# Defining an acid-base reaction from the perspective of the Arrhenius theory of acids and bases

$$\ce{NH3 + HCl -> NH4Cl}$$

The above equation is the reaction of ammonia with hydrochloric acid (all substances are aqueous). I understand that ammonia counts as a base because it can react with water to give hydroxide ions, and $\ce{HCl}$ is an acid. By logic, this should be an 'acid-base' reaction and this link says that it is indeed an acid-base reaction but how is this so if no water is produced?

I am learning acid-base theory from the beginning and can understand why this is an acid base reaction from the perspective of the Brønsted-Lowry theory and the Lewis theory, but I am now attempting to understand the Arrhenius theory, so please explain to me, in terms of the Arrhenius theory, why this is an acid-base reaction and a 'neutralistaion' if there is no water produced and no hydroxide ions which take place in the reaction what so ever?

• Unless you are considering this equation as a gas phase reaction then substances are by convention always dissolved in water, unless some other solvent is specifically stated. Thus you know immediately that there are $\ce{ H^+}$ (or $\ce{H3O^+), NH4^+ , Cl^- and OH^-}$ ions present. – porphyrin Aug 12 '16 at 14:12

I understand that ammonia counts as a base because it can react with water to give hydroxide ions, and HCl is an acid.

This is where you went wrong.

The Arrhenius theory is the oldest of the common acid-base theories. According to Arrhenius:

An acid is a compound that will dissociate in aquaeous solution forming $\ce{H+}$ ions.

A base is a compound that will dissociate in aquaeous solution forming $\ce{OH-}$ ions.

As per the Arrhenius theory, ammonia is not a base. Also, contrary to all later theories (Brønsted-Lowry, Lewis and Pearson), Arrhenius did not account for any acid-base reactions; instead, he intended to define only those substances whose solutions reacted acidicly or basicly. It is therefore meaningless to define an acid-base reaction according to the Arrhenius theory.

What Arrhenius did account for was neutralisation reactions. Indeed, if an Arrhenius acid and an Arrhenius base are mixed, the form water and a salt, which can be easily shown in the equation below:

$$\ce{(H+ + A^-) + (B+ + OH^-) -> HOH + A^+B^-}$$

But it was not until Brønsted’s and Lowry’s theory that it was realised how a reaction should always be acid-base and not one or the other.

• I'll just add that there are multiple theories about how to characterize acids and bases. By learning the different theories and where they are applicable you learn about the nuances of chemistry. – MaxW Sep 11 '16 at 19:05

Arrhenius theory is based on the interaction of the two substances in water. Since there is no water in your equation, it can't be applied directly. However, we can make some partial equations to work around it:

$$\ce{NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq)}$$ $$\ce{HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq)}$$

Now the neutralization equation:

$$\ce{H3O+(aq) + OH-(aq) -> 2H2O(l)}$$

If you add the three equations you end up with your original equation:

$$\ce{NH3 + HCl -> NH4Cl}$$

• Thank you very much, so is it just a case of considering more than one reaction taking place, then adding together the equations? The way that you've done it, it seems to me like the 3 equations can be regarded as the mechanism by which the reaction takes place. – Mathematician Aug 12 '16 at 10:01
• It's a neat trick in chemistry: you can break down a reaction equation into as many partial equations as you want in order to explain it better. The cool part is that they don't need to be able to happen in the real world in the way you represent them. The ion-electron method is a popular case where half the equations are mere fantasy, but in the end it all wraps nicely (chemistry.bd.psu.edu/jircitano/redox.html). Just remember that this way of doing chemistry doesn't represent the mechanism by which a reaction occurs. It's only a trick to understand better what's going on. – Variax Aug 12 '16 at 10:10
• But how can I possibly understand better what's going on, when it isn't even going on? And the ion electron equations, aren't they merely a representative of what happens when you have specific ions in a solution, irrespective of the compound? Also, if the ammonia and hydrochloric acid reaction doesn't happen in the way in which you represented it, then this isn't actually an acid base reaction. Where am I going wrong? I'm so confused, all of chemistry seems wrong 😂😂 – Mathematician Aug 12 '16 at 11:59
• I used this method to show you how it's an acid base reaction according to the Arrhenius theory. If you apply the Lewis theory, an acid is a substance capable of accepting a pair of electrons to form a coordinate covalent bond, while a base donates that pair of electrons to form that bond. That is what happens between the H+ of HCl and NH3, to form NH4+ (+Cl- since it's a salt). Don't worry about seeing on paper things that aren't real. Every field of science has its own little tricks, like when physicists assume that lenses are negligibly thin. It's wrong but it makes math easier. – Variax Aug 12 '16 at 12:42
• Another question, if I may. I understand that the Arrhenius theory was rejected because it meant that a substance which was described as an acid in water, wouldn't be described as an acid in gas phase (for example), which was silly. Firstly, if the reaction takes place directly as a collision between NH3 and HCl, then why can't the gas phase reaction just be regarded as an acid-base reaction? Secondly, why can't we just dissolve a substance in water in order to see if it increases the concentration of H+ or OH- ions then permanently define them as acids and bases, irrespective of the phase? – Mathematician Aug 14 '16 at 3:42