Study of Compounds — Nitric Acid

What is aqua fortis ?

Answer

Nitric acid (HNO₃) was formerly known aqua fortis, meaning strong water. It is so called because it reacts with nearly all metals. It can even dissolve silver which does not dissolve in other acids.

What is aqua regia ?

Answer

Conc. nitric acid (1 part by volume) when mixed with conc. hydrochloric acid (3 parts by volume) gives the mixture called Aqua Regia (meaning royal water).

What is fixation of nitrogen?

Answer

The conversion of free atmospheric nitrogen (N₂) into useful nitrogenous compounds in the soil is known as fixation of nitrogen.

During thunderstorms, rain water contains nitric acid. Explain with reactions.

Answer

During thunderstorms, the nitrogen present in the atmosphere reacts with oxygen to form nitric oxide.

N₂ + O₂ ⟶ 2NO

Nitric oxide is further oxidized to nitrogen dioxide.

2NO + O₂ ⟶ 2NO₂

The nitrogen dioxide dissolves in atmospheric moisture or rain water in the presence of oxygen of the air and forms nitric acid which is washed down during rain and combines with the salt present on the surface of the earth.

4NO₂ + 2H₂O + O₂ ⟶ 4HNO₃

Ammonia is used in the Ostwald process,

(a) Give the source of reactants used in this process.

(b) Name the catalyst used in the process.

(c) Name the oxidising agent used in this process.

(d) What is the ratio of ammonia and air taken in this process?

(e) Why is quartz used in this process?

Answer

(a) Dry air (free from carbon dioxide and dust particles) and dry ammonia produced in Haber’s process.

(b) Platinum gauze

(c) Oxygen

(d) 1:10 i.e., 1 volume of ammonia and 10 volume of air.

(e) Quartz is acid resistant and when packed in layers, it slows down the movement of the gaseous NO₂ so that it can dissolve in water uniformly.

Write a balanced chemical equation for the laboratory preparation of nitric acid.

Answer

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

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

In the preparation of nitric acid from KNO₃, Concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why.

Answer

Conc. HCl is not used in place of conc. Sulphuric Acid (H₂SO₄) because HCl is volatile and hence nitric acid vapous will carry HCl vapours.

Conc. nitric acid prepared in the laboratory is yellow in colour. Why? How is this colour removed?

Answer

Concentrated nitric acid prepared in the laboratory is yellow in color due to dissolution of reddish brown nitrogen dioxide gas in the acid. This gas is produced due to the thermal decomposition of a portion of nitric acid.

4HNO₃ ⟶ 2H₂O + 4NO₂ + O₂

The yellow color of the acid is removed :

1. If dry air or CO₂ is bubbled through the yellow acid, the latter turns colourless because it drives out NO₂ gas from warm acid, which is further oxidised to nitric acid.
2. By adding excess water, nitrogen dioxide gas dissolves in water and thus yellow color of acid is removed.

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.

Answer

The temperature of the reaction should not increase above 200°C, because sodium sulphate formed at higher temperature, forms a hard crust which sticks to the walls of the retort and is difficult to remove, although the yield of nitric acid is higher.

2NaNO₃ + H₂SO₄ $\xrightarrow{\gt 200 \degree\text{C}}$ Na₂SO₄ + 2HNO₃

The higher temperature

• may damage the glass apparatus.
• decomposition of nitric acid can also occur.
• wastage of fuel.

(a) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of HNO₃. State the reason.

(b) What is passive iron? How is passivity removed?

Answer

(a) An aqueous solution of nitric acid (68% concentration) forms a constant boiling mixture at 121°C. A constant boiling mixture is one which boils without change in composition.

Hence on boiling further, the mixture evolves out the vapours of both acid and water in the same proportion as in the liquid.

Therefore, dilute HNO₃ cannot be concentrated beyond 68% by boiling.

(b) Iron becomes passive or inert when treated with pure concentrated nitric acid due to the formation of the extremely thin layer of insoluble iron oxide (passivity) which stops the reaction.

Fe + 6HNO₃ ⟶ Fe(NO₃)₃ + 3H₂O + 3NO₂

This inert iron is known as passive iron.

Passivity can be removed by rubbing the surface layer with sandpaper or by treating with strong reducing agent.

Name the products formed when:

(a) carbon and conc. nitric acid is heated,

(b) dilute HNO₃ is added to copper.

Answer

(a) The products formed when carbon and conc. nitric acid is heated are carbon dioxide, nitrogen dioxide and water

C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

(b) The products formed when dilute HNO₃ is added to copper are copper nitrate, nitric oxide and water

3Cu + 8HNO₃ ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

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 oxidising agent.

Answer

(a) Action on non-metals:

C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

(b) Reaction with alkalis:

K₂O + 2HNO₃ ⟶ 2KNO₃ + H₂O

CuO + 2HNO₃ ⟶ Cu(NO₃)₂ + H₂O

(c) Nitric acid acting as oxidising agent:

C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

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

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

Answer

(a) Sodium hydrogen carbonate when treated with nitric acid forms sodium nitrate, carbon dioxide and water.

NaHCO₃ + HNO₃ ⟶ NaNO₃ + H₂O + CO₂ ↑

(b) Cupric oxide reacts with dilute nitric acid, it forms copper nitrate and water.

CuO + 2HNO₃ ⟶ Cu(NO₃)₂ + H₂O

(c) Zinc reacts with dil. nitric acid to form zinc nitrate, nitric oxide and water

3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO

(d) When concentrated nitric acid is heated it forms nitrogen dioxide gas, water and oxygen gas

4HNO₃ $\xrightarrow{\Delta}$ 4NO₂ + 2H₂O + O₂

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

Answer

(a) NaOH + HNO₃ [dil.] ⟶ NaNO₃ + H₂O

(b) CuO + 2HNO₃ [dil.] ⟶ Cu(NO₃)₂ + H₂O

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

(d) Mg(OH)₂ + 2HNO₃ [dil.] ⟶ Mg(NO₃)₂ + 2H₂O

(e) Fe + 6HNO₃ [conc.] ⟶ Fe(NO₃)₃ + 3H₂O + 3NO₂

(f) Aqua regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid mixed in the ratio of 3:1 by volume.

3HCl + HNO₃ ⟶ NOCl + 2H₂O + 2[Cl]

Write equation for the following conversions A, B, C and D.

Answer

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

B : 2Cu(NO₃)₂ $\xrightarrow{\Delta}$ 2CuO + 4NO₂ + O₂

C : 2Cu + O₂ ⟶ 2CuO

D : CuO + H₂ $\xrightarrow{\Delta}$ Cu + H₂O

Correct the following, if required:

(a) HNO₃ is a strong reducing agent.

(b) NaNO₃ gives NO₂ and O₂ on heating.

(c) Constant boiling nitric acid contains 80% nitric acid by weight.

(d) Nitric acid remains colourless even when exposed to light.

Answer

(a) HNO₃ is a strong oxidizing agent

(b) NaNO₃ gives NaNO₂ and O₂ on heating.

(c) Constant boiling nitric acid contains 68% nitric acid by weight

(d) Nitric acid turns yellow when exposed to light.

The nitrate salt which does not give a mixture of NO₂ and O₂ on heating is:

1. AgNO₃
2. KNO₃
3. Cu(NO₃)₂
4. Zn(NO₃)₂

Answer

KNO₃

Reason — The alkali metal nitrates such as sodium or potassium nitrates when heated melts into colourless liquids which decompose on heating to give oxygen gas.

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

The chemical used in the brown ring test is:

1. CuSO₄
2. FeSO₄
3. Fe₂(SO₄)₃
4. ZnSO₄

Answer

FeSO₄

Reason — 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.

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

FeSO₄ + NO ⟶ FeSO₄.NO

[Nitroso Ferrous sulphate, a brown compound]

Lead nitrate decomposes on heating to give:

1. NO
2. N₂O
3. NO₂
4. N₂O₅

Answer

NO₂

Reason — Lead nitrate decomposes on heating to give lead oxide, nitrogen dioxide and oxygen.

2Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂

Name :

(a) a nitrate of metal which on heating does not give nitrogen dioxide.

(b) a nitrate which on heating leaves no residue behind.

(c) a metal nitrate which on heating is changed into metal oxide.

(d) a metal nitrate which on heating is changed into metal.

(e) a solution which absorbs nitric oxide.

(f) the oxide of nitrogen which turns brown on exposure to air. How is it prepared?

Answer

(a) Sodium nitrate
2NaNO₃ $\xrightarrow{\Delta}$ 2NaNO₂ + O₂↑

(b) Ammonium nitrate
NH₄NO₃ $\xrightarrow{\Delta}$ N₂O + 2H₂O

(c) Calcium nitrate
2Ca(NO₃)₂ $\xrightarrow{\Delta}$ 2CaO + 4NO₂ + O₂

(d) Silver nitrate
2AgNO₃ $\xrightarrow{\Delta}$ 2Ag + 2NO₂ + O₂

(e) Freshly prepared ferrous sulphate
6FeSO₄ + 3H₂SO₄ + 2HNO₃ (dil.) ⟶ 3Fe₂(SO₄)₃ + 4H₂O +2NO

FeSO₄ + NO ⟶ FeSO₄.NO

(f) Nitric oxide
2NO + O₂ ⟶ 2NO₂ (brown gas)

Nitric oxide can be prepared by catalytic oxidation of ammonia as shown below:

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

Mention three important uses of nitric acid. Give the property of nitric acid involved in the use.

Answer

1. To etch designs on copper and brassware.
   Property : Acts as solvent for a large number of metals except noble metals
2. To purify gold.
   Property : Gold may contain Cu, Ag, Zn, Pb etc. as impurities which dissolve in nitric acid.
3. To prepare aqua regia.
   Property : It dissolves noble metals

(a) Explain with the help of a balanced equation, the brown ring test for nitric acid.

(b) Why is freshly prepared ferrous sulphate solution used for testing the nitrate radical in the brown ring test

Answer

(a) Brown ring test for nitric acid

To the aq. solution of a nitrate or nitric acid :

1. Add freshly prepared saturated solution of iron [II] sulphate.
2. Now add conc. sulphuric acid carefully from the sides of the test tube, so that it should not fall drop-wise in the test tube.
3. Cool the test tube in water.
4. A brown ring appears at the junction of the two liquids.

Equations for brown ring test:

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

FeSO₄ + NO ⟶ FeSO₄.NO
[Nitroso Ferrous sulphate, a brown compound]

(b) A freshly prepared ferrous sulphate solution is used, because on exposure to the atmosphere, it is oxidised to ferric sulphate, which will not give the brown ring test.

From the following list of substances, choose one substance in each case which matches the description given below:

Ammonium nitrate, calcium hydrogen carbonate, copper carbonate, lead nitrate, potassium nitrate, sodium carbonate, sodium hydrogen carbonate, zinc carbonate.

(a) A substance which gives off only oxygen when heated.

(b) A substance which on heating decomposes into dinitrogen oxide (nitrous oxide) and steam.

(c) A substance which gives off oxygen and nitrogen dioxide when heated.

(d) A substance which on heating leaves yellow residue.

Answer

(a) Potassium nitrate

(b) Ammonium nitrate

(c) Calcium nitrate

(d) Lead nitrate

The action of heat on the blue crystalline solid X, gives a reddish brown gas Y, a gas which re-lights a glowing splint and leaves a black residue. When gas Z, which has a rotten egg smell, is passed through a solution of X, a black ppt. is formed.

(a) Identify X, Y and Z.

(b) Write equation for action of heat on X.

(c) Write equation between solution of X and gas Z.

Answer

(a) X is copper nitrate [Cu(NO₃)₂],
     Y is nitrogen dioxide [NO₂] and
     Z is hydrogen sulphide [H₂S].

(b) 2Cu(NO₃)₂ $\xrightarrow{\Delta}$ 2CuO + 4NO₂ + O₂

(c) When a H₂S gas, which has a rotten egg smell, is passed through a solution of Cu(NO₃)₂, a black ppt. of CuS is formed.
Cu(NO₃)₂ + H₂S ⟶ CuS + 2HNO₃

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 observations. Copy and complete the corresponding inferences in (a) to (e).

(a) A reddish-brown gas is obtained when X, Y and Z are separately warmed with concentrated sulphuric acid and copper turning added to the mixture.

INFERENCE 1: The common anion is the ............... ion.

(b) When X is heated, it melts and gives off only one gas which re-lights a glowing splint.

INFERENCE 2: The cation in X is either ............... or ...............

(c) The action of heat on Y produces a reddish-brown gas and a yellow residue which fuses with the glass of the test tube.

INFERENCE 3: The metal ion present in Y is the ............... ion.

(d) When Z is heated, it leaves no residue. Warming Z with sodium hydroxide solution liberates a gas which turns moist red litmus paper blue.

INFERENCE 4: Z contains the ............... cation.

(e) Write the equations for the following reactions.

1. X and concentrated sulphuric acid (below 200° C). (One equation only for either of the cations given in INFERENCE 2).
2. Action of heat on Y.
3. Concentrated nitric acid is added to copper turnings kept in a beaker.

Answer

(a) The common anion is the nitrate ion.

(b) The cation in X is either sodium or potassium

(c) The metal ion present in Y is the lead ion.

(d) Z contains the ammonium ion

(e) The balanced equations are:

1. Reaction between Potassium nitrate and concentrated sulphuric acid (below 200° C):

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

2. 2Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂

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

Explanation

(a) On adding copper to acidified nitrates and heating, dense reddish brown fumes of nitrogen dioxide [NO₂] are evolved.

(b) The alkali metal nitrates such as sodium or potassium nitrates when heated melts into colourless liquids which decompose on heating to give oxygen gas.

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

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

(c) Lead nitrate decomposes on heating to give nitrogen dioxide, oxygen gas and yellow residue of lead oxide which fuses with the glass of the test tube.

2Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂

(d) Ammonium nitrate decomposes explosively on heating leaving behind no residue.

NH₄NO₃ $\xrightarrow{\Delta}$ N₂O + 2H₂O

Ammonium nitrate reacts with sodium hydroxide to give sodium nitrate, ammonia, and water. Ammonia gas turns moist red litmus paper blue.

NH₄NO₃ + NaOH ⟶ NaNO₃ + NH₃ + H₂O

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

(b) Write the equation for the reaction of dilute nitric acid and conc. nitric acid with copper.

Answer

(a) Dilute nitric acid is considered a typical acid except for it's reaction with metals since it does not liberate hydrogen. It is a powerful oxidising agent and the nascent oxygen formed oxidises the hydrogen to water.

3Cu + 8HNO₃ ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO

3Fe + 8HNO₃ ⟶ 3Fe(NO3)2 + 4 H₂O + 2NO

(b) Reaction with dil. HNO₃:

3Cu + 8HNO₃ ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

Reaction with conc. HNO₃:

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

Explain why

(a) Only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate.

(b) Nitric acid is kept in a reagent bottle for a long time.

Answer

(a) 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.

(b) 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.

4HNO₃ ⟶ O₂ + 2H₂O + 2NO₂

The figure given below illustrates the apparatus used in the laboratory preparation of nitric acid.

(a) Name A (a liquid), B (a solid) and C (a liquid). (Do not give the formulae).

(b) Write an equation to show how nitric acid undergoes decomposition.

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

Answer

(a) A — concentrated sulphuric acid,

B — sodium nitrate or potassium nitrate and

C — is nitric acid.

(b) Nitric acid decomposes to liberate nitrogen dioxide along with oxygen and form water.

4HNO₃ $\xrightarrow{\Delta}$ 4NO₂ + 2H₂O + O₂

(c) Copper metal is oxidised by concentrated nitric acid to copper nitrate.

Cu + 4HNO₃ $\xrightarrow{\Delta}$ Cu(NO₃)₂ + 2H₂O + 2NO₂

(a) A dilute acid B does not normally give hydrogen when reacted with metals but does give a gas when reacts with copper. Identify B. Write equation with copper.

(b) Completed the table:

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

(d) State one observation :

(i) Concentrated nitric acid is reacted with - sulphur.

(ii) Lead nitrate is heated strongly in a test tube.

Answer

(a) HNO₃ (Nitric acid)

3Cu + 8HNO₃ ⟶ 3Cu(NO₃)₂ + 4H₂O + 2NO

(b) Complete table is as follows:

(c) 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.

(d) (i) When concentrated nitric acid is reacted with sulphur, dense brown fumes of nitrogen dioxide are observed.

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

(ii) Lead nitrate decomposes on heating to give nitrogen dioxide, oxygen gas and yellow residue of lead oxide which fuses with the glass of the test tube.

2Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂

Name - the gas produced when copper reacts with conc. HNO₃.

Answer

Nitrogen dioxide.

Cu + 4HNO₃ ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂

State observation : Zinc nitrate crystals are strongly heated.

Answer

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

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

Correct the statement :

Magnesium reacts with nitric acid to liberate hydrogen gas.

Answer

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

Iron is rendered passive with fuming HNO₃. Give reason.

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

Give balanced equation for dilute nitric acid and copper carbonate.

Answer

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

Identify the gas evolved when:

(i) Sulphur is treated with conc. 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

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

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.

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

Give a balanced equations for: oxidation of carbon with conc. HNO₃.

Answer

C + 4HNO₃ ⟶ CO₂ + 2H₂O + 4NO₂

Fill in the blank :

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.

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:

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

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

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

Identify the acid

(i) Which is used for the preparation of non-volatile acid.

(ii) The acid which is prepared by cata­lytic oxidation of ammonia.

Answer

(i) Nitric acid

S + 6HNO₃ ⟶ H₂SO₄ + 2H₂O + 6NO₂

(ii) Nitric acid

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

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

4NO₂ + 2H₂O + O₂ ⟶ 4HNO₃

State one appropriate observation : 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.

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

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.