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Nitric Acid (HNO3) – Structure, Preparation, Physical and Chemical Properties, Uses, Practice Problems and FAQ

"Taj Mahal," the beautiful and pristine symbol of love, has always been a centre of attraction for global visitors. The Taj Mahal is said to be pinkish in the morning, milky white in the evening, and golden when the moon shines. Though this may have been true for the renowned spotless marble monument in the past, pollution and poor management have now left the Taj with a 24-hour covering of yellowy-brown.

This is primarily because of acid rain. Nitric acid, also known by ancient alchemists as 'aqua fortis' (meaning strong water), is a major component of acid rain. The emissions from nearby industries contain acidic oxides of nitrogen and sulphur, which mix with rain water and fall to earth as 'acid rain,' destroying such beautiful monuments as well as causing other environmental destruction.

But nitric acid is not entirely the villain. It does have its own characteristics and traits. However, the wise use of this acid has numerous glorifying benefits as well. Let's learn more about this intriguing one!


TABLE OF CONTENTS

  • Nitric Acid - Introduction
  • Nitric Acid - Structure
  • Nitric Acid - Laboratory Preparation
  • Nitric Acid - Manufacturing by Ostwald Process
  • Nitric Acid - Physical Properties
  • Nitric Acid - Chemical Properties
  • Brown Ring Test
  • Nitric Acid - Uses
  • Practice Problems
  • Frequently Asked Questions - FAQ

Nitric Acid - Introduction

Nitric acid (HNO3) is a strong acid. It is also known by several other names like engraver’s acid, azotic acid, the spirit of nitre and aqua fortis. It is colourless in its purest form, but as it ages, it develops a yellow hue. The breakdown of nitric acid into nitrogen oxides and water produces this colour. It is extremely caustic and poisonous. It produces severe burns on the skin. It forms nitrate salts when it interacts with hydroxides, metals, and oxides. Other names for it include aqua fortis, the spirit of nitre, engraver's acid, and azotic acid. In its most pure state, it is colourless, but as it ages, it takes on a yellow tint. This colour is created when nitric acid breaks down into nitrogen oxides and water. It is exceedingly toxic and caustic. The skin suffers severe burns as a result. When it comes into contact with hydroxides, metals, and oxides, it produces nitrate salts.

When exposed to light, the acid decomposes, generating nitrogen dioxide (NO2), a brownish gas. The presence of modest quantities of nitrogen dioxide causes the yellowish hue frequently seen in nitric acid. Nitric acid is one of the most powerful oxidising agents known, attacking practically every metal except gold and platinum.

Henry Cavendish, an English scientist and physicist, was the first to discover the chemical nature and composition of nitric acid in 1784.

Ammonia is catalytically oxidised to produce nitric acid. It is a popular laboratory reagent as well as an important chemical in the production of explosives and fertilisers. Nitric acid has a pH of roughly 3.01. The acid breaks down when it is exposed to light, releasing nitrogen dioxide (NO2), a brownish gas. Small amounts of nitrogen dioxide are what give nitric acid its characteristically yellow colour. With the exception of gold and platinum, nitric acid is one of the most potent oxidising agents known to man.

Nitric acid's chemical origin and composition were first discovered in 1784 by English scientist and physicist Henry Cavendish.

Nitric acid is created by catalytically oxidising ammonia. It is a common laboratory reagent and a crucial chemical in the creation of fertilisers and explosives. The pH of nitric acid is around 3.01.

Nitric Acid - Structure

Nitric acid molecule consists of three oxygen atoms, one nitrogen atom, and one hydrogen one nitrogen atom, one hydrogen atom, and three oxygen atoms. One of the oxygen atoms in HNO3 molecules are doubly linked to the central nitrogen atom. One oxygen atom is singly connected to the central nitrogen atom, as well as a single hydrogen atom. HNO3 molecules have a double bond between the core nitrogen atom and one of the oxygen atoms. A single hydrogen atom and one oxygen atom are both individually coupled to the central nitrogen atom.

One other oxygen atom has a charge of -1 and is singly linked to the central nitrogen atom. The nitrogen atom at the core of the molecule has a charge of +1 because it is involved in four covalent bonds (with three oxygen atoms). As a result, the molecule has a net charge of 0 (the positive charge on the nitrogen atom cancels out the negative charge on the oxygen atom). The core nitrogen atom is singly connected to one additional oxygen atom, which has a charge of -1. Due to its four covalent bonds, the nitrogen atom at the centre of the molecule has a charge of +1. (with three oxygen atoms). The molecule thus has a net charge of 0. (the positive charge on the nitrogen atom cancels out the negative charge on the oxygen atom).


In a gaseous state, it exists in a planar structure. 


Nitric Acid - Laboratory Preparation

An acid that is more volatile, can be displaced from its salt with the help of a less volatile acid. This is the basic principle in the laboratory preparation of nitric acid. It is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 in a glass retort. A less volatile acid can enable a more volatile acid in displacing itself from its salt. This is the fundamental idea behind the nitric acid synthesis process in the lab. It is made by heating concentrated H2SO4 with KNO3 or NaNO3 in a glass retort.

image

  • Nitric acid is a more volatile acid than sulphuric acid and hence is displaced by sulphuric acid from metal nitrates. Sulphuric acid replaces nitric acid in metal nitrates because the former is a more volatile acid.

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

  • Collection of nitric acid: The vapours of nitric acid are cooled and condensed for collection.

Nitric Acid - Manufacturing by Ostwald Process

Large scale production of nitric acid was initiated by the Ostwald process. Ammonia is oxidised over a platinum catalyst or a platinum-rhodium combination in the Ostwald process. Catalytic oxidation of NH3, by atmospheric oxygen takes place in this process.

Step 1: image

Step 2: The nitric oxide so formed, combines with oxygen to give NO2.

2NO(g) + O2(g) ⇌ 2NO2 (g)

Step 3: The nitrogen dioxide so formed formed nitrogen dioxide gives HNO3 when dissolved in water. dissolves in water to give HNO3. The nitric oxide (NO) obtained as a by-product is recycled.

image

Step 4: By distillation, aqueous, HNO3 can be concentrated up to ~68 % by mass.

Step 5: It can be further concentrated with concentrated H2SO4 to up to 98 % by mass due to dehydration.

Nitric Acid - Physical Properties

  • It is a colourless liquid. 
  • Its freezing point is 231.4 K and its boiling point is 355.6 K.
  • The density of HNO3 is 1.51 g cm-3.
  • Nitric acid is miscible with water. It also decomposes in ethyl alcohol and reacts violently with most of the organic solvents.
  • The molar mass of nitric acid is 63.01 g mol-1.

Nitric Acid - Chemical Properties

  • Standing nitric acid decomposes into brown nitrogen dioxide. This is why, despite the fact that fresh nitric acid is colourless, it becomes dark with time.

4HNO3 → 4NO2 + O2 + 2H2O

  • Nitric acid is strongly acidic and easily turns blue litmus red.
  • Nitric acid behaves as a strong acid, releasing hydronium and nitrate ions.

image

  • Concentrated nitric acid is a strong oxidising agent. It attacks most metals except gold (Au) and platinum (Pt) Nitric acid in concentrated form has a powerful oxidising effect. Other than gold (Au) and platinum (Pt), it doesn't attack most metals.
  • The products of oxidation depend upon the concentration of the acid, temperature and nature of the material undergoing oxidation.
  • Metals like copper and zinc which are insoluble in HCl, dissolve in HNO3.
  • The oxidation's end products are influenced by the acid's concentration, temperature, and the type of material being oxidised.
  • HNO3 allows for the dissolution of metals like copper and zinc that are insoluble in HCl.

Example:

image


  • Some metals, such as gold, are insoluble even in HNO3, but dissolves in aqua regia.
  • Aqua regia: A mixture of 25% concentrated HNO3 and 75% concentrated HCl.
  • The enhanced ability of aqua regia to dissolve metals arises from the oxidising power of HNO3, and the ability of Cl- to form complexes with metal ions.
  • Some metals like chromium and aluminium do not react with nitric acid as they develop a thin passive layer of oxide.

  • Nitric acid reacts with iodine and oxidises it to iodic acid. 

image

  • Carbon is oxidised to carbon dioxide by nitric acid.

image

  • Sulphur is oxidised to sulphuric acid by nitric acid.

image

  • Nitric acid oxidises phosphorus to phosphoric acid.

image

Brown Ring Test

Brown ring test is a qualitative test done to detect the presence of nitrate ions in a sample. It is also known as the nitrate test. It is a common qualitative test performed on any solution which results in the formation of the brown-coloured ring in order to confirm the presence of nitrate ions (NO3-).

A brown ring is formed due to the addition of ferrous sulphate (FeSO4) to the solution that contains nitrate ions and is further acidified by adding concentrated sulphuric acid to the entire mixture. A brown coloured ring is generated at the junction point of sulphuric acid and ferrous sulphate. This ring confirms the presence of nitrates in the given sample or solution.

This follows a reduction reaction. The nitrate is reduced to nitric oxide by iron (II) ion, which gets oxidised to iron (III). The nitric oxide thus formed reacts with the iron (II) left behind to form a nitrosyl complex. Here, nitrate ion is reduced to NO.image

Brown Complex


Nitric Acid - Uses

  • Nitric acid is majorly used in the manufacturing of ammonium nitrates which is further used to produce fertilisers.
  • Nitric acid is used to obtain other nitrates which are used to make explosives and pyrotechnics.
  • It is also used in the preparation of nitroglycerine, trinitrotoluene (TNT) and other organic nitro compounds.
  • It is used in the etching of metals, and as oxidizers in rocket fuels.
  • It is used to produce certain dyes.
  • In its pure form, nitric acid is used in the removal of the wart.
  • Nitric acid is a good chemical doping agent used in electrochemistry.




Practice Problems

Q1. Concentrated HNO3 upon standing long develops a yellowish-brown colour due to the formation of

A. N2O
B. N2O4
C. NO
D. NO2
Answer: On standing long, nitric acid decomposes to give nitrogen dioxide and oxygen. Nitrogen dioxide (NO2) is a brownish colour gas. Hence, it gets slightly dissolved in the acid and the overall colour of the acid becomes yellowish-brown. 

So, option D) is the correct answer.

Q2. When copper is heated with concentrated HNO3, the product obtained is

image
Answer: When copper is heated with concentrated HNO3,  copper nitrate and nitrogen dioxide are obtained as the products. 

image

So, option D) is the correct answer.

Q3. The mixture of concentrated HCl and concentrated HNO3 in 3:1 ratio, i.e., aqua regia, contains

A. ClO2
B. NCl3
C. N2O
D. NOCl

Answer: A mixture of concentrated HCl and concentrated HNO3 in 3:1 ratio contains nitrosyl chloride, which is a strong oxidising agent.

image

So, option D) is the correct answer.

Frequently Asked Questions - FAQ

Question 1. What happens when skin is exposed to nitric acid?
Answer: Nitric acid is corrosive to the skin, causing severe burns, ulcers, scarring, dermatitis, and skin discolouration.

Question 2. What happens when alcohol is mixed with nitric acid?
Answer: Nital is formed, which is a term used to describe a nitric acid and alcohol solution. It is often used for the etching of metals. It is ideal for showing the microstructure of carbon steels in particular. Methanol, ethanol, or methylated spirits can all be used as alcohol.

Ethanol and nitric acid mixtures have the potential to be explosive especially if the nitric acid concentration exceeds 10%. This is most frequently accomplished by gas evolution, however, ethyl nitrate can also be produced. Methanol is not explosive, but it is poisonous.

Question 3. How harmful is nitric acid to humans?
Answer: Nitric acid has a suffocating odour and is very toxic upon inhalation. On exposure, it can induce eye, skin, and mucous membrane irritation, as well as delayed pulmonary oedema, pneumonitis, bronchitis, and tooth erosion. Nitric acid is a very corrosive substance. Workers may be affected by nitric acid exposure.

Question 4. How do you identify nitric acid?
Answer: Nitric acid is a colourless to yellowish liquid with a strong, sour, acidic odour. Concentrated nitric acid generates 85-100 % nitrogen dioxide and nitrogen oxide gases.

Question 5. What does nitric acid taste like? What is the taste of nitric acid?
Answer: Nitric acid has strong acidic properties. It shows a pH of much less than 7. It has a bitter-sour taste, and sometimes a violent reaction to other metals. Strong acidic properties characterise nitric acid. The pH value is substantially lower than 7. It tastes bitter-sour and can occasionally react violently with other metals.

Related Topics

Allotropy and Allotropes of Phosphorus

Oxygen

Phosphorus halides

Potassium

Phosphine

Alkali Metals

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