CBSE Class 9 Chemistry Chemistry of Nitrogen and Halogens MCQs Set 07

Answer: (b) (NH₄)₂Cr₂O₇

Explanation:
1. Ammonium dichromate decomposes on heating: \( (NH_4)_2Cr_2O_7 \rightarrow N_2 + Cr_2O_3 + 4H_2O \).
2. Ammonium nitrate gives nitrous oxide, ammonium sulphate gives ammonia and ammonium carbonate gives ammonia and carbon dioxide.

Teacher's Note:
1. This is the 'volcano experiment' because the green chromium(III) oxide swells up like ash.
2. In ammonium dichromate nitrogen is in the -3 state and chromium in +6, so they react internally.

Answer: (a) NH₄NO₃

Explanation:
1. \( NH_4NO_3 \rightarrow N_2O + 2H_2O \): both products are gases.
2. NaNO₃ leaves NaNO₂, Cu(NO₃)₂ leaves CuO and AgNO₃ leaves silver.

Teacher's Note:
1. Nitrates of ammonium decompose completely into gaseous products.
2. Heating ammonium nitrate is hazardous as it can explode.

Answer: (b) Sn

Explanation:
1. Zinc, silver and mercury all dissolve in concentrated nitric acid to form their nitrates.
2. Tin is oxidised to a white insoluble hydrated stannic oxide (metastannic acid): \( Sn + 4HNO_3 \rightarrow H_2SnO_3 + 4NO_2 + H_2O \).

Teacher's Note:
1. Nitric acid is a strong oxidising agent.
2. Tin is oxidised to a higher oxidation state and forms an oxide, not a nitrate.

Answer: (c) aluminium foil is boiled with sodium hydroxide and sodium nitrate

Explanation:
1. Ammonium nitrate: \( NH_4NO_3 \rightarrow N_2O + 2H_2O \).
2. Zinc with very dilute nitric acid gives N₂O, and hyponitrous acid decomposes as \( H_2N_2O_2 \rightarrow H_2O + N_2O \).
3. Aluminium with NaOH and nitrate reduces the nitrate to ammonia, not N₂O.

Teacher's Note:
1. The product of nitric acid reduction depends on the metal and the concentration of the acid.
2. Nitrate in alkaline medium with Al or Zn gives ammonia.

Answer: (a) ferric ion

Explanation:
1. Chlorine oxidises H₂S to sulphur, Sn²⁺ to Sn⁴⁺ and H₂SO₃ to H₂SO₄.
2. Fe³⁺ is already the higher oxidation state of iron and cannot be oxidised by chlorine under ordinary conditions.

Teacher's Note:
1. An oxidising agent can oxidise only a species which can lose electrons.
2. Fe(II) is oxidised by chlorine to Fe(III), but not Fe(III) further.

Answer: (d) NaNO₃

Explanation:
1. \( 2NaNO_3 \rightarrow 2NaNO_2 + O_2 \): oxygen is colourless.
2. Nitrates of Ag, Pb and Cu give brown nitrogen dioxide along with oxygen.

Teacher's Note:
1. Nitrates of very reactive metals (Na, K) give nitrite and oxygen.
2. Nitrates of heavy metals give oxide or metal, NO₂ and O₂.

Answer: (c) Pb(NO₃)₂

Explanation:
1. Lead nitrate crackles (decrepitates) on heating: \( 2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2 \).
2. NO₂ is brown and PbO is reddish-brown when hot and yellow when cold.

Teacher's Note:
1. Decrepitation means crackling because of loss of water of crystallisation or trapped water.
2. Colour change of the residue is a good test for lead oxide.

Answer: (b) Hg₂Cl₂

Explanation:
1. Mercurous chloride (calomel) reacts with ammonia to give a black mixture of mercury and mercury(II) amino chloride.
2. AgCl dissolves in ammonia, PbCl₂ and HgCl₂ give white precipitates.

Teacher's Note:
1. The name calomel means 'beautiful black' because of this reaction.
2. The black colour is due to finely divided mercury.

Answer: (b) chromyl chloride

Explanation:
1. This is the chromyl chloride test for chlorides.
2. Red-brown vapours of chromyl chloride, CrO₂Cl₂, are formed.

Teacher's Note:
1. The test is not given by fluorides, bromides or iodides.
2. Chromyl chloride vapours turn sodium hydroxide solution yellow.

Answer: (c) Cl₂

Explanation:
1. Scheele prepared chlorine by heating manganese dioxide with muriatic (hydrochloric) acid.
2. He thought it to be an oxide of muriatic acid, so it was called oxymuriatic acid gas, but it is chlorine.

Teacher's Note:
1. Davy later showed that the gas is an element and named it chlorine.
2. Chlorine is a greenish-yellow gas.

Answer: (b) moisture

Explanation:
1. Moist chlorine forms hypochlorous acid: \( Cl_2 + H_2O \rightarrow HCl + HOCl \).
2. HOCl gives nascent oxygen which bleaches coloured matter: \( HOCl \rightarrow HCl + [O] \).

Teacher's Note:
1. Dry chlorine does not bleach dry cloth.
2. Bleaching by chlorine is permanent because it is an oxidation.

Answer: (c) platinum

Explanation:
1. Hot concentrated nitric acid oxidises iodine to iodic acid, sulphur to sulphuric acid and phosphorus to phosphoric acid.
2. Platinum (and gold) is not attacked by nitric acid.

Teacher's Note:
1. Gold and platinum dissolve only in aqua regia.
2. Nitric acid is a strong oxidising agent for non-metals.

Answer: (d) I⁻

Explanation:
1. Reducing power depends on the ease with which an ion loses an electron.
2. The size increases from F⁻ to I⁻, so the electron is held less tightly in I⁻, making it the strongest reducing agent.

Teacher's Note:
1. Iodide is oxidised to iodine even by mild oxidising agents.
2. Fluorine is the strongest oxidising agent among halogens.

Answer: (a) HIO₃

Explanation:
1. Iodic acid has iodine in the +5 state: HIO₃.
2. HIO is hypoiodous acid, HI is hydroiodic acid and HIO₄ is periodic acid.

Teacher's Note:
1. Remember the series for chlorine: HOCl, HClO₂, HClO₃, HClO₄.
2. '-ic' acid with three oxygen atoms is the usual halogen oxoacid pattern.

Answer: (c) a dehydrating agent

Explanation:
1. Nitric acid is a strong acid and a powerful oxidising agent.
2. With sulphuric acid it nitrates benzene.
3. It does not remove water from compounds the way concentrated H₂SO₄ does.

Teacher's Note:
1. Concentrated sulphuric acid is the common dehydrating agent.
2. A mixture of conc. HNO₃ and H₂SO₄ is the nitrating mixture.

Answer: (b) p-block elements

Explanation:
1. The halogens F, Cl, Br, I have the outer configuration \( ns^2np^5 \).
2. Their last electron enters a p orbital, so they belong to the p-block (group 17).

Teacher's Note:
1. Group 17 elements are one electron short of the noble gas configuration.
2. They are highly reactive non-metals.

Answer: (d) hydrogen fluoride

Explanation:
1. Stability of H-X falls as the size of X increases because the bond becomes longer and weaker.
2. The H-F bond is the shortest and strongest, so HF is the most stable (HI the least).

Teacher's Note:
1. Order of stability: HF > HCl > HBr > HI.
2. Bond strength decreases down the group.

Answer: (a) sodamide and hydrogen

Explanation:
1. Sodium replaces hydrogen of ammonia: \( 2Na + 2NH_3 \rightarrow 2NaNH_2 + H_2 \).
2. The products are sodamide and hydrogen.

Teacher's Note:
1. Ammonia behaves as a very weak acid towards a strong metal like sodium.
2. Sodamide is a strong base.

Answer: (b) high temperature

Explanation:
1. By Le Chatelier's principle an increase of temperature favours the endothermic (backward) reaction, so less ammonia is obtained.
2. High pressure favours the forward reaction because 4 moles of gas change to 2 moles.
3. A catalyst changes only the rate, not the equilibrium yield.

Teacher's Note:
1. Low temperature favours yield but is too slow, so about 450 °C is used as a compromise.
2. Iron is the catalyst and molybdenum is the promoter.

Answer: (c) hydrogen chloride

Explanation:
1. Hydrogen chloride is a polar gas that reacts with water to form ions (hydrochloric acid) and dissolves extremely well (fountain experiment).
2. Nitrogen and nitric oxide are only slightly soluble, and chlorine is moderately soluble.

Teacher's Note:
1. About 450 volumes of HCl dissolve in one volume of water at room temperature.
2. The fountain experiment is also done with ammonia.

Answer: (a) AgBr

Explanation:
1. Silver bromide is the most light-sensitive of the common silver halides and is used on photographic plates and films.
2. Light changes it to silver, forming the image.

Teacher's Note:
1. Hypo (sodium thiosulphate) dissolves the unchanged AgBr in the fixing step.
2. AgCl is used in printing paper.

Answer: (b) AgF

Explanation:
1. AgCl, AgBr and AgI are insoluble in water.
2. Silver fluoride is the only silver halide that dissolves readily in water.

Teacher's Note:
1. Fluoride is small and has a high hydration energy.
2. AgNO₃ is also a soluble silver salt.

Answer: (b) a powerful explosive

Explanation:
1. Gun cotton is cellulose nitrate formed by the action of a mixture of concentrated nitric and sulphuric acids on cotton (cellulose).
2. It burns very rapidly and explodes.

Teacher's Note:
1. It is highly nitrated cellulose.
2. Dynamite and TNT are other explosives.