# Why is Nitric Acid more reactive and caustic than Sulfuric or Hydrochloric?

Nitric acid reacts much more vigorously with living tissue and is a obviously more immediately hazardous on contact than sulfuric or hydrochloric.

What is the reason for this? Is it due to the oxidizing potential of the nitrate anion?

Does the greater reactivity of nitric acid also hold for reactions of acids with metals? Perhaps the factors related to the acid anions that determine dissociation are not the same ones that determine their reactivity?

• As you state, it is the oxidizing potential of the NO3- radical. Neutral nitrates also react with organic compounds. For example AgNO3 was called "lunar caustic", though the pH+ of an aqueous is about 6 or 7 (depending on dissolved CO2). – DrMoishe Pippik Dec 28 '16 at 6:34

Nitric Acid is indeed what most often is referred to as a "strong acid" (at ambient temperatures), and its $\mathrm{pK_a} \approx -1$, i.e nitric acid in diluted solution is almost always fully dissociated except in extremely acidic solutions.

First of all, how does it compare to other "strong acids", for example sulfuric acid

$\ce{HNO3 + 2 H2SO4 ⇌ NO2+ + H3O+ + 2HSO4−}$

Clearly, in this reaction sulfuric acid acts as the "acid", while nitric acid acts as the "base", hence sulfuric acid is indeed a stronger acid.

Reactions of dilute nitric acid with some metals are typical of an acid, namely:

$\ce{Mg + 2 HNO3 → Mg(NO3)2 + H2\uparrow}$

Other metals that react in this fashion are manganese and zinc.

You do correctly note that a special feature of nitric acid's reactivity is its strength as an oxidising agent, and this is also evident in its reactions with metals, for example. However, do note, that the exact details depend on the concentration of the acid and the nature of the metal

For example, nitric acid can oxidise non-active metals such as copper and silver, however, the products depend on temperature and the acid concentration.

For instance, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry:

$\ce{3 Cu + 8 HNO3 → 3 Cu^2+ + 2 NO + 4 H2O + 6 NO3−}$ The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide.

However, with more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry:

$\ce{Cu + 4 H+ + 2 NO3− → Cu^2+ + 2 NO2 + 2 H2O}$

Metals like chromium, iron and aluminium readily dissolve in dilute nitric acid, but the concentrated acid forms a metal-oxide layer that protects the bulk of the metal from further oxidation; thus, concentrated nitric acid serves to passivate these metals towards further reactions. Common metals that are passivated by concentrated nitric acid are iron, cobalt, chromium, nickel, and aluminium.

• I'm upvoting. As far as the answer, what is nitric acid doing to organic material then? Is it oxidizing them to more oxidized organic compounds or is it actually causing combustion. And outside of the answer, is Silver Nitrate similarly formed with dilute Nitric acid. Also, wondering as I know that Potassium Chlorate can react similarly to Potassium Nitrate as a rapid oxidizer, how does Perchloric acid compare to Nitric as an oxidizer. – Joseph Hirsch Dec 28 '16 at 17:38
• @JosephHirsch Yes, it would act as an oxidant when it reacts with organic matter. Yes, silver nitrate can be formed by reacting metallic silver with nitric acid. However, I don't know how perchloric acid and nitric acid compare as oxidants, but I believe you can find relevant data online. – getafix Dec 28 '16 at 17:47