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Chemistry [3 Mark Question Answer]

9.2 Nitric Acid · ICSE STD 10 (ICSE - English Medium). 14 questions.

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[3 Mark Question Answer]

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Question 13 Marks
Explain the following :
Concentrated nitric acid appears yellow when it is left standing in a glass bottle.
Answer
Although pure concentrated nitric acid is colourless, it appears yellow when left standing in a glass bottle due to the dissolution of reddish brown nitrogen dioxide gas in the acid. Nitrogen dioxide is produced because of the thermal decomposition of a portion of nitric acid.
$4HNO_3 → 2H_2O + 4NO_2 + O_2$​​​​​​​
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Question 23 Marks
The figure given below illustrates the apparatus used in the laboratory preparation of nitric acid:

(i) Name A (a liquid), B( a Solid) and C(a liquid).
(ii) write the balanced chemical equation to show how nitric acid undergoes decomposition.
(iii) Write the balanced chemical equation for the reaction in which copper is oxidized by concentrated nitric acid.
Answer
(i) $A=$ Sulphuric acid, $B=$ potassium nitrate, $C=$ nitric acid
(ii)
$
4 HNO _3 \longrightarrow 4 NO _2+2 H _2 O + O _2
$
(iii)
$
Cu +4 HNO _3 \xrightarrow{\Delta} Cu \left( NO _3\right)_2+2 NO _2+2 H _2 O
$
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Question 33 Marks
During a thunderstorm, the rainwater contains nitric acid. The nitric acid is formed as a result of three chemical reactions. Describe or write balanced chemical equation for these three reactions.
Answer
During a thunderstorm, the rainwater contains nitric acid. The nitric acid is formed as a result of three chemical reactions which is described as follows:
1. During lightning discharge, nitrogen in atmosphere combaines with oxygen to form nitric oxide.
$
N _2+ O _2 \xrightarrow{\text { lightning }} 2 NO
$
2. Nitric oxide is further oxidized to nitrogen dioxide.
$
2 NO + O _2 \longrightarrow 2 NO _2
$
3.Nitrogen dioxide dissolves in moisture or rain water to form nitric acid.
$
4 NO _2+2 H _2 O + O _2 \longrightarrow 4 HNO _3
$
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Question 43 Marks
Name the product, catalyst and approximate temperature maintained in Ostwalds process. How is the temperature maintained in the Ostwalds process?
Answer
The nitric acid is manufactured by ostwald process. In ostwald process ammonia gas and air are taken in 1:10 ratio. Temperature is maintained at 800°C I the catalytic chamber, as the reaction is exothermic so the heat evolved maintains the temperature in the catalytic chamber.
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Question 53 Marks
Give reason for the following:
In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitatre should not be heated very strongly, above 200°C
Answer
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 because a higher temperature can cause following problems:
1. Nitric acid would decompose to form nitrogen dioxide.
2. Sodium sulphate is formed which may stick to the glass and cannot be removed easily.
3. The glass apparatus may break.
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Question 63 Marks
Give reason for the following:
Commercial concentrated nitric acid is yellow in colour, but when it is dilute with water, it turns colourless.
Give one chemical test for nitric acid.
Answer
Commercial nitric acid is yellow in colour because of presence of nitrogen dioxide but when it is dilute with water, it turns colourless because nitrogen dioxide gets dissloved in water.
Brown ring test can be used to test nitric acid:
Brown ring test :
Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated $H _2 SO _4$ is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.
$
6 FeSO _4+3 H _2 SO _4+2 HNO _3 \longrightarrow 3 Fe _2\left( SO _4\right)_3+2 NO +4 H _2 O
$
$
FeSO _4+ NO +5 H _2 O \longrightarrow \quad\left[ Fe ( NO )\left( H _2 O \right)_5\right] SO _4
$
Hydrated nitrosoferrous sulphate(brown ring)
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Question 73 Marks
Give two chemical tests for nitric acid.
Answer
Two tests for nitric acid are-
(i) Brown ring test :
Nitric acid in a test tube is taken and then freshly prepared ferrous sulphate solution is added. Concentrated $H_2SO_4$​​​​​​​ is then added carefully down the sides of the test tube, dark brown ring is formed at the
junction of two layers.
Image
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Question 83 Marks
Give reason for the following:
Aqua regia dissolves gold and platinum.
Answer
$
3 HCl + HNO _3 \longrightarrow 2 H _2 O + NOCl +2[ Cl ]
$
Gold in Aqua regia :
$
Au +4[ Cl ] \longrightarrow AuCl _3
$
Platinum in Aqua regia :
$
Pt +4[ Cl ] \longrightarrow PtCl _4
$
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Question 93 Marks
Explain with the help of a balanced equation, the brown ring test for nitric acid.
Answer
Brown ring test -
Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated $H _2 SO _4$ is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.
$
6 FeSO _4+3 H _2 SO _4+2 HNO _3 \longrightarrow 3 Fe _2\left( SO _4\right)_3+2 NO +4 H _2 O
$
$
FeSO _4+ NO +5 H _2 O \longrightarrow \underset{\text { Hydrated nitrosoferrous sulphate(brown ring) }}{\left[ Fe ( NO )\left( H _2 O \right)_5\right] SO _4}
$
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Question 103 Marks
Mention three important uses of nitric acid; Give the property of nitric acid involved in the use.
Answer
Uses of nitric acid are -
(i) Nitric acid is used in the purification of silver, gold, platinum etc. because impurities of other metals are dissolved in it.
(ii) Nitric acid is used as an oxidiser in rocket fuel because it can supply large amount of oxygen.
(iii) Nitric acid is used in the manufacture of dyes, perfumes, drugs etc. from coal tar products since nitrobenzene is one of the raw materials of it which is manufactured from nitric acid.
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Question 113 Marks
Give three equations of the reaction to prove that nitric acid is an acid.
Answer
Three equations to prove acidic nature of nitric acid are-
(a) Reaction with basic oxide :-
$
CaO +2 HNO _3 \longrightarrow Ca \left( NO _3\right)_2+ H _2 O
$
(b) Reaction with carbonates and hydrogen carbonates :-
$
\begin{gathered}
Na _2 CO _3+2 HNO _3 \longrightarrow 2 NaNO _3+ H _2 O ( I )+ CO _2 \\
NaHCO _3+ HNO _3 \longrightarrow NaNO _3+ H _2 O + CO _2
\end{gathered}
$
(c) Reaction with metallic sulphites :-
$
Na _2 SO _3+2 HNO _3 \longrightarrow 2 NaNO _3+ SO _2+ H _2 O
$
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Question 123 Marks
Write the equations involved in Ostwald process.
Answer
$\begin{aligned} & 4 NH _3+5 O _2 \xrightarrow[800^{\circ} C ]{\text { Platinum gangue }} 4 NO +6 H _2 O +\text { Heat } \\ & 2 NO + O _2 \xrightarrow{50^{\circ} C } 2 NO _2 \\ & 4 NO _2+ O _2+2 H _2 O \longrightarrow 4 HNO _3 \\ & \end{aligned}$
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Question 133 Marks
What happens when (Given balanced equation):
Nitric acid is added to Kl (aq)?
Answer
Nitric acid act as oxidizing agent and oxidizes iodide to iodine.
$
HNO _3+ H _2 O \longrightarrow H _3 O ^{+}+ NO _3^{-}
$
$
KI \longrightarrow I ^{-}+ K ^{+}
$
$
2 I ^{-} \xrightarrow{-2 e ^{-}} I _2
$
$
2 HNO _3+ H _2 O +2 KI \longrightarrow NO _3^{-}+ I _2+2 KNO _3+ H _3 O ^{+}
$
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Question 143 Marks
Why is the temperature maintained at 200°C in the above reaction?
Answer
The temperature is maintained at 200°C in the above reaction to avoid following problems:
  • Nitric acid would decompose at high temperature.
  • Potassium or sodium sulphate is formed which will stick to the walls of glass and cannot be removed be easily.
  • The glass apparatus may break at high temperature.
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