Nitric Acid ($HNO_3$)
Ostwald Process, Oxidizing Power, Metal Reactivity, and Passivity.
Nitric acid ($HNO_3$), previously known as aqua fortis (strong water), is a highly corrosive mineral acid. It is one of the most important industrial chemicals, used extensively in the manufacture of fertilizers (ammonium nitrate) and explosives (TNT, nitroglycerin). In competitive exams, its unique concentration-dependent oxidizing properties are tested heavily.
1. Preparation of Nitric Acid
A. Laboratory Preparation
In the lab, nitric acid is prepared by heating a nitrate salt (like $KNO_3$ or $NaNO_3$) with concentrated sulphuric acid in a glass retort.
B. Industrial: The Ostwald Process
On a large scale, nitric acid is manufactured by the catalytic oxidation of Ammonia. This is known as the Ostwald Process and involves three distinct steps:
A mixture of $NH_3$ and air is passed over a Platinum / Rhodium gauge catalyst at 500 K and 9 bar pressure.
The Nitric Oxide ($NO$) thus formed readily combines with oxygen to form Nitrogen dioxide ($NO_2$).
Nitrogen dioxide gas is dissolved in water to give Nitric acid. The NO produced here is recycled back to step 2.
2. Chemical Properties: Powerful Oxidizing Agent
Nitric acid contains nitrogen in its maximum oxidation state (+5). Therefore, it acts exclusively as a very strong oxidizing agent. It oxidizes most non-metals, metals, and inorganic compounds.
A. Oxidation of Non-Metals
Concentrated $HNO_3$ oxidizes non-metals to their corresponding higher oxoacids, while the nitric acid itself is reduced to Nitrogen dioxide ($NO_2$).
| Non-Metal | Oxidized Product | Reaction (with Conc. $HNO_3$) |
|---|---|---|
| Iodine ($I_2$) | Iodic Acid ($HIO_3$) | $I_2 + 10HNO_3 \rightarrow 2HIO_3 + 10NO_2 + 4H_2O$ |
| Carbon ($C$) | Carbon Dioxide ($CO_2$) | $C + 4HNO_3 \rightarrow CO_2 + 2H_2O + 4NO_2$ |
| Sulfur ($S_8$) | Sulfuric Acid ($H_2SO_4$) | $S_8 + 48HNO_3 \rightarrow 8H_2SO_4 + 48NO_2 + 16H_2O$ |
| Phosphorus ($P_4$) | Phosphoric Acid ($H_3PO_4$) | $P_4 + 20HNO_3 \rightarrow 4H_3PO_4 + 20NO_2 + 4H_2O$ |
B. Reaction with Metals (Copper & Zinc)
Unlike standard acids ($HCl$, $H_2SO_4$) that release Hydrogen gas ($H_2$) upon reacting with metals, Nitric acid rarely produces $H_2$ gas. Because it is a powerful oxidizer, any $H_2$ produced is immediately oxidized into water ($H_2O$), while the $HNO_3$ is reduced to various nitrogen oxides ($NO$, $NO_2$, $N_2O$).
The products depend strictly on the Concentration of the acid and the Nature of the metal:
$3Cu + 8HNO_3(\text{dilute}) \rightarrow 3Cu(NO_3)_2 + 2\mathbf{NO} + 4H_2O$
$Cu + 4HNO_3(\text{conc.}) \rightarrow Cu(NO_3)_2 + 2\mathbf{NO_2} + 2H_2O$
$4Zn + 10HNO_3(\text{dilute}) \rightarrow 4Zn(NO_3)_2 + 5H_2O + \mathbf{N_2O}$
$Zn + 4HNO_3(\text{conc.}) \rightarrow Zn(NO_3)_2 + 2H_2O + 2\mathbf{NO_2}$
Only two metals, Magnesium (Mg) and Manganese (Mn), react with very dilute (1-2%) cold nitric acid to liberate Hydrogen gas ($H_2$).
$Mg + 2HNO_3(\text{v. dilute}) \rightarrow Mg(NO_3)_2 + \mathbf{H_2 \uparrow}$
C. Passivity of Metals
Some metals like Iron ($Fe$), Aluminum ($Al$), Chromium ($Cr$), Nickel ($Ni$), and Cobalt ($Co$) do not dissolve in concentrated nitric acid.
Reason: The strong oxidizing nature of concentrated $HNO_3$ forms an extremely thin, tough, and insoluble protective layer of metallic oxide on the surface of the metal. This layer halts further reaction, rendering the metal chemically inert or "passive".
3. The Brown Ring Test (For Nitrate Ion)
The presence of the nitrate ion ($NO_3^-$) in any salt or solution is confirmed in qualitative analysis using the famous Brown Ring Test.
Procedure: A freshly prepared solution of Iron(II) sulphate ($FeSO_4$) is added to an aqueous solution containing the nitrate ion. Then, concentrated Sulphuric acid ($H_2SO_4$) is slowly added down the inner side of the test tube.
Observation: A distinct brown ring forms at the junction of the two liquid layers.
The Chemistry (Reactions):
First, the nitrate ion is reduced to Nitric Oxide ($NO$) by $Fe^{2+}$ ions (which are oxidized to $Fe^{3+}$) in the acidic medium.
Second, this $NO$ gas is trapped by the excess $Fe^{2+}$ ions (present as a hydration complex) to form a brown-colored coordination complex.
(Pentaaquanitrosyliron(I) ion - The Brown Ring)
Figure 1: The Brown Ring forms exactly at the junction of the heavier sulfuric acid layer and the lighter aqueous layer.
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