Get notes, summary, questions and answers, MCQs, extras, competency-based questions and PDFs of Study of Compounds-Nitric Acid: ICSE Class 10 Chemistry (Concise/Selina). However, the notes should only be treated as references, and changes should be made according to the needs of the students.
Summary
Nitric acid (HNO3) is a compound where nitrogen has a valency of five. It occurs in small amounts in rainwater after lightning and as mineral salts. Atmospheric lightning helps nitrogen and oxygen form nitric oxide, which becomes nitrogen dioxide. This gas dissolves in rainwater with oxygen to create nitric acid. This natural process is called atmospheric nitrogen fixation. Nitric acid was once called “aqua fortis,” meaning “strong water,” for its ability to react with many metals.
In labs, nitric acid is made by heating potassium nitrate or sodium nitrate with concentrated sulphuric acid in a glass retort. The nitric acid vapor is cooled to become a liquid. This lab-made acid is often yellow due to dissolved nitrogen dioxide, a brown gas formed from some acid decomposing. Bubbling air through the warm acid or adding water can remove this color. All-glass apparatus is used, and heating is kept below 200°C.
Industrially, the Ostwald process makes nitric acid. First, ammonia and air pass over a hot platinum catalyst, producing nitric oxide and heat. Second, nitric oxide cools and reacts with more air to form nitrogen dioxide. Third, nitrogen dioxide is absorbed in water in a tower, often packed with quartz, to form nitric acid. This acid can be concentrated further.
Pure nitric acid is a colorless liquid; commercial acid can be yellowish. It has a sharp smell and sour taste. It is corrosive, staining skin yellow by reacting with proteins. Nitric acid is a strong acid, reacting with bases to form salts and water, and with carbonates to release carbon dioxide. A key feature is its strong oxidizing power. Its reaction with metals varies with acid concentration and the metal. Dilute acid usually gives metal nitrates and nitric oxide gas. Concentrated acid gives metal nitrates and nitrogen dioxide gas. Very dilute acid with magnesium or manganese can produce hydrogen. Metals like iron become passive with concentrated nitric acid due to a protective oxide layer. “Aqua regia,” a mix of nitric and hydrochloric acids, dissolves gold and platinum. The term “royal water” refers to its ability to dissolve these noble metals.
Nitric acid is used to etch metals, purify gold, and as a rocket fuel oxidant. It is vital for making fertilizers like ammonium nitrate. It’s also used for explosives, dyes, plastics, and synthetic fibers. The brown ring test detects nitric acid or nitrates. Fresh iron(II) sulphate solution is added to the test sample, followed by concentrated sulphuric acid down the test tube’s side. A brown ring at the liquid junction confirms nitrate presence. Heating most metal nitrates releases brown nitrogen dioxide gas.
Workbook solutions (Concise/Selina)
Intext Questions and Answers I
1. Fill in the blank: Cold dil. nitric acid reacts with copper to form ____ (hydrogen, nitrogen dioxide, nitric oxide).
Answer: Cold dil. nitric acid reacts with copper to form nitric oxide.
2. What is: (a) aqua fortis, (b) aqua regia (c) Fixation of Nitrogen?
Answer: (a) Aqua fortis was the former name for nitric acid, meaning strong water.
(b) Aqua regia is a mixture formed when conc. nitric acid (1 part by volume) is mixed with conc. hydrochloric acid (3 parts by volume). It means royal water.
(c) Fixation of atmospheric nitrogen is the conversion of free atmospheric nitrogen into useful nitrogenous compounds in the soil.
3. During thunderstorm, rain water contains nitric acid. Explain with reactions.
Answer: In the free state, nitric acid is found in rain water, where it occurs in traces after lightning. The formation of nitric acid in the atmosphere occurs as follows:
- During lightning discharge, the nitrogen present in the atmosphere reacts with the oxygen to form nitric oxide.
N₂ + O₂ –(3000°C)–> 2NO - Nitric oxide is further oxidised to nitrogen dioxide.
2NO + O₂ → 2NO₂ - This nitrogen dioxide dissolves in atmospheric moisture or rain water in the presence of oxygen of the air and forms nitric acid in the free state.
4NO₂ + 2H₂O + O₂ → 4HNO₃
4. 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) The reactants used in the Ostwald process are a mixture of dry air (free from carbon dioxide and dust particles) and dry ammonia.
(b) The catalyst used in the process is platinum gauze.
(c) The oxidising agent used in this process is oxygen, present in the air.
(d) The ratio of dry air to dry ammonia taken in this process is 10 : 1 by volume.
(e) Quartz is used in this process because it is acid resistant and when packed in layers it helps in dissolving nitrogen dioxide uniformly in water in the absorption tower.
5. (a) Write a balanced chemical equation for the laboratory preparation of nitric acid.
Answer: (a) A balanced chemical equation for the laboratory preparation of nitric acid is:
KNO₃ + H₂SO₄ (conc.) –(<200°C)–> KHSO₄ + HNO₃
(b) In the preparation of nitric acid from KNO₃, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why ?
Answer: (b) In the preparation of nitric acid from KNO₃, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid because HCl is volatile and hence nitric acid vapours will carry HCl vapours.
(c) Conc. nitric acid prepared in laboratory is yellow in colour. Why? How is this colour removed?
Answer: (c) Conc. nitric acid prepared in the laboratory is slightly yellow. The yellow colour is due to dissolution of reddish brown coloured 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 colour of the acid is removed:
(i) If dry air or CO₂ is bubbled through the yellow acid, the latter turns colourless because it drives out NO₂ from warm acid which is further oxidised to nitric acid.
(ii) By addition of excess of water, nitrogen dioxide gas dissolves in water and thus the yellow colour of the acid is 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.
Answer: (d) 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 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 HNO₃ is higher. The higher temperature (i) may damage the glass apparatus, (ii) decomposition of nitric acid can also occur, (iii) Wastage of fuel.
6. (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) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of HNO₃ because an aqueous solution of nitric acid (68% concentration) forms a constant boiling mixture at 121°C.
(b) Passive iron is iron that has become passive (inert) when treated with pure concentrated nitric acid. This is due to the formation of an extremely thin layer of insoluble metallic oxide (passivity) which stops the reaction.
Passivity can be removed by rubbing the surface layer with sand paper, or by treating with strong reducing agents.
7. Name the products formed when :
(a) carbon and conc. nitric acid is heated,
(b) dilute HNO₃ is added to copper.
Answer: (a) When carbon and conc. nitric acid are heated, the products formed are carbon dioxide (CO₂), water (2H₂O), and nitrogen dioxide (4NO₂).
(b) When dilute HNO₃ is added to copper, the products formed are copper(II) nitrate (3Cu(NO₃)₂), water (4H₂O), and nitric oxide (2NO).
8. 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) Two chemical equations for reactions of nitric acid with non-metals are:
1. C + 4HNO₃ [conc.] → CO₂ + 2H₂O + 4NO₂ (Reaction with Carbon)
2. S + 6HNO₃ [conc.] → H₂SO₄ + 2H₂O + 6NO₂ (Reaction with Sulphur)
(b) Two chemical equations for nitric acid showing as acidic character are:
1. HNO₃(aq) ⇌ H⁺ + NO₃⁻ (Ionisation in aqueous solution)
2. CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O (Reaction with a metallic oxide, e.g., copper(II) oxide, to form salt and water)
(c) Two chemical equations for nitric acid acting as an oxidising agent are:
1. 2HNO₃ (conc.) → 2NO₂ + H₂O + [O] (Decomposition to produce nascent oxygen)
2. Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂ (Oxidation of a metal, e.g., copper)
9. Write balanced equations and name the products formed when:
(a) sodium hydrogen carbonate is added to nitric acid,
Answer: When sodium hydrogen carbonate is added to nitric acid:
NaHCO₃ + HNO₃ → NaNO₃ + H₂O + CO₂↑
The products formed are sodium nitrate (NaNO₃), water (H₂O), and carbon dioxide (CO₂).
(b) cupric oxide reacts with nitric acid,
Answer: When cupric oxide reacts with nitric acid:
CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O
The products formed are copper(II) nitrate (Cu(NO₃)₂) and water (H₂O).
(c) zinc reacts with dilute nitric acid,
Answer: When zinc reacts with dilute nitric acid:
3Zn + 8HNO₃ → 3Zn(NO₃)₂ + 4H₂O + 2NO
The products formed are zinc nitrate (3Zn(NO₃)₂), water (4H₂O), and nitric oxide (2NO).
(d) concentrated nitric acid is heated.
Answer: When concentrated nitric acid is heated:
4HNO₃ –(Δ)–> 2H₂O + 4NO₂ + O₂
The products formed are water (2H₂O), nitrogen dioxide (4NO₂), and oxygen (O₂).
10. How will you prepare the following from nitric acid ?
(a) Sodium nitrate
Answer: (a) Sodium nitrate can be prepared from nitric acid by reacting nitric acid with sodium hydroxide or sodium carbonate:
NaOH + HNO₃ → NaNO₃ + H₂O
or Na₂CO₃ + 2HNO₃ → 2NaNO₃ + H₂O + CO₂↑
(b) Copper nitrate
Answer: (b) Copper nitrate can be prepared from nitric acid by reacting nitric acid with copper metal, copper oxide, or copper carbonate:
3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 4H₂O + 2NO
or CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O
(c) Lead nitrate
Answer: (c) Lead nitrate can be prepared from nitric acid by reacting nitric acid with lead metal or lead oxide. For example, with lead metal (similar to copper):
3Pb + 8HNO₃ (dilute) → 3Pb(NO₃)₂ + 4H₂O + 2NO (This specific reaction for lead is a general application of nitric acid’s properties with metals, as detailed for copper and zinc).
(d) Magnesium nitrate
Answer: (d) Magnesium nitrate can be prepared from nitric acid by reacting very dilute nitric acid with magnesium metal or nitric acid with magnesium hydroxide:
Mg + 2HNO₃ (V. dilute) → Mg(NO₃)₂ + H₂
or Mg(OH)₂ + 2HNO₃ → Mg(NO₃)₂ + 2H₂O
(e) Ferric nitrate
Answer: (e) Ferric nitrate can be prepared from nitric acid by reacting concentrated nitric acid with iron metal or nitric acid with ferric hydroxide:
Fe + 6HNO₃ (conc.) → Fe(NO₃)₃ + 3H₂O + 3NO₂
or Fe(OH)₃ + 3HNO₃ → Fe(NO₃)₃ + 3H₂O
(f) Aqua regia
Answer: Aqua regia is prepared from nitric acid by mixing concentrated nitric acid (1 part by volume) with concentrated hydrochloric acid (3 parts by volume):
HNO₃ + 3HCl → NOCl + 2H₂O + 2[Cl]
11. Write equation for the following conversions A, B, C and D.
Answer:
A: Copper to Copper nitrate
3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 4H₂O + 2NO
(Alternatively, with concentrated nitric acid: Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂)
B: Copper nitrate to Copper oxide
2Cu(NO₃)₂ –(Δ)–> 2CuO + 4NO₂ + O₂
C: Copper to Copper oxide
2Cu + O₂ –(Heat)–> 2CuO
(Oxygen for this reaction can be obtained from the decomposition of nitric acid: 4HNO₃ → 2H₂O + 4NO₂ + O₂)
D: Copper oxide to Copper
CuO + H₂ –(Heat)–> Cu + H₂O
(This is a reduction reaction, typically not involving nitric acid as nitric acid is an oxidising agent.)
12. Correct the following, if required :
(a) HNO₃ is a strong reducing agent.
Answers: HNO₃ is a strong reducing agent.
Correction: HNO₃ is a strong oxidising agent.
(b) NaNO₃ gives NO₂ and O₂ on heating.
Answers: HNO₃ is a strong reducing agent.
Correction: HNO₃ is a strong oxidising agent.
(c) Constant boiling nitric acid contains 80% nitric acid by weight.
Answers: HNO₃ is a strong reducing agent.
Correction: HNO₃ is a strong oxidising agent.
(d) Nitric acid remains colourless even when exposed to light.
Answers: HNO₃ is a strong reducing agent.
Correction: HNO₃ is a strong oxidising agent.
(e) Magnesium reacts with nitric acid to liberate hydrogen gas.
Answer: Magnesium reacts with nitric acid to liberate hydrogen gas.
Correction (or clarification): This statement is correct under specific conditions. Very dilute (about 1%) nitric acid reacts with magnesium at room temperature to give magnesium nitrate and hydrogen gas.
MCQs
1. The nitrate salt which does not give a mixture of NO₂ and O₂ on heating is :
(a) AgNO₃
(b) KNO₃
(c) Cu(NO₃)₂
(d) Zn(NO₃)₂
Answer: (b) KNO₃
2. The chemical used in the brown ring test is :
(a) CuSO₄
(b) FeSO₄
(c) Fe₂(SO₄)₃
(d) ZnSO₄
Answer: (b) FeSO₄
3. Lead nitrate decomposes on heating to give :
(a) NO
(b) N₂O
(c) NO₂
(d) N₂O₅
Answer: (c) NO₂
4. In the laboratory preparation of nitric acid, nitre is heated with:
(a) conc. HCl
(b) conc. H₂SO₄
(c) dil. H₂SO₄
(d) conc. H₃PO₄
Answer: (b) conc. H₂SO₄
5. Nitric acid is manufactured by:
(a) Bayer’s process
(b) Haber’s process
(c) Ostwald process
(d) Contact process
Answer: (c) Ostwald process
6. The catalyst used in the manufacture of HNO₃ is :
(a) Pt
(b) V₂O₅
(c) Fe
(d) Pd
Answer: (a) Pt
7. Cold and dilute HNO₃ reacts with Cu to give :
(a) NO₂
(b) NO
(c) N₂
(d) O₂
Answer: (b) NO
8. The brown ring test is used for the detection of:
(a) CO₃²⁻
(b) Cl⁻
(c) NO₃⁻
(d) SO₃²⁻
Answer: (c) NO₃⁻
9. Sulphur reacts with conc. HNO₃ to give :
(a) SO₂
(b) H₂
(c) H₂SO₄
(d) H₂S
Answer: (c) H₂SO₄
10. The solvent for NO is :
(a) Freshly prepared FeSO₄
(b) Fe₂(SO₄)₃
(c) FeCl₃
(d) FeCl₂
Answer: (a) Freshly prepared FeSO₄
11. The acid which can be used to etch designs on brassware is:
(a) HCl
(b) HNO₃
(c) H₂SO₄
(d) H₂CO₃
Answer: (b) HNO₃
12. HNO₃ is kept in reagent bottles because it is :
(a) an oxidising agent
(b) a reducing agent
(c) hygroscopic in nature
(d) decomposed in the presence of light
Answer: (d) decomposed in the presence of light
13. The nitrate which does not leave any residue behind on heating is:
(a) NaNO₃
(b) NH₄NO₃
(c) AgNO₃
(d) Ca(NO₃)₂
Answer: (b) NH₄NO₃
14. On heating zinc nitrate, the products formed are zinc oxide, nitrogen dioxide and oxygen. Zinc oxide is :
P a black solid
Q brown when hot, yellow when cold
R yellow when hot, white when cold
Which of the following is true ?
(a) Only P
(b) Only Q
(c) Only R
(d) Both Q and R
Answer: (c) Only R
15. Assertion (A) : KNO₃ and conc. H₂SO₄ are the reactants for the preparation of nitric acid.
Reason (R): Conc. H₂SO₄ being a non volatile acid is required to prepare volatile acids.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
16. Assertion (A): Nitric acid prepared in the laboratory is yellow in colour.
Reason (R) : Brown gas is used in its preparation.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
17. Assertion (A): Catalytic oxidation of Ammonia does not require any external heat in Oswald process.
Reason (R): Catalytic oxidation of ammonia is an endothermic process.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
18. Assertion (A): The yellow colour of nitric acid is removed by using carbon dioxide gas or dry air.
Reason (R): Carbon dioxide gas or dry air drives away NO₂ gas from the warm nitric acid.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
19. Assertion (A): Nitric acid reacts with NaOH/KOH to form salt and water.
Reason (R): Alkalies react with acids to form salt and water.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
20. Assertion (A): Cold and dilute nitric acid oxidise metals to their nitrates and nitric oxide.
Reason (R): Hot and concentrated nitric acid liberates nitrogen dioxide.
(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
Very Short Answer Type
1. Name:
(a) a nitrate of metal which on heating does not give nitrogen dioxide.
Answer: A nitrate of metal which on heating does not give nitrogen dioxide is potassium nitrate (KNO₃) or sodium nitrate (NaNO₃).
(b) a nitrate which on heating leaves no residue behind.
Answer: A nitrate which on heating leaves no residue behind is ammonium nitrate (NH₄NO₃).
(c) a metal nitrate which on heating is changed into metal oxide.
Answer: A metal nitrate which on heating is changed into metal oxide is, for example, calcium nitrate [Ca(NO₃)₂], zinc nitrate [Zn(NO₃)₂], lead nitrate [Pb(NO₃)₂], or copper nitrate [Cu(NO₃)₂].
(d) a metal nitrate which on heating is changed into metal.
Answer: A metal nitrate which on heating is changed into metal is silver nitrate (AgNO₃) or mercuric nitrate [Hg(NO₃)₂].
(e) a solution which absorbs nitric oxide.
Answer: A solution which absorbs nitric oxide is a freshly prepared ferrous sulphate solution.
(f) the oxide of nitrogen which turns brown on exposure to air.
Answer: The oxide of nitrogen which turns brown on exposure to air is nitric oxide (NO).
(g) the gas produced when copper reacts with conc. HNO3.
Answer:The gas produced when copper reacts with conc. HNO₃ is nitrogen dioxide (NO₂).
2. Identify the gas evolved when :
(a) Sulphur is treated with conc. nitric acid.
Answer: When sulphur is treated with conc. nitric acid, the gas evolved is nitrogen dioxide (NO₂).
(b) A few crystals of KNO3 are heated in a hard glass test tube.
Answer: When a few crystals of KNO₃ are heated in a hard glass test tube, the gas evolved is oxygen (O₂).
3. Identify the acid :
(a) Which is used for the preparation of non-volatile acid.
Answer: The acid which is used for the preparation of a non-volatile acid (sulphuric acid from sulphur) is concentrated nitric acid.
(b) The acid which is prepared by catalytic oxidation of ammonia.
Answer: The acid which is prepared by catalytic oxidation of ammonia is nitric acid (HNO₃).
4. 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) Asubstancewhich gives off only oxygenwhen heated.
Answer: A substance which gives off only oxygen when heated is potassium nitrate.
(b) A substance which on heating decomposes into dinitrogen oxide [nitrous oxide] and steam.
Answers: A substance which on heating decomposes into dinitrogen oxide [nitrous oxide] and steam is ammonium nitrate.
(c) A substance which gives off oxygen and nitrogen dioxide when heated.
Answers: A substance which gives off oxygen and nitrogen dioxide when heated is lead nitrate.
(d) A substance which on heating leaves yellow residue,
Answers: A substance which on heating leaves a yellow residue is lead nitrate.
5. Complete the table :
Answer:
Short Answer Type
1. Explain why
(a) Only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate.
Answer: Only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate because nitric acid vapours attack rubber and cork.
(b) Nitric acid is kept in a reagent bottle for along time.
Answers: Nitric acid is kept in a reagent bottle for a long time, specifically in coloured bottles, because pure nitric acid is unstable to sunlight and decomposes. To avoid the decomposition, nitric acid is normally stored in coloured bottles. It is decomposed in the presence of light.
(c) Iron is rendered passive with fuming HNO3. Give reason.
Answers: Iron is rendered passive with fuming HNO₃. This is because metals like iron become passive (inert) when treated with pure concentrated nitric acid. It is due to the formation of an extremely thin layer of insoluble metallic oxide (passivity) which stops the reaction.
(d) Dil. HNO3 is generally considered a typical acid but not so in the reaction with metals..
Answers: Dil. HNO₃ is generally considered a typical acid but not so in its reaction with metals because, unlike typical acids, it does not liberate hydrogen. Dilute nitric acid is a powerful oxidising agent, and the nascent oxygen formed oxidises the hydrogen (that would have been formed) to water.
(e) When it is left standing in a glass bottle concentrated nitric acid appears yellow.
Answers: When it is left standing in a glass bottle, concentrated nitric acid appears yellow. This is because nitric acid decomposes, even at room temperature or in the presence of sunlight, to form nitrogen dioxide. The yellow colour is due to dissolved NO₂ in HNO₃. This nitrogen dioxide gas is produced due to the thermal decomposition of a portion of nitric acid (4HNO₃ → 2H₂O + 4NO₂ + O₂).
2. 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.
Answer: Acid B is dilute nitric acid.
The equation for the reaction of dilute nitric acid with copper is:
3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 4H₂O + 2NO
3. State one observation:
(a) dilute nitric acid and copper carbonate.
Answer: When concentrated nitric acid is reacted with sulphur, reddish-brown fumes of nitrogen dioxide are evolved.
(b) oxidation of carbon with conc. HNO3.
Answer: When lead nitrate is heated strongly in a test tube, the colourless crystalline solid decrepitates, giving off reddish-brown fumes of nitrogen dioxide and leaving a yellow residue which fuses with glass.
(c) Laboratory preparation of nitric acid.
Answer: When zinc nitrate crystals are strongly heated, the very deliquescent crystals give off reddish-brown fumes of nitrogen dioxide and leave a residue which is yellow when hot and white when cold.
(d) Action of heat on a mixture of copper and nitric acid.
Answer: When copper nitrate crystals are heated in a test tube, the blue crystalline solid gives off reddish-brown fumes of nitrogen dioxide and leaves a black residue.
4. What is the property of nitric acid which allows it to react with copper?
Answer: The property of nitric acid that allows it to react with copper is its oxidising property. Nitric acid is a powerful oxidising agent.
5. Give balanced equations for the following :
(a) dilute nitric acid and copper carbonate.
Answers: CuCO₃ + 2HNO₃ (dilute) → Cu(NO₃)₂ + H₂O + CO₂↑
(b) Oxidation of carbon with conc. HNO₃:
Answers: C + 4HNO₃ (conc.) → CO₂ + 2H₂O + 4NO₂
(c) Laboratory preparation of nitric acid (using sodium nitrate):
Answers: NaNO₃ + H₂SO₄ (conc.) <200°C → NaHSO₄ + HNO₃
(d) Action of heat on a mixture of copper and nitric acid (assuming concentrated nitric acid or hot dilute nitric acid):
Answers: Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂
Long Answer Type
1. 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).
Answer:
(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 nitrate 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 K⁺ or Na⁺.
(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 Pb²⁺ 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 ammonium (NH₄⁺) 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).
KNO₃ + H₂SO₄ (conc.) → KHSO₄ + HNO₃ (below 200°C)
or
NaNO₃ + H₂SO₄ (conc.) → NaHSO₄ + HNO₃ (below 200°C)
(2) Action of heat on Y.
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(3) Concentrated nitric acid is added to copper turnings kept in a beaker.
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂
2. The diagram given below is a representation of the Industrial preparation of Nitric acid by Ostwald’s process. With respect to the process, answer the following questions:
(a) Write the temperature and the catalyst required during the catalytic oxidation of ammonia.
(b) Give balanced chemical equation for the reaction occurring during the conversion of nitrogen dioxide to nitric acid.
Answer: (a) The temperature and the catalyst required during the catalytic oxidation of ammonia are about 800°C and platinum gauze, respectively.
(b) The balanced chemical equation for the reaction occurring during the conversion of nitrogen dioxide to nitric acid is:
4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃ (aq.)
3. (a) Mention three important uses of nitric acid. Give the property of nitric acid involved in the use.
Answer: Three important uses of nitric acid and the property involved are:
(i) To etch designs on copper and brassware.
Reason: Nitric acid acts as a solvent for a large number of metals except noble metals.
(ii) To purify gold.
Reason: Gold may contain Cu, Ag, Zn, Pb, etc., as impurities which dissolve in nitric acid.
(iii) It acts as a rocket fuel oxidant.
(b) Explain with the help of a balanced equation, the brown ring test for nitric acid.
Answer: To the aqueous solution of a nitrate or nitric acid:
(i) Add freshly prepared saturated solution of iron [II] sulphate.
(ii) Now add conc. sulphuric acid carefully from the sides of the test tube, so that it should not fall dropwise in the test tube.
(iii) Cool the test tube in water.
(iv) A brown ring appears at the junction of the two liquids.
The reactions are:
6FeSO₄ + 3H₂SO₄ + 2HNO₃ → 3Fe₂(SO₄)₃ + 4H₂O + 2NO
FeSO₄ + NO → FeSO₄·NO (Nitroso ferrous sulphate, a brown compound)
(c) Why is freshly prepared ferrous sulphate solution used for testing the nitrate radical in the brown ring test?
Answer: 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.
4. 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.
Answer: (a) X is Copper nitrate (Cu(NO₃)₂). Y is Nitrogen dioxide (NO₂). Z is Hydrogen sulphide (H₂S).
(b) The equation for the action of heat on X is:
2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂
(c) The equation between solution of X and gas Z is:
Cu(NO₃)₂(aq) + H₂S(g) → CuS(s) + 2HNO₃(aq)
5. (a) 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 its 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. However, very dilute (about 1%) acid reacts with magnesium and manganese at room temperature to give their nitrates and hydrogen gas. Since the oxidising action of the acid is much reduced due to dilution.
(b) Write the equation for the reaction of dilute nitric acid and conc. nitric acid with copper.
Answer: The equation for the reaction of dilute nitric acid with copper is:
3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 4H₂O + 2NO
The equation for the reaction of conc. nitric acid with copper is:
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂
6. 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 (a liquid) is Concentrated Sulphuric Acid. B (a solid) is Sodium or Potassium Nitrate. C (a liquid) is Nitric Acid.
(b) An equation to show how nitric acid undergoes decomposition is:
4HNO₃ → 2H₂O + 4NO₂ + O₂
(c) The equation for the reaction in which copper is oxidized by concentrated nitric acid is:
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2H₂O + 2NO₂
7. With respect to the brown ring test for nitrates, explain :
(a) Freshly prepared ferrous sulphate is used.
(b) Brown ring disappear if the test tube is disturbed.
(c) Lead nitrate does not respond properly to the brown ring test.
(d) What is the name and formula of the brown ring formed ?
Answer: (a) Freshly prepared ferrous sulphate is used because on exposure to the atmosphere, it is oxidised to ferric sulphate which will not give the brown ring.
(b) The brown ring disappears if the test tube is disturbed because the brown ring of nitroso ferrous sulphate decomposes on disturbing the test tube. The heat evolved decomposes the unstable brown ring.
(c) Lead nitrate does not respond properly to the brown ring test. Salt solutions forming insoluble sulphates interfere with the result of the Brown ring test.
(d) The name of the brown ring formed is nitroso ferrous sulphate, and its formula is FeSO₄·NO.
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