Which one is nitric Acid, HNO3 by itself, or when it is included in H2O?

Question

This is a quote from my Chemistry textbook:

Nitric acid ionises according to the following equation: $$\ce{HNO3(l) + H2O(l) -> H3O+(aq) + NO3- (aq)}$$

I know that the formula for nitric acid is $\ce{HNO3}$. But I cannot understand what is the product of this equation. What is $\ce{H3O+(aq) + NO3- (aq)}$? Is it a diluted acid with less concentration?

This is another quote (from my textbook) that confuses me:

Nitric acid and sulfuric acid are not quite as strong as hydrochloric acid. Chemical and spectroscopic studies have shown that concentrated solutions of both these acids do contain some unionised molecules. For our purpose, however, their ionisation is effectively complete. This is particularly true in dilute solutions.

I cannot understand the part that is in the bold. What are these unionised molecules? I know that these acids are considered strong acids which completely ionise in water solution. If that is the case, then why does the text indicate that there are some unionised molecules?

Answer 1

In your first reaction, $\ce{H2O}$ acts as a base to abstract an $\ce{H+}$ from the nitric acid. The resulting $\ce{H3O+}$, called hydronium, is the conjugate acid, while the $\ce{NO3-}$, called nitrate, is the conjugate base (this is the molecule of nitric acid, but stripped of its $\ce{H+}$). You could view this as a dilution, in the sense that the resulting solution is less acidic than highly concentrated nitric acid would be, but the process of dilution of acid by mixing with water inevitably involves an acid-base reaction.

As to your second question, the unionized molecules are the neutral water and nitric acid on the reactant side, while the ions are the products resulting from the acid-base reaction. Nitric acid is relatively strong, meaning it tends to ionize almost completely in aqueous solution. Additionally, the less acidic the solution already is prior to addition of the acid, the more extensively the added acid will dissociate into ions (to understand this, please read up on acid-base equilibria and Le Chatelier's principle, as the topics are too lengthy and complex to cover adequately here). This is why the author emphasizes that the dissociation is more complete in dilute solutions. Statistically, however, some proportion of the nitric acid molecules will inevitably not dissociate into ions. The stronger an acid, the greater the extent of dissociation, but there is always some statistical chance that a proportion of the molecules will remain unreacted. For strong acids, however, the overwhelming preponderance of the molecules will dissociate, while only a tiny fraction remain unreacted. Hence, for many (if not most) purposes, it's reasonable when dealing with strong acids to proceed as if the dissociation is truly complete, since the unreacted portion is typically negligible in quantity.

Answer 2

Different compounds and mixtures are described as nitric acid

Depending on context and use several chemicals are described as "nitric acid".

The common laboratory chemical usually called concentrated nitric acid is a 68% solution of nitric acid in water (the composition is set as this is an azeotropic mixture that is easier to manufacture).

But other reagents consist of mostly nitric acid with little water (as lower water levels are often useful in rocket fuels or as nitrating or oxidising agents). White fuming nitric acid is mainly pure HNO3 with less than 2% water. Red fuming nitric acid is basically relatively pure HNO3 but will a large proportion of NO2 in the mix (which might be thought to reduce the water content as it reacts with water to give nitric acid). But those fuming variants are more complicated than they look as complex equilibria exist with various nitrogen oxides and some of these equilibria create water even when the initial mixture doesn't contain any. So it is impossible to get bulk nitric acid that is free of any water.

So the question of which reagent is nitric acid is moot as several reagents are called nitric acid and even the "pure" substance isn't completely HNO3 because of a range of side reactions that can't be prevented.

Answer 3

The product is simply the ionized form of $\text{HNO}_3$; if you prefer it is equally correct to write $$\text{HNO}_3(aq)\rightarrow \text{H}^+(aq)+\text{NO}_3^-(aq)$$

Note that technically nitric acid is $\text{HNO}_3(aq)$and not $\text{HNO}_3(l)$ (which would presumably be the salt hydrogen nitrate in liquid form) As hydrogen nitrate will simply decompose under standard conditions if not dissolved so $\text{HNO}_3$ is sufficient to refer to nitric acid; however, it is understood to refer to $\text{HNO}_3$ dissolved in water, not just $\text{HNO}_3$ alone.