Study of Compounds — Nitric Acid

Equation Worksheet

Question 1

Preparation of nitric acid

Answer

Question 2

Chemical properties of nitric acid

Answer

Question 3

Tests for nitric acid

Answer

Questions

Question 1(2002)

Give equation for the action of heat on –

1. NH₄Cl
2. NH₄NO₃

State whether the reaction is an example of thermal decomposition or thermal dissociation.

Answer

1. NH₄Cl ⇌ NH₃ + HCl
   This reaction is an example of thermal dissociation.
2. NH₄NO₃ ⟶ N₂O + 2H₂O
   This reaction is an example of thermal decomposition.

Question 2(2002)

What compounds are required for the laboratory preparation of nitric acid.

Answer

Potassium nitrate [nitre] or sodium nitrate [chile salt petre] and conc. sulphuric acid are the products required for the laboratory preparation of nitric acid.

Question 3(2002)

State why pure nitric acid takes on a yellowish brown colour when exposed to light.

Answer

Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight.

The decomposition results in formation of reddish brown nitrogen dioxide [NO₂] which remains dissolved in the acid thus imparting a slight yellowish brown colour.

Question 4(2002)

Write an equation for the following reaction: Copper and concentrated nitric acid.

Answer

Cu + 4HNO₃ (conc.) ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂.

Question 5(2002)

The first step in the manufacture of HNO₃ is the catalytic oxidation of NH₃. Name the catalyst used.

Answer

Platinum gauze

Question 1(2003)

Name a solution which gives nitrogen dioxide with copper.

Answer

Concentrated nitric acid (HNO₃) gives nitrogen dioxide with copper

Cu + 4HNO₃ (conc.) ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂.

Question 2(2003)

When nitric acid is prepared by the action of concentrated sulphuric acid on potassium nitrate, what is the special feature of the apparatus used.

Answer

All glass apparatus is used in the laboratory preparation of nitric acid since the vapours of nitric acid being highly corrosive attack rubber, cork, etc.

Question 3(2003)

Write the equation for the lab. preparation of HNO₃ from potassium nitrate and conc. H₂SO₄

Answer

Below is the equation for the lab. preparation of HNO₃ from potassium nitrate and conc. H₂SO₄:

$\underset{[conc.]}{\text{KNO}_3} + \underset{[conc.]}{\text{H}_2\text{SO}_4} \xrightarrow{\lt 200 \degree\text{C}} \underset{[acid \space salt]}{\text{KHSO}_4} + \text{HNO}_3$

Question 4(2003)

Potassium nitrate is prepared from KOH and nitric acid. State the type of reaction involved.

Answer

Neutralization reaction.

KOH + HNO₃ [dil.] ⟶ KNO₃ + H₂O

Question 5(2003)

State the conc. acid which will oxidise sulphur directly to H₂SO₄. Write the equation for the same.

Answer

Hot and conc. nitric acid.

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 1(2004)

X, Y and Z are three crystalline solids which are soluble in water and have a common anion. To help you to identify X, Y and Z, you are provided with the following experimental observation. Copy and complete the corresponding inference.

A reddish-brown gas is obtained when X, Y and Z are separately warmed with concentrated sulphuric acid and copper turnings added to the mixture. The common anion is the .............. ion.

Answer

A reddish-brown gas is obtained when X, Y and Z are separately warmed with concentrated sulphuric acid and copper turnings added to the mixture. The common anion is the [NO^–] Nitrate ion.

Question 2(2004)

Write a balanced equation for the reaction of conc. HNO₃ when added to copper turnings kept in a beaker.

Answer

Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 1(2005)

Write a balanced equation for the reaction of – sulphur and hot concentrated nitric acid.

Answer

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 2(2005)

Dilute nitric acid is generally considered a typical acid except for its reaction with metals. In what way is dilute nitric acid different from other acids when it reacts with metals.

Answer

Dilute nitric acid is generally considered a typical acid except for it's reaction with metals since it generally does not liberate hydrogen on reaction with metals. Nitric oxide on decomposition forms nascent oxygen which oxidizes the hydrogen to water.

Question 3(2005)

Write the equation for the reaction of dilute nitric acid with copper.

Answer

3Cu + 8HNO₃ [cold and dil.] ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

Question 4(2005)

State why a yellow colour appears in conc. nitric acid when left standing in an ordinary glass bottle.

Answer

Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight. The decomposition results in formation of reddish brown nitrogen dioxide [NO₂] which remains dissolved in the acid thus imparting a slight yellowish brown colour.

Question 1(2006)

From the substances – Ammonium sulphate, Lead carbonate, Chlorine, Copper nitrate, Ferrous sulphate — State:

A compound which releases a reddish brown gas on reaction with conc. H₂SO₄ and copper turnings.

Answer

Copper nitrate

Question 2(2006)

State what is observed when nitric acid is kept in a reagent bottle for a long time.

Answer

When nitric acid is kept in a reagent bottle for a long time, it turns dark yellowish brown in colour.

This is because nitric acid is unstable hence, it decomposes slightly even at ordinary temperatures and in the presence of sunlight. When it is kept for a long time, the decomposition is complete resulting in the formation of reddish brown nitrogen dioxide [NO₂] that dissolves in the acid giving it a darker yellowish brown colour.

Question 3(2006)

Explain why only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate.

Answer

All glass apparatus is used in the laboratory preparation of nitric acid since the vapours of nitric acid being highly corrosive attack rubber, cork, etc.

Question 1(2007)

In the laboratory preparation of HNO₃ - Name the reactants A [a liquid] and B [a solid] used.

Answer

(A) Liquid → Conc. sulphuric acid

(B) Solid → Potassium Nitrate (Nitre)

Question 2(2007)

Write an equation to show how nitric acid undergoes decomposition.

Answer

4HNO₃ ⟶ 4NO₂ + 2H₂O + O₂

Question 3(2007)

Write the equation for the reaction in which copper is oxidized by concentrated nitric acid.

Answer

Cu + 4HNO₃ [conc.] ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 1(2008)

Identify the following substances: a dilute acid 'B' which does not normally give hydrogen when reacted with metals but does give a gas when it reacts with copper.

Answer

HNO₃ (Nitric acid)

Question 2(2008)

Copy and complete the following table relating to an important industrial process. Output refers to the product of the process not the intermediate steps.

Answer

Question 3(2008)

What is the property of nitric acid which allows it to react with copper ?

Answer

When nitric acid undergoes decomposition it yields nascent oxygen. This nascent oxygen oxidises copper allowing it to react with nitric acid. Hence, the oxidising nature of nitric acid helps it to react with copper.

Question 4(2008)

Write the equation for the following reaction:- Dilute nitric acid and copper.

Answer

3Cu + 8HNO₃ [cold & dil.] ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

Question 1(2009)

Name the gas evolved [formula is not acceptable]. The gas produced by the action of dilute nitric acid on copper.

Answer

Nitric oxide (NO)

3Cu + 8HNO₃ [cold and dil.] ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

Question 2(2009)

Match each substance A to E listed below with the appropriate description given below:

(A) Sulphur

(B) Silver chloride

(C) Hydrogen chloride

(D) Copper [II] sulphate

(E) Graphite.

A non-metal which reacts with concentrated nitric acid to form it's own acid as one of the product.

Answer

(A) Sulphur

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 3(2009)

Correct the statement — Copper reacts with nitric acid to produce nitrogen dioxide.

Answer

Copper reacts with concentrated nitric acid to produce nitrogen dioxide.

Cu + 4HNO₃ [conc.] ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 1(2010)

Select the correct answer from A, B, C, D and E —

(A) Nitroso Iron [II] sulphate

(B) Iron [III] chloride

(C) Chromium sulphate

(D) Lead [II] chloride

(E) Sodium chloride.

The compound responsible for the brown ring in the brown ring test for identifying the nitrate ion.

Answer

(A) Nitroso Iron [II] sulphate

Question 2(2010)

A blue crystalline solid 'X' on heating gave a reddish brown gas 'Y', a gas which relits a glowing splint and a residue which is black. Identify X and Y, and write the equation for the action of heat on X.

Answer

X is Copper nitrate [Cu(NO₃)₂]

Y is Nitrogen dioxide gas (NO₂)

$2\text{Cu(NO}_3)_2 \xrightarrow{\Delta} 2\text{CuO} + \text{O}_2 + 4\text{NO}_2$

Question 1(2011)

Choose from the list of substances – Acetylene gas, aqua fortis, coke, brass, barium chloride, bronze, platinum. A catalyst used in the manufacture of nitric acid by Ostwald's process.

Answer

Platinum

Question 2(2011)

State your observation when —

Copper is heated with concentrated nitric acid in a hard glass test tube.

Answer

Dense brown fumes of nitrogen dioxide can be seen.

$\text{Cu} + 4\text{HNO}_3 \xrightarrow{\Delta} \text{Cu(NO}_3)_2 + 2\text{H}_2\text{O} + 2\text{NO}_2$

Question 3(2011)

Choose the correct answer from the choices given – The brown ring test is used for detection of:

(A) CO₃^2-
(B) NO₃^1–
(C) SO₃^2-
(D) C1^1-

Answer

NO₃^1–

6FeSO₄ + 3H₂SO₄ [conc.] + 2HNO₃ [dil.] ⟶ 3Fe₂(SO₄)₃ + 4H₂O + 2NO

FeSO₄ + NO ⟶ FeSO₄.NO [Nitroso Iron [II] Sulphate] - brown compound

Question 4(2011)

(i) State the special feature of the apparatus used in the laboratory preparation of nitric acid?

(ii) State why the temperature of the reaction mixture of HNO₃ is not allowed to rise above 200°C.

Answer

(i) All glass apparatus is used since, the vapours of nitric acid being highly corrosive attack rubber, cork. etc.

(ii) Temperature above 200°C may cause:

1. Damage to the glass apparatus.
2. Further decomposition of nitric acid
3. Formation of hard residual crust of the corresponding sulphate [Na₂SO₄ or K₂SO₄] which being a poor conductor of heat, sticks to the glass and cannot be easily removed from the apparatus.

Question 5(2011)

Give a balanced equation for – Ferric hydroxide reacts with nitric acid.

Answer

Fe(OH)₃ + 3HNO₃ [dil.] ⟶ Fe(NO₃)₃ + 3H₂O

Question 1(2012)

Name the gas produced when copper reacts with concentrated nitric acid.

Answer

Nitrogen dioxide.

Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 2(2012)

State one observation for the following: Zinc nitrate crystals are strongly heated.

Answer

Reddish brown nitrogen dioxide gas is evolved on heating zinc nitrate crystals.

$2\text{Zn(NO}_3)_2 \xrightarrow{\Delta} 2\text{ZnO} + \text{O}_2 + 4\text{NO}_2$

Question 3(2012)

Rewrite the correct statement with the missing word/s:

Magnesium reacts with nitric acid to liberate hydrogen gas.

Answer

Magnesium reacts with cold, very dilute nitric acid to liberate hydrogen gas.

Question 4(2012)

Give reasons for the following: Iron is rendered passive with fuming nitric acid.

Answer

Iron or aluminium are rendered passive on reaction with fuming HNO₃ due to formation of a thin oxide coating on the surface of the metal which prevents further reaction

Question 5(2012)

Give a balanced equation for the reaction: Dilute nitric acid and copper carbonate.

Answer

CuCO₃ + 2HNO₃ [dil.] ⟶ Cu(NO₃)₂ + H₂O + CO₂

Question 1(2013)

Identify the gas evolved when:

(i) Sulphur is treated with concentrated nitric acid.

(ii) A few crystals of KNO₃ are heated in a hard glass test tube.

Answer

(i) Nitrogen dioxide gas

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

(ii) Oxygen gas

$2\text{KNO}_3 \xrightarrow{\Delta} 2\text{KNO}_2 + \text{O}_2$

Question 2(2013)

State two relevant observations for : Lead nitrate crys­tals are heated in a hard glass test tube.

Answer

Reddish brown nitrogen dioxide gas is evolved.

Buff coloured residue of PbO is obtained in the test tube.

$\text{Pb(NO}_3)_2 \xrightarrow{\Delta} 2\text{PbO} + \text{O}_2 + 4\text{NO}_2$

Question 3(2013)

Give a balanced equations for: Oxidation of carbon with concentrated nitric acid.

Answer

C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

Question 1(2014)

Fill in the blank from the choices given in the bracket:

Cold, dil. nitric acid reacts with copper to form ............... [Hydrogen, nitrogen dioxide, nitric oxide].

Answer

Cold, dilute nitric acid reacts with copper to form nitric oxide.

Question 2(2014)

Give balanced equations for the following:

(i) Laboratory preparation of nitric acid.

(ii) Action of heat on a mixture of copper and concentrated nitric acid.

Answer

(i) Balanced equation for laboratory preparation of nitric acid:

$\underset{[conc.]}{\text{KNO}_3} + \underset{[conc.]}{\text{H}_2\text{SO}_4} \xrightarrow{\lt 200 \degree\text{C}} \underset{[acid \space salt]}{\text{KHSO}_4} + \text{HNO}_3$

(ii) Balanced equation for action of heat on a mixture of copper and concentrated nitric acid:

$\text{Cu} + 4\underset{[conc.]}{\text{HNO}_3} \longrightarrow \text{Cu(NO}_3 \text{)}_2 + 2\text{H}_2 \text{O} + 2\text{NO}_2$

Question 1(2015)

State one appropriate observation for — When crystals of copper nitrate are heated in a test tube.

Answer

Reddish brown nitrogen dioxide gas is evolved.

Black coloured residue of CuO is obtained in the test tube.

$2\text{Cu(NO}_3)_2 \xrightarrow{\Delta} 2\text{CuO} + \text{O}_2 + 4\text{NO}_2$

Question 2(2015)

Identify the acid — The acid which is prepared by cata­lytic oxidation of ammonia.

Answer

Nitric acid

Question 3(2015)

Explain the following:

(i) Dil. HNO₃ is generally considered a typical acid but not so in it's reaction with metals.

(ii) When it is left standing in a glass bottle, concentrated nitric acid appears yellow.

(iii) In the laboratory preparation of nitric acid, an all glass apparatus is used.

Answer

(i) Dilute nitric acid is generally considered a typical acid except for it's reaction with metals since it generally does not liberate hydrogen on reaction with metals. Nitric oxide on decomposition forms nascent oxygen which oxidizes the hydrogen to water.

(ii) Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight.

The decomposition results in formation of reddish brown nitrogen dioxide [NO₂] which remains dissolved in the acid thus imparting a slight yellowish brown colour.

(iii) All glass apparatus is used since, the vapours of nitric acid being highly corrosive attack rubber, cork. etc.

Question 4(2015)

From the list of the following salts — AgCl, MgCl₂, NaHSO₄, PbCO₃, ZnCO₃, KNO₃, Ca(NO₃)₂. State the salt which on heating, evolves a brown coloured gas.

Answer

Ca(NO₃)₂

$2\text{Ca(NO}_3)_2 \xrightarrow{\Delta} 2\text{CaO} + \text{O}_2 + 4\text{NO}_2$

Question 1(2016)

Write balanced chemical equation for: Action of hot and concentrated nitric acid on copper.

Answer

Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 2(2016)

Fill in the blanks using the appropriate words given in the bracket below:

[sulphur dioxide, nitrogen dioxide, nitric oxide, sulphuric acid]

(i) Cold, dilute nitric acid reacts with copper to give ...............

(ii) Hot, concentrated nitric acid reacts with sulphur to form ...............

Answer

(i) Cold, dilute nitric acid reacts with copper to given nitric oxide.

(ii) Hot, concentrated nitric acid reacts with sulphur to form nitrogen dioxide.

Question 1(2017)

Write the balanced chemical equation for –

(i) Action of cold and dilute nitric acid on copper.

(ii) Action of conc. nitric acid on sulphur.

(iii) Laboratory preparation of nitric acid.

Answer

(i) 3Cu + 8HNO₃ [dil.] ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO(g)

(ii) S + 6HNO₃ [conc.] ⟶ H₂SO₄ + 2H₂O + 6NO₂

(iii) KNO₃ [conc.] + H₂SO₄ [conc.] $\xrightarrow{\lt 200 \degree\text{C}}$ KHSO₄ + HNO₃

Question 1(2018)

Name the gas that is produced.

(i) When sulphur is oxidized by concentrated nitric acid.

(ii) During action of cold and dilute ntric acid on copper.

Answer

(i) Nitrogen dioxide

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

(ii) Nitric oxide

3Cu + 8HNO₃ (dil.) ⟶ 3Cu(NO₃)₂ + 2NO(g) + 4H₂O

Question 2(2018)

State the type of salt formed when - The reactants are heated at a suitable temperature for the preparation of nitric acid in the laboratory.

Answer

Acid salt

$\underset{[conc.]}{\text{NaNO}_3} + \underset{[conc.]}{\text{H}_2\text{SO}_4} \xrightarrow{\lt 200 \degree\text{C}} \underset{[acid \space salt]}{\text{NaHSO}_4} + \text{HNO}_3$

Question 3(2018)

Give a reason why - For the preparation of nitric acid in the laboratory, the complete apparatus is made up of glass.

Answer

The vapours of nitric acid are highly corrosive and attack rubber, cork. etc. For this reason, all glass apparatus is used in the laboratory preparation of nitric acid.

Question 1(2019)

State one observation for the following : Concentrated nitric acid is reacted with - sulphur.

Answer

S + 6HNO₃ [conc.] ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 2(2019)

Complete the following equations:

(i) S + Conc. HNO₃ ⟶

(ii) Cu + dil. HNO₃ ⟶

Answer

(i) S + 6HNO₃ [conc.] ⟶ H₂SO₄ + 2H₂O + 6NO₂

(ii) 3Cu + 8HNO₃ [dil.] ⟶ 3Cu(NO₃)₂ + 2NO (g) + 4H₂O

Question 3(2019)

Identify the substance italicized : The dilute acid which is an oxidizing agent.

Answer

Nitric acid

Question 1(2020)

State one relevant observation for the reaction : Action of concentrated nitric acid on copper.

Answer

Reddish brown fumes of nitrogen dioxide gas are liberated when copper reacts with conc. nitric acid.

Cu + 4HNO₃ [conc.] ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 2(2020)

Write a balanced chemical equation for : Reaction of carbon powder and concentrated nitric acid.

Answer

C + 4HNO₃ [conc.] ⟶ CO₂ + 2H₂O + 4NO₂

Question 3(2020)

State one relevant reason for the following : Concentrated nitric acid appears yellow, when it is left for a while in a glass bottle.

Answer

Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight.

The decomposition results in formation of reddish brown nitrogen dioxide [NO₂] which remains dissolved in the acid thus imparting a slight yellowish brown colour.

Question 4(2020)

Match the gas given in column I to the identification of the gases mentioned in column II:

Answer

Additional Questions

Question 1

State how atmospheric nitrogen converts itself to nitric acid.

Answer

During lightning discharge, nitrogen in the atmosphere reacts with oxygen to form nitric oxide which is further oxidised to nitrogen dioxide.

$\text{N}_2 + \text{O}_2 \xrightarrow[\text{discharge}]{\text{lightning}} \text{2NO}$

2NO + O₂⟶ 2NO₂

The nitrogen dioxide dissolves in atmospheric moisture and in the presence of oxygen of the air forming nitric acid in the free state.

4NO₂ + 2H₂O + O₂ ⟶ 4HNO₃

Question 2

Give a word equation and balanced molecular equation for the laboratory preparation of nitric acid from :

(i) KNO₃

(ii) NaNO₃.

Answer

(i) Potassium Nitrate [Nitre] + Sulphuric acid [conc.] ⟶ Potassium bisulphate + Nitric acid [vapours]

$\text{KNO}_3 [\text{conc.}] + \text{H}_2\text{SO}_4 [\text{conc.}]\xrightarrow{\lt 200 \degree\text{C}} \text{KHSO}_\text{4} + \text{HNO}_\text{3}$

(ii) Sodium Nitrate [Chile salt petre]+ Sulphuric acid [conc.] ⟶ Sodium bisulphate + Nitric acid [vapours]

$\text{NaNO}_3 [\text{conc.}] + \text{H}_2\text{SO}_4 [\text{conc.}]\xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_\text{4} + \text{HNO}_\text{3}$

Question 3

In the laboratory preparation of nitric acid from – KNO₃ or NaNO₃ state :

(i) the acid used

(ii) the type of apparatus used

(iii) the precautions to be taken during the preparation

(iv) the method of collection of the acid

(v) the method of identification of the product i.e., acid formed.

Answer

(i) Conc. Sulphuric Acid [H₂SO₄]

(ii) All glass apparatus is used

(iii) Precautions are:

1. Complete apparatus is made of glass since HNO₃ vapours are corrosive.
2. Conc. HCl is not used in place of conc. Sulphuric Acid (H₂SO₄) as a reactant, since being volatile itself it is not used to displace another volatile acid.
3. The reaction temperature is properly maintained and controlled to protect the glass apparatus, prevent further decomposition of the nitric acid and to avoid formation of hard residual crust of the corresponding sulphate.

(iv) Conc. HNO₃ vapours condense and are collected in water cooled receiver.

(v) The vapours obtained in the receiver on heating alone or with copper turnings evolve reddish brown fumes of nitrogen dioxide which turns acidified iron [II] sulphate solution brown proving that the vapours are of nitric acid.

Question 4

Give reasons for the following — pertaining to the above laboratory preparation of nitric acid

(i) concentrated hydrochloric acid is not used as a reactant in the laboratory preparation.

(ii) the complete apparatus in the laboratory preparation does not contain parts made of rubber or cork.

(iii) the reaction temperature is maintained below 200°C

(iv) at high temperatures the sodium sulphate or potassium sulphate formed, forms a crust and sticks to the glass apparatus.

Answer

(i) Conc. HCl is not used in place of conc. sulphuric acid, since being volatile itself it is not used to displace another volatile acid.

(ii) All glass apparatus is used in the laboratory preparation of nitric acid since the vapours of nitric acid being highly corrosive attack rubber, cork, etc.

(iii) High temperatures above 200°C are not used as they may cause :

1. Damage to the glass apparatus.
2. Further decomposition of nitric acid.
3. Formation of a hard residual crust of the corresponding sulphate [Na₂SO₄ and K₂SO₄] which being a poor conductor of heat, sticks to the glass and cannot be easily removed from the apparatus.

(iv) At high temperatures above 200°C, a hard residual crust of the corresponding sulphate [Na₂SO₄ and K₂SO₄] is formed which being a poor conductor of heat, sticks to the glass and cannot be easily removed from the apparatus.

$\text{NaNO}_3 + \text{NaHSO}_4 \xrightarrow{\gt 200 \degree\text{C}} \text{Na}_\text{2}\text{SO}_\text{4} + \text{HNO}_\text{3}$

Question 5

State the colour of :

(i) pure nitric acid

(ii) nitric acid obtained in the laboratory

(iii) nitric acid obtained in the laboratory after passage of air or addition of water to it.

Answer

(i) Colourless

(ii) Slightly yellowish brown due to decomposition of the acid which is negligible at ordinary temperature but substantial at high temperature

(iii) Colourless — bubbling of air drives out the reddish brown nitrogen dioxide gas from the warm acid and further oxidises nitrogen dioxide to nitric acid. Addition of water causes dissolution of the nitrogen dioxide gas which is soluble in water.

Question 6

State which reaction of ammonia forms the first step of Ostwald's process.

Answer

Catalytic oxidation of ammonia to nitric oxide and water is the first step of Ostwald's process.

$4\text{NH}_3 + 5\text{O}_2 \xrightarrow[700-800 \degree\text{C}]{\text{Pt}} \text{4NO} + \text{6H}_\text{2}\text{O} + 21.5 \text{K cals}$

Question 7

Convert ammonia to nitric acid by Ostwald's process giving all conditions.

Answer

Step I : $4\text{NH}_3 + 5\text{O}_2 \xrightarrow[700-800 \degree\text{C}]{\text{Pt}} \text{4NO} + \text{6H}_\text{2}\text{O} + 21.5 \text{K cals}$

Step II : $2\text{NO} + \text{O}_2 \xrightarrow{50 \degree\text{C}} \text{2NO}_\text{2}$

Step III : $4\text{NO}_\text{2} + 2\text{H}_\text{2}\text{O} + \text{O}_2 \longrightarrow 4\text{HNO}_3$

Question 8

State how —

(i) a higher ratio of the reactant air

(ii) exothermicity of the catalytic reaction

(iii) use of low temp. in the conversion of NO to NO₂ — affects each related step in Ostwald's process.

Answer

(i) Air is used in all the three main reactions of Ostwald's process i.e., the reaction in the catalytic, oxidation and absorption tower including oxidation of nitric oxide to nitrogen dioxide which requires excess of air. Hence, a higher ratio of the reactant air is used.

(ii) Since the reaction is exothermic in nature, the electrical heating is done initially only. The heat evolved i.e., heat of reaction further maintains the required temperature of the reaction.

(iii) Low temperatures are used since they :

• Facilitate easy oxidation of nitric oxide to nitrogen dioxide
• Minimize the chances of any decomposition of nitrogen dioxide which may take place at higher temperatures.

Question 9

State why nitric acid

(i) stains the skin

(ii) cannot be concentrated beyond 68% by boiling.

Answer

(i) Nitric acid brings about nitration of protein of the skin forming a yellow compound Xanthoproteic acid which stains the skin yellow.

(ii) A constant boiling mixture is maintained at 121°C, it is when the mixture boils without change in composition. Hence, on boiling, the above mixture evolves out the vapours of both acid and water in the same proportion as in the liquid. Thus, nitric acid cannot be concentrated beyond 68% by boiling.

Question 10

State two conditions which affect the decomposition of nitric acid.

Answer

Two conditions which affect the decomposition of nitric acid are:

1. Nitric acid is unstable and it decomposes slightly even at ordinary temperatures and in the presence of sunlight.
2. It decomposes completely at higher temperatures or after a prolonged period of time.

Question 11

State the change in colour of pure concentrated nitric acid on initial and prolonged decomposition.

Answer

Pure nitric acid is colourless. The decomposition results in formation of reddish brown nitrogen dioxide which remains dissolved in the acid thus imparting a slight yellowish brown colour.

At higher temperatures or after a prolonged period of time the decomposition is complete and the acid turns a darker yellowish brown colour.

Question 12

State the cation responsible for turning moist neutral litmus red on reaction with dil. HNO₃.

Answer

The high concentration of H⁺ ions imparts acidic nature to dilute Nitric Acid (HNO₃), hence it turns moist neutral litmus red.

HNO₃ + H₂O ⇌ H₃O¹⁺ + NO₃^1-

$\text{HNO}_3 \xrightleftharpoons {\text{H}_2\text{O}} \text{H}^{1+} + \text{NO}_3^{1-}$

H⁺ + H₂O ⇌ H₃O¹⁺

Question 13

State why nitric acid is a strong oxidizing agent and yields varying products such as NO, NO₂ on reaction with metals, non-metals etc.

Answer

The oxidizing property of nitric acid is based on the fact that when nitric acid undergoes decomposition, it yields nascent oxygen, which is very reactive.

2HNO₃ (hot conc.) ⟶ H₂O + 2NO₂ + [O]

2HNO₃ (mod. conc.) ⟶ H₂O + 2NO + 3[O]

The nascent oxygen oxidises non-metals, metals, inorganic and organic compounds. Nitric acid itself undergoes reduction in a number of ways hence a number of reduction products [NO, NO₂, N₂O etc.] are formed depending on the acid concentration and the temperature of the reaction.

Question 14

Give an equation for reaction of conc. HNO₃ with :

(i) carbon

(ii) copper.

Answer

(i) C + 4HNO₃ ⟶ CO₂+ 2H₂O + 4NO₂

(ii) Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 15

Convert nitric acid to sulphuric acid using a non-metal.

Answer

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

Question 16

State how you would obtain:

(i) hydrogen

(ii) nitric oxide

(iii) nascent chlorine – from nitric acid.

State the concentration of nitric acid used in each case.

Answer

(i) Cold, very dilute (1%) nitric acid reacts with metals like Mg and Mn to give hydrogen
Mg + 2HNO₃ [cold, very dil.] ⟶ Mg(NO₃)₂ + H₂ [g]

(ii) Dilute nitric acid reacts with metals like Cu, Ag to give nitric oxide (NO).
3Cu + 8HNO₃ [dil.] ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

(iii) Aqua Regia which is a mixture of conc. nitric acid [1 part] + conc. hydrochloric acid [3 parts] [by volume] liberates nascent chlorine by oxidising hydrochloric acid to chlorine.
HNO₃ (conc.) + 3HCl (conc.) ⟶ 2H₂O + NOCl + 2[Cl]

Question 17

State why hydrogen is liberated when zinc reacts with dil. HCl but not with dil. HNO₃.

Answer

As zinc displace hydrogen from dil. HCl, hence, hydrogen is liberated when zinc reacts with dil. HCl

Zn + 2HCl (dil.) ⟶ ZnCl₂ + H₂

However, when zinc reacts with dil HNO₃, no hydrogen is obtained as nitric acid being a strong oxidising agent decomposes to form nascent oxygen which oxidizes the hydrogen produced to water.

Question 18

State a reason for the inactivity of iron and aluminium on reaction with fuming HNO₃.

Answer

Iron and Aluminium are rendered passive on reaction with fuming HNO₃ due to formation of a thin oxide coating on the surface of the metal which prevents further reaction.

Question 19

State your observation when:

(i) nitric acid is added to saw dust

(ii) conc. nitric acid is heated (a) in absence of copper (b) in presence of copper.

Answer

(i) Saw dust bursts into flames when hot conc. nitric acid is poured on it. The reason is that Nitric acid is a strong oxidising agent. It decomposes to give nascent oxygen which oxidises organic compounds to carbon dioxide and water.

Saw dust [C₆H₁₀O₅]_n + Nitric acid [hot conc.] ⟶ carbon dioxide + water + nitrogen dioxide.

(ii) (a) When conc. nitric acid is heated in absence of copper, reddish brown fumes of nitrogen dioxide are evolved.

$4\text{HNO}_3 \xrightarrow{\Delta} \text{4NO}_2 + \text{2H}_\text{2}\text{O} + \text{O}_2$

(ii) (b) Cu reacts with conc. nitric acid liberating reddish brown fumes of nitrogen dioxide along with blue coloured solution of copper nitrate.

Cu + 4HNO₃ [conc.] ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

Question 20

State how addition of nitric acid to acidified FeSO₄, serves as a test for the former.

Answer

When nitric acid is added to acidified FeSO₄, conc. sulphuric acid being heavier settles down and the iron [II] sulphate layer remains above it resulting in the formation of the brown ring at the junction.

6FeSO₄ + 3H₂SO₄ + 2HNO₃ (dil. ) ⟶ 3Fe₂(SO₄)₃ + 4H₂O +2NO

FeSO₄ + NO ⟶ FeSO₄.NO

Question 21

Name three chemical products manufactured from nitric acid. Give two general uses of HNO₃.

Answer

Three chemical products manufactured from nitric acid are:

1. Tri-nitro toluene [C₇H₅(NO₂)₃]
2. Tri-nitro cellulose [C₆H₇O₂(NO₂)₃]_n
3. Tri-nitro glycerine [C₃H₅O₃(NO₂)₃]

Two general uses of nitric acid are:

1. For etching designs on copper and brassware
2. In purification of gold.

Unit Test Paper 7C — Nitric Acid

Question 1

Select the letters A, B, C, D or E, which form the gaseous products of the reactions from 1 to 5.

A: Nitrogen dioxide only
B: Nitric oxide only
C: Hydrogen
D: Nitrogen dioxide and oxygen
E: Nitrogen dioxide and carbon dioxide.

1. Reaction of manganese with cold, very dil. nitric acid.
2. Reaction of sulphur with conc. nitric acid.
3. Reaction of zinc with dil. nitric acid.
4. Reaction of carbon with conc. nitric acid.
5. Heat on nitric acid.

Answer

1. Reaction of manganese with cold, very dil. nitric acid — (C) Hydrogen
2. Reaction of sulphur with conc. nitric acid — (A) Nitrogen dioxide only
3. Reaction of zinc with dil. nitric acid — (B) Nitric oxide only
4. Reaction of carbon with conc. nitric acid — (E) Nitrogen dioxide and carbon dioxide.
5. Heat on nitric acid — (D) Nitrogen dioxide and oxygen.

Question 2

Select the correct word from the list in bracket to complete each statement.

1. The oxidised product obtained on reaction of H₂S gas with dil. HNO₃ is ................ [sulphur dioxide / sulphur / sulphuric acid].
2. Aqua regia is a mixture of one part of ............... and three parts of ............... [conc. hydrochloric acid / conc. nitric acid] in which nitric acid ............... [reduces/oxidises] hydrochloric acid to chlorine.
3. Pure conc. nitric acid or fuming nitric acid renders the metal ............... [zinc/copper/iron] passive or inactive.
4. A mineral acid obtained from conc. nitric acid on reaction with a non-metal is ............... [hydrochloric acid / sulphuric acid / carbonic acid].
5. The reaction of ............... [calcium carbonate / calcium oxide / calcium sulphite] with dilute nitric acid is an example of a neutralization reaction.

Answer

1. The oxidised product obtained on reaction of H₂S gas with dil. HNO₃ is sulphur .
2. Aqua regia is a mixture of one part of conc. nitric acid and three parts of conc. hydrochloric acid in which nitric acid oxidises hydrochloric acid to chlorine.
3. Pure conc. nitric acid or fuming nitric acid renders the metal iron passive or inactive.
4. A mineral acid obtained from conc. nitric acid on reaction with a non-metal is sulphuric acid.
5. The reaction of calcium oxide with dilute nitric acid is an example of a neutralization reaction.

Question 3

Give balanced equations for the following conversions A to E.

1. $\text{Copper} \xrightarrow{\text{A}} \text{Copper nitrate} \xrightarrow{\text{B}} \text{Copper oxide} \xrightarrow{\text{C}} \text{Copper}$

2. $\text{Sulphur} \xrightarrow{\text{D}} \text{Sulphuric acid} \xleftarrow{\text{E}} \text{Sulphur dioxide}$

Answer

A : Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

B : $2\text{Cu(NO}_3)_2 \xrightarrow{\Delta} 2\text{CuO} + \text{O}_2 + 4\text{NO}_2$

C : $\text{CuO} + \text{C} \xrightarrow{\Delta} \text{Cu} + \text{CO}$

D : S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

E : 3SO₂ + 2H₂O + 2HNO₃ ⟶ 3H₂SO₄ + 2NO

Question 4

Name the oxidised product when the following 1 to 5 react with nitric acid

1. Sulphur [with conc. acid]
2. Zinc [with dil. acid]
3. Aqueous soln. of SO₂ [with dil. acid]
4. Acidified iron [II] sulphate [with dil. acid]
5. Carbon [with conc. acid]

Answer

1. H₂SO₄
   S + 6HNO₃ [conc.] ⟶ H₂SO₄+ 2H₂O + 6NO₂

2. Zn(NO₃)₂
   3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO

3. H₂SO₄
   3SO₂ + 2H₂O + 2HNO₃ [dil.] ⟶ 3H₂SO₄ + 2NO

4. Fe₂(SO₄)₃
   6FeSO₄ + 3H₂SO₄ + 2HNO₃ ⟶ 3Fe₂(SO₄)₃ + 4H₂O + 2NO

5. CO₂
   C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

Question 5.1

Give reasons for the following:

Nitric acid is not manufactured from atmospheric nitrogen.

Answer

Direct conversion of atmospheric N₂ into HNO₃ is highly energy intensive process and it is very expensive. Therefore, nitric acid is not manufactured from atmospheric nitrogen.

Question 5.2

Give reasons for the following:

Nitric acid affects the skin if it accidently falls on it, staining the skin yellow.

Answer

Nitric acid brings about nitration of protein of the skin forming a yellow compound Xanthoproteic acid which stains the skin yellow.

Question 5.3

Give reasons for the following:

The yellow colour of nitric acid obtained in the laboratory is removed by bubbling air through it.

Answer

Yellow colour of the nitric acid is due to dissolved NO₂. Bubbling of air drives out the reddish brown nitrogen dioxide gas from the warm acid and further oxidises nitrogen dioxide to nitric acid.

Question 5.4

Give reasons for the following:

Nitric acid finds application in the purification of gold.

Answer

Crude gold contains Cu, Ag, Zn, Pb etc., as impurities which dissolve in HNO₃ while gold is unaffected. Hence, nitric acid finds application in the purification of gold.

Question 5.5

Give reasons for the following:

Nitric acid is a stronger oxidising agent in the conc. state of the acid than in the dilute state.

Answer

The oxidizing property of nitric acid is based on the fact that when nitric acid undergoes decomposition, it yields nascent oxygen, which is very reactive.

Due to the ease with which conc. HNO₃ decomposes to give nascent oxygen, it acts as a powerful oxidising agent compared to nitric acid in dilute state.

Question 6

Answer the following questions pertaining to the brown ring test for nitric acid:

1. Name the chemical constituent of the brown ring 'Y'.
2. Which of the two solutions – iron [II] sulphate or conc. sulphuric acid, do 'X' and 'Z' represent.
3. State why the unstable brown ring decomposes completely on disturbing the test tube.
4. Give a reason why the brown ring does not settle down at the bottom of the test tube.
5. Name the gas evolved when acidified iron [II] sulphate reacts with dilute nitric acid in the brown ring test.

Answer

1. Nitroso Iron (II) Sulphate (FeSO₄.NO).
2. X is Iron (II) Sulphate (FeSO₄) and Z is conc. Sulphuric Acid (H₂SO₄).
3. When the test tube is shaken the conc. sulphuric acid may further mix with water and the heat evolved assists in decomposing the unstable brown ring. Hence, unstable brown ring of nitroso ferrous sulphate decomposes completely on disturbing.
4. In the brown ring test the conc. sulphuric acid being heavier settles down and the iron [II] sulphate layer remains above it resulting in formation of the brown ring at the junction.
5. Nitric oxide
   6FeSO₄ + 3H₂SO₄ + 2HNO₃ ⟶ 3Fe₂(SO₄)₃ + 4H₂O + 2NO