Acids and bases question

ClovisI

Hi, I'm a 5th year student of (among other things) chemistry.

We recently covered acids and bases and I have to say there is something that I don't fully understand.

We are told that Arrhenius' theory states that:

an Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+)

and

an Arrhenius base is a substance that dissociates in water to form hydroxide (OH−) ions

So as far as I can tell, an acid is anything which, if put in water will increase the concentration of hydronium. And vice versa for bases and OH-.

However the textbook (and wikipedia, http://en.wikipedia.org/wiki/Acid#Br.C3.B8nsted-Lowry_acids), says that a certain reaction, (CH3COOH + NH3 ===> (CH3COO-) + (NH4+)) cannot be described as acid-base in terms of Arrhenius' Theory because " the reaction does not produce hydronium" (in my textbook it says "because the reaction doesn't involve water"). Hence the need for the Bronsted-Lowry theory.

But nowhere as far as I can see in the Arrhenius' definitions does it mention that all acid-base reactions must produce hydronium, nor involve water. As far as I can see, all that needs to be true for this to be an acid-base reaction is for both reactants to behave in a certain way if they were put in water. And this is true, as far as I know.

Could someone please clarify this for me as my teacher doesn't seem to be able to explain it very well and I refuse to write up any notes on the topic until I understand it!

Woodward

The proper definition of an Arhenius acid is one that increase the H3O+ concentration in water and a base is one which increases the OH- concentration

ClovisI

Thanks, I agree with you that that definition makes more sense, but I still don't think that it answers my question.

ianobrien

Just a couple of things to keel in mind
1. Both Bronsted-Lowry and Arrhenius models use the transfer of components from one molecule to another
2. A proton is simply a H+
3. Bronsted-Lowry involves the transfer of protons and as such is a more general case when compared to Arrhenius Acid mechanism. Adding a proton to a water molecule produces a H3O+, removing a proton from a water molecule produces a OH-

The easiest thing that I found was to understand how the Bronsted-Lowry worked and that the Arrhenius was a specific example of the Bronsted-Lowry

Pace2008

ClovisI wrote: »

But nowhere as far as I can see in the Arrhenius' definitions does it mention that all acid-base reactions must produce hydronium, nor involve water. As far as I can see, all that needs to be true for this to be an acid-base reaction is for both reactants to behave in a certain way if they were put in water. And this is true, as far as I know.

Could someone please clarify this for me as my teacher doesn't seem to be able to explain it very well and I refuse to write up any notes on the topic until I understand it!

The Arrhenius definition of acids and bases refers only to reactions in aqueous solution ie. where the solvent is water.

Check your textbook again, it might not specifically mention water and hydronium but it'll probably refer to aqueous solutions and H+ (shorthand for hydronium when dealing with Arrhenius acids and bases).