you seem to understand half of it, and the other half is a bit twisted. Most people understand your confusion, since you've most likely only have been taught a very narrow and limited definition of what an acid or a base is.

I would say the reaction you gave: 2Na + 2H20 = 2NaOH + H2
is not an acid/base reaction, because there is no donation of an electron pair. However, the end product of NaOH will cause the aqueous solution to become basic.
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Offline Borek

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Re: Water as an acid
« Reply #7 on: October 21, 2007, 05:17:11 AM »
acids only produce hydronium ions in the presence of water

Hardly surprising when water is responsible for 18/19 of the hydronium mass :) But acids are acids even when there is no water around and no hydronium ions are produced.

Quote
but this is 'water in water', so to speak. cant you see that hydrogen ions are only produced when water acts as a base in the presence of another acid? and that hyroxide ions are always produced when water acts as an acid?
Water as a base: HCl + H2O = Cl(-) + H3O(+)
Water as an acid: NH3 + H2O = NH4(+) + OH(-)    (note: no H(+) or H3O(+) present)

We are in murky waters here. First of all - define an acid and a base precisely, than we can think whether water fits or not. Stating

Quote
that definition does not apply to this situation

and then calling something an acid without telling us WHICH definition applies is just juggling with names. Nature doesn't like our simple definitions so you will most likely hit the wall sooner or later.

Note also, that a lot will depend on the detailed mechanism of the reaction.

Quote
NH3 + H2O = NH4+ + OH-

How do you know it is not two steps process involving H+:

H2O = H+ + OH-
NH3 + H+ = NH4+

or (no H+)

NH3 + H2O = NH4OH
NH4OH = NH4+ + OH-
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Offline schmitgreg

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Re: Water as an acid
« Reply #8 on: October 21, 2007, 09:24:19 AM »
Not to ask dumb questions, but my bio teacher always taught me that acid means more H+ and base means more OH-. I'm not sure if that is a correct definition for all acids/bases, because I just thought that if an atom lacked an electron then it would take it from another compound as long as the other compound didn't "want" it as much. Then that atom/molecule that ripped off the electron acted as an acid.

So what defines an acid or a base??

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Re: Water as an acid
« Reply #9 on: October 21, 2007, 01:53:08 PM »
Not to ask dumb questions, but my bio teacher always taught me that acid means more H+ and base means more OH-. I'm not sure if that is a correct definition for all acids/bases, because I just thought that if an atom lacked an electron then it would take it from another compound as long as the other compound didn't "want" it as much. Then that atom/molecule that ripped off the electron acted as an acid.

So what defines an acid or a base??

That is likely the same definition dirt is using, but it is a very limting definition and obviously is now leading to confusion.
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Offline enahs

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Re: Water as an acid
« Reply #10 on: October 21, 2007, 05:39:24 PM »

So what defines an acid or a base??

You do. When you are talking about acids and bases. Now, in order to communicate with other people you can not just make up random stuff and expect to understand. So we as chemists agree to accept the definitions of acids and bases, more specifically the two most common are the "Lewis" definition and the "Bronsted-Lowry" definition, that was in the webpage I linked to previously.

What dirt is trying to argue basically that in his opinion it is behaving as a typical acid, but it does not produce H+.  However, his teachers definition of acid "increase H+ concentration and a base increase OH- concentration" is valid, and the most typical simplified definition of acid/base chemistry.



Now, to try and help dirt and clear up his confusion.
Quote
i dont see how it isnt acting as an acid. hydroxide is always the product when water acts as an acid, it donates a proton to form a hydroxide ion. hydrogen ions are only present when water acts as a base.

HCl is and acid, because it produces H+ in water, making some H3O+, the H+ comes from HCl, not the water.
In water there is always some H+ and OH- present. It is a very small concentration and can typically be ignored, it is as CT linked to earlier, amphoteric.


Your statement is clear you are confusing, in the typical definition of acid/base your teacher is using and teaching you, as to what generates the H+ or OH-. It is not the water, the water accepts the additional ions.



Offline dirt

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Re: Water as an acid
« Reply #11 on: October 24, 2007, 08:11:30 AM »
the definition that i am working with, my simplistic year 11 definition, is : an acid donates a proton or H+, a base accepts a proton. 
I see water acting as an acid because in order for hydroxide to form, the water has to lose the hydrogen ion.
Quote
I would say the reaction you gave: 2Na + 2H20 = 2NaOH + H2
is not an acid/base reaction.
-  i never said that it was.
 
Quote
How do you know it is not two steps process involving H+:
it obviously is a two step process involving H+ but the quote from my teacher states that H+ must be one of the final products.
My problem is that i see reactions involving acids that do not produce H+ as a product, so this confuses me.
for example: 2Na + H2SO4 = Na2SO4 + H2.
i can only see that Hydrogen ions are present in the acid solution on the reactant side of the equation.
« Last Edit: October 24, 2007, 08:24:44 AM by dirt »

Offline enahs

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Re: Water as an acid
« Reply #12 on: October 24, 2007, 09:15:05 AM »
Quote
the definition that i am working with, my simplistic year 11 definition, is : an acid donates a proton or H+, a base accepts a proton.
I see water acting as an acid because in order for hydroxide to form, the water has to lose the hydrogen ion.

Your definition of acid your teacher is talking about is slightly skewed.
An acid increases the concentration of H+.
A base increases the concentration of OH-.

If a water donates a H+, it by definition also donates a OH-. You can not have one without the other.

To be acidic by the definition you are supposed to be using, there has to be more free H+ in solution then OH-.


If water donates a proton it creates OH- ,which can accept a proton, and so by your definition you are saying water is and acid and base at the same time. Which it is. And why this is not an acid-base reaction by the definition of increasing the H+ concentration or increasing the OH- concentration.








« Last Edit: October 24, 2007, 12:04:25 PM by enahs »

Offline dirt

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Re: Water as an acid
« Reply #13 on: October 24, 2007, 05:28:48 PM »
thank you. a response that that has actually made things clearer.
Quote
Your definition of acid your teacher is talking about is slightly skewed.
An acid increases the concentration of H+.
A base increases the concentration of OH-.
i understand that by this definition, water cant be truely acidic because the final solution is very basic, but how does this definition apply to the example i mentioned before?
: 2Na + H2SO4 = Na2SO4 + H2
Obviously the sulfuric acid on the reactant side has a high concentration of H+ but what about the products? does that matter?
 

Offline enahs

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Re: Water as an acid
« Reply #14 on: October 24, 2007, 07:32:06 PM »
Quote
: 2Na + H2SO4 = Na2SO4 + H2
Obviously the sulfuric acid on the reactant side has a high concentration of H+ but what about the products? does that matter?

No, it does not matter. Just because something is an "acid" does not mean it can only be involved in acid/base reactions in the sense of increasing H+ or OH-.
This is just a single displacement reaction.

It is true that in this case the pH will go up if you measure the pH before adding the Na and after. It goes up because the chemical that acts as an acid is used up in a chemical reaction. The acid is removed from solution through a single displacement reaction.



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