1. Preparation (Ostwald Process):
Nitric acid is manufactured commercially by the Ostwald Process. In this method, ammonia is mixed with air and passed over a heated platinum-rhodium gauze catalyst. This oxidizes ammonia to nitric oxide, which is further oxidized and dissolved in water to form nitric acid.
2. Physical Properties:
- It is a fuming, volatile liquid.
- It is colourless in its pure form but often turns yellow due to the dissolution of nitrogen dioxide.
- It has a characteristic pungent odour.
- It is highly corrosive and miscible with water.
3. Chemical Properties:
- Acidic Nature: It turns blue litmus red and reacts with bases and carbonates to form salts.
- As an Oxidizing Agent: Nitric acid is a very strong oxidizing agent. It oxidizes non-metals like carbon and sulphur, and metals to their respective nitrates.
- Reaction with Metals: It does not typically liberate hydrogen gas with metals (except Mg and Mn). Instead, it produces nitrogen oxides (like NO₂ or NO) depending on the concentration of the acid and the reactivity of the metal.
- Aqua Regia: A mixture of concentrated nitric acid and hydrochloric acid in a 1:3 ratio is called Aqua Regia. It can dissolve even noble metals like gold and platinum.
4. Uses:
- Manufacturing of fertilizers (ammonium nitrate).
- Production of explosives (TNT, nitroglycerin).
- In the making of dyes, plastics, and drugs.
- As a laboratory reagent.
EXERCISE .1
1) What is: (a) aqua fortis, (b) aqua regia (c) Fixation of Nitrogen?
Ans: (a) Aqua Fortis
Aqua Fortis is the historical (alchemical) name for concentrated nitric acid (HNO₃). The name translates to “strong water,” and it was known for its powerful ability to dissolve metals like silver and copper.
(b) Aqua Regia
Aqua Regia is a highly corrosive mixture of concentrated nitric acid and concentrated hydrochloric acid, typically in a 1:3 ratio. Its name means “royal water,” referring to its unique ability to dissolve noble metals, such as gold and platinum, which single acids cannot.
(c) Fixation of Nitrogen
Fixation of Nitrogen is the chemical process of converting unreactive atmospheric nitrogen gas (N₂) into useful nitrogen compounds, such as ammonia (NH₃) or nitrates. This process is crucial for life because it makes nitrogen available for plants to use in creating essential proteins and nucleic acids.
2) During thunderstorms, rain water contains nitric acid. Explain with reactions.
Ans: During thunderstorms, the intense energy from lightning causes the main gases in the air—nitrogen (N₂) and oxygen (O₂)—to react.
Step 1: Formation of Nitric Oxide
The high temperature of a lightning bolt forces nitrogen and oxygen to combine, forming nitric oxide (NO).
Reaction: N₂ (g) + O₂ (g) → 2NO (g)
Step 2: Formation of Nitrogen Dioxide
In the air, nitric oxide (NO) quickly reacts with more oxygen to form nitrogen dioxide (NO₂), a brownish gas.
Reaction: 2NO (g) + O₂ (g) → 2NO₂ (g)
Step 3: Formation of Nitric Acid
Nitrogen dioxide (NO₂) then dissolves and reacts with rainwater (H₂O) to form nitric acid (HNO₃), making the rainwater slightly acidic.
Reaction: 3NO₂ (g) + H₂O (l) → 2HNO₃ (aq) + NO (g)
This is why rainwater collected during a thunderstorm is found to contain traces of nitric acid.
3) (a) Write a balanced chemical equation for the laboratory preparation of nitric acid
(b) In the preparation of nitric acid from KNO3 concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why?
(c) Conc. Nitric acid prepared in the laboratory is yellow in colour. Why? How is this colour removed?
(d) Give reasons for the following: In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitrate should not be heated very strongly above 200°C.
Ans: (a) Balanced chemical equation for laboratory preparation of nitric acid:
NaNO₃ + H₂SO₄ (conc.) → NaHSO₄ + HNO₃
(b) Why concentrated HCl is not used instead of concentrated H₂SO₄?
Concentrated hydrochloric acid is not used because:
It is volatile, so it does not displace the less volatile nitric acid effectively.
The reaction would produce nitrosyl chloride (NOCl) and chlorine gas as side products, resulting in an impure acid.
(c) Why conc. HNO₃ is yellow and how is the colour removed?
Reason for yellow colour: On standing or heating, concentrated nitric acid slightly decomposes to form nitrogen dioxide (NO₂), a brown gas. This gas dissolves in the acid, giving it a yellow colour.
Removal of colour: The yellow colour can be removed by bubbling a stream of dry air through the warm acid. The air expels the dissolved nitrogen dioxide gas.
(d) Why should the mixture not be heated strongly above 200°C?
If the mixture of concentrated sulphuric acid and sodium nitrate is heated very strongly (above 200°C), the sodium bisulphate (NaHSO₄) formed can react further with sodium nitrate.
NaNO₃ + NaHSO₄ → Na₂SO₄ + HNO₃
This reaction occurs at a higher temperature. The nitric acid produced at this high temperature decomposes immediately into nitrogen dioxide, resulting in a loss of the product.
4) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of HNO3 State.
Ans: Nitric acid and water form a special mixture called a constant-boiling azeotrope at approximately 68% HNO₃ and 32% H₂O.
When this 68% solution is heated, both the acid and the water evaporate together in this fixed ratio. The vapor produced has the same concentration (68%) as the liquid.
Therefore, no matter how long you distill a dilute nitric acid solution, the distillate (the condensed vapor) will never be stronger than 68%. The water cannot be fully separated from the acid through simple distillation.
5) What is passive iron? How is passivity removed?
Ans: Passive iron is ordinary iron or steel that has been made highly corrosion-resistant by forming a thin, invisible, and protective oxide layer on its surface. This layer, often made of ferric oxide (Fe₂O₃), acts as a shield that prevents the underlying metal from reacting with water and oxygen in the environment.It is not a special type of iron, but rather a state that iron enters when this protective film is stable and intact.
How is Passivity Removed?
Passivity is removed by destroying this protective oxide layer. This can be done through several methods:
- Mechanical Damage: Scratching, abrading, or grinding the surface physically removes the passive film.
- Chemical Reduction: Using strong reducing agents that can break down the oxide layer.
- Cathodic Protection: Applying an external electrical current that suppresses the formation of the passive layer.
- Introduction of Aggressive Ions: The most common method involves exposing the passive iron to specific ions, especially chloride ions (found in seawater and salt). These ions penetrate the oxide film, break it down locally, and cause pitting corrosion, thus removing the passive state.
6) Name the products formed when:
(a) carbon and conc. Nitric acid is heated
(b) dilute HNO3 is added to copper.
Ans: (a) C + 4HNO3—> CO2 + 2H2O +4NO2
(b) 3Cu + 8HNO3—> 3Cu(NO3) 2 + 4H2O + 2NO
7) Give two chemical equations for each of the following:
(a) Reactions of nitric acid with non-metals
(b) Nitric acid showing as acidic character
(c) Nitric acid acting as oxidizing agent
Ans: (a) Reaction of nitric acid with non-metals:
C + 4HNO3→ CO2 + 2H2O + 4 NO2
S + 6 HNO3 → H2SO4 + 2H2O + 6 NO2
(b) Nitric acid showing acidic character:
K2O + 2HNO3→ 2KNO3 + H2O
ZnO + 2HNO3 → Zn(NO3)2 + H2O
(c) Nitric acid acting as oxidizing agent
P4 +20HNO3 → 4H3PO4 + 4H2O + 20NO2
3Zn +8HNO3→ 3Zn(NO3)2 + 4H2O + 2NO
8) Write balanced equations and name the products formed when:
(a) Sodium hydrogen carbonate is added to nitric acid
(b) cupric oxide reacts with nitric acid
(c) zinc reacts with dilute nitric acid
(d) concentrated nitric acid is heated
Ans: (a) When sodium hydrogen carbonate is added to nitric acid sodium nitrate, carbon dioxide and water is formed.
NaHCO3 + HNO3 —> NaNO3 + H2O + CO2
(b) When Cupric oxide reacts with dilute nitric acid, it forms Copper nitrate.
CuO + 2HNO3—> Cu(NO3)2 + H2O
(c) Zinc reacts with nitric acid to form Zinc nitrate, nitric oxide and water.
3 Zn + 8HNO3 —> 3Zn(NO3)2 + 4H2O + 2NO
(d) 4HNO3—> 2H2O + 4NO2 + O
9) Write an equation for the following conversions A, B, C and D.
Ans:
A: Copper can be converted into copper nitrate.
3Cu + 8HNO3—> 3Cu(NO3)2 + 4H2O + 2NO
B: 2Cu(NO3)2 —> 2CuO + 4NO2 + O2
C: 2Cu + O2—> 2CuO
D: By reduction 2CuO + C—> 2Cu + CO2
10) How will you prepare the following from nitric acid?
(a) Sodium nitrate
(b) copper nitrate
(c) Lead nitrate
(d) Magnesium nitrate
(e) Ferric nitrate
(f) Aqua regia
Ans: (a) Sodium nitrate: NaOH + HNO3 —> NaNO3 + H2O Sodium hydroxide reacts with nitric acid to form sodium nitrate.
(b) Copper nitrate: CuO + 2HNO3 —> Cu(NO3)2 + H2O Copper oxide reacts with nitric acid to form copper nitrate.
(c) Lead nitrate: Pb + 4HNO3—> Pb(NO3)2 + 2H2O + 2NO2 Lead reacts with conc. nitric acid to form lead nitrate.
(d) Magnesium nitrate: Mg + 2HNO3—> Mg(NO3)2 + H2 Magnesium with dil. nitric acid to form magnesium nitrate.
(e) Ferric nitrate: Fe + 6HNO3—> Fe(NO3)3 + 3H2O + 3NO2
Iron reacts with conc. nitric acid to form ferric nitrate.
(f) Aqua regia: HNO3 + 3HCl —> NOCl + 2H2O + 2[Cl] Nitric acid reacts with hydrochloric acid to form a mixture called aqua regia.
