Nitric Acid (HNO3) – Formula, Properties, Molar Mass & Uses

Nitric acid is a strong oxidizing agent containing hydrogen, oxygen, and nitrogen atoms. It is manufactured by the catalytic oxidation of ammonia. Nitric acid is a highly useful chemical reagent in laboratories and in the manufacturing of explosives and fertilizers. It is a strong acid with a pH of approximately 3.01. It occurs as a liquid that is colorless to yellow. With a strong, distinct acidic odor, it is easy to identify. It is very corrosive but finds applications in several industries. The following sections explore the nitric acid Lewis structure, molecular mass, nitric acid formula, properties, and more.

What is Nitric Acid?

Nitric acid is a strong acid. It is also known by the following names:

• spirit of nitre
• aqua fortis

It is a colorless liquid in its pure form, but it turns yellowish as it gets older. The yellow cast is due to the decomposition of nitric acid into water and nitrogen oxides. Proper safety measures must be adopted while handling nitric acid because it is highly corrosive and toxic. It can cause severe burns on contact with the skin.

Nitric Acid (Hno3) Formula and Structure

The formula for nitric acid is HNO₃. It contains the atoms of the following elements:

• Three oxygen atoms
• One nitrogen atom
• One hydrogen atom

The nitric acid molecule has one of its three oxygen atoms doubly bonded to the central nitrogen atom. In contrast, the other oxygen atom bonds to the central nitrogen atom via a single bond. An oxygen atom is also bonded to a hydrogen atom via a single covalent bond.

Charge on the Nitric Acid Molecule: The third oxygen atom has a charge of -1. It bonds to the central nitrogen atom. The central nitrogen atom forms four covalent bonds (3 with oxygen atoms and one with hydrogen); it has a +1 charge. Thus, the net charge on the molecule is zero.

The positive charge on the nitrogen is canceled by the negative charge on the oxygen atom. Also, the charges in this molecule can be delocalized due to resonance.

Nitric Acid (HNO3) Lewis Structure

The following image illustrates the nitric acid Lewis structure with covalent bonds and lone pairs. The molecule contains 3 single covalent bonds and a double covalent bond. It also has seven lone pairs of electrons.

Nitric Acid (HNO3) Molar Mass

The molecular mass of nitric acid can be easily calculated using the chemical formula and the atomic masses of its constituent elements.

The atomic mass of Hydrogen = 1u

The atomic mass of Nitrogen = 14 u

Atomic mass of Oxygen = 16u

The molar mass of nitric acid (HNO₃) will be calculated as follows:

Molar mass = (1 × atomic mass of hydrogen) + (1  × atomic mass of nitrogen) +( 3  × atomic mass of oxygen)

Molar mass = (1 × 1) +(1 × 14) + (3 × 16)

Nitric Acid Molar Mass = 63 u

Laboratory Preparation of Nitric Acid (HNO3)

In the lab, nitric acid is prepared by the reaction of sulphuric acid with potassium nitrate.

Principle: An acid that is more volatile can be displaced from its salt using a less volatile acid.

Illustration: Sulphuric acid is less volatile than nitric acid. Therefore, it can be displaced from metal nitrates using sulphuric acid.

Reactants: The following reactants are used in the fraction:

• Potassium nitrate – 50gm
• Concentrated sulphuric acid – 25ml

Procedure: The reactants are placed in a round bottom flask and heated to about 200°C. It is vital to ensure that the temperature does not rise above 200°C.

Reaction: KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

(Salt of more volatile acid + less volatile acid → Salt + more volatile acid)

Collection: The nitric acid vapors are collected. They are condensed to give the nitric acid solution.

Commercial Preparation of Nitric Acid (HNO3)

Commercial nitric acid solutions have a  52% to 68% concentration. They are prepared via the Ostwald process, wherein anhydrous ammonia gets oxidized to nitric oxide. Platinum or rhodium is used as a catalyst for the reaction. A high temperature of about 500 K and a pressure of 9 atm are maintained.

Reactants: Ammonia and oxygen

Catalyst: Platinum

Reaction: 4 NH₃(g) + 5 O₂(g) → 4 NO(g) + 6 H₂O(g)

The reaction involves −905.2 kJ/mol energy.

The nitrogen monoxide produced in the first reaction again reacts with oxygen to form nitrogen dioxide.

2 NO(g) + O₂(g) → 2 NO₂(g)

The reaction involves −114 kJ/mol energy.

Nitrogen dioxide gets absorbed in water to produce nitric acid as follows:

3 NO₂(g) + H₂O(l) → 2 HNO₃(aq) + NO(g)

The reaction involves −135.74 kJ/mol energy.

The nitric oxide produced in the previous step is again cycled back for reoxidation.

4 NO₂(g) + O₂(g) + 2 H₂O(l) → 4 HNO₃(aq)

The aqueous nitric acid obtained is distilled to attain a 68% mass concentration. It can be further concentrated up to 98% by dehydration with concentrated sulphuric acid.

Physical Properties of Nitric Acid 

The important physical properties of nitric acid are as follows:

• Nitric Acid Molar Mass: 63.01 g/mol
• Density: 1.51 g/cm³
• Color: Colourless to yellow
• Odor: Strong acidic smell
• Boiling Point: 83 °C
• Melting Point: -42 °C

Chemical Properties of Nitric Acid 

It is a strong acid that turns blue litmus red. It gives all the tests for acids. Following are some of the important chemical reactions involving nitric acid.

Decomposition

Nitric acid readily decomposes to nitrogen dioxide standing. It turns from colorless to yellow to brown.

4 HNO₃ → 4 NO₂ + O₂ + 2 H₂O

Reaction with Metals

When nitric acid is made to react with metals, it liberates hydrogen gas. However, this reaction is possible only with the metals above hydrogen in the reactivity series.

Reaction with Magnesium: Mg + 2 HNO₃ → Mg(NO₃)₂ + H₂

Reaction with Manganese: Mn + 2 HNO₃ → Mn(NO₃)₂ + H₂

Nitric acid can also react with non-active metals like silver and copper, oxidizing them in the process. The reaction largely depends on the temperature and the acid concentration.

Reaction with Copper: 3 Cu + 8 HNO₃ → 3 Cu²⁺ + 2 NO + 4 H₂O + 6 NO⁻³

Reactions With Non-metals

As Nitric acid is a powerful oxidizing acid, it reacts with several organic materials. These reactions can be explosive too. The hydroxyl group of the acid will typically remove hydrogen from the given organic molecule and result in water formation, While the nitro group jumps to the hydrogen’s place. The primary synthesis procedure of many explosives involves nitrating organic compounds with nitric acids, such as trinitrotoluene (TNT) and nitroglycerin.

In reaction with non-metallic elements, except nitrogen, oxygen, noble gases, halogens (other than iodine), and silicon, it oxidizes them to their highest oxidation states. For instance,

Reaction of concentrated nitric acid with carbon: Gives nitrogen dioxide.

C (graphite) + 4 HNO₃ → CO₂ + 4 NO₂ + 2 H₂O

Reaction of dilute nitric acid with carbon: It gives nitrogen monoxide.

3 C (graphite) + 4 HNO3 → 3 CO₂ + 4 NO + 2 H₂O

Xanthoproteic test

A specific type of yellow nitrated product is formed when Nitric acid reacts with proteins. This reaction is called the xanthoproteic reaction. In this test, concentrated nitric acid is added to the tested substance, and the mixture is heated. The mixture turns yellow if the added proteins contain amino acids with aromatic rings. The color changes to orange when a base is added to the mixture, such as ammonia. The nitrated aromatic rings present in the protein are responsible for these colour changes. When nitric acid comes into contact with the epithelial cells, Xanthoproteic acid is formed. Therefore, owing to these local skin color changes, proper safety precautions must be taken when handling Nitric acid.

Uses of Nitric Acid

• Nitric acid is used in the production of ammonium nitrates, which in turn are used to manufacture dye, plastic, and fertilizers.
• HNO₃ is used in the production of explosives such as TNT.
• Nitric acid is used as an oxidizer in liquid-fueled rockets.
• It is used to remove warts when in pure form.
• Nitric acid is also used in electrochemistry as a chemical doping agent.

Frequently Asked Questions

1. What happens when nitric acid reacts with water?

A. H₂ of water is a basis for removing the H+ from nitric acid. The resulting hydronium H₃O+ is a conjugate acid. Here, the base of the conjugate is NO₃, i.e., Nitrate. It is a molecule of nitric acid whose H+ is removed.

2. Does Nitric Acid dissolve any metal?

A. Yes, Nitric acid oxidizes some metals and alloys. However, when treated with concentrated nitric acid, gold and platinum are not affected. At the same time, certain metals are passivated by Nitric acid. These metals are dissolved using a dilute nitric solution or a mixture of acids.

3. Is there anything that neutralizes nitric acid?

A. Yes, Nitric acid is neutralized by Sodium bicarbonate. It is considered that one part of sodium bicarbonate neutralizes one part of nitric acid. As a fact, 5 percent of nitric is not much heavier than H₂O. Therefore, 100 gallons of this solution equals 834 pounds of nitric acid.