Study of Compounds: Nitric Acid — ICSE Class 10 Chemistry Important Questions

Answer: (c) Platinum (platinum-rhodium gauze).

Ammonia is catalytically oxidised over hot platinum: $4NH_3+5O_2\xrightarrow{Pt}4NO+6H_2O$4NH₃+5O₂→ (Pt)4NO+6H₂O.

Answer: (b) Decomposes in sunlight turning yellow.

$4HNO_3\xrightarrow{light}4NO_2+O_2+2H_2O$4HNO₃→ (light)4NO₂+O₂+2H₂O. The dissolved brown $NO_2$NO₂ gives the acid a yellow colour, so it is kept in dark bottles.

Answer: (b) Reddish-brown nitrogen dioxide ($NO_2$NO₂).

$Cu+4HNO_3(conc.)\rightarrow Cu(NO_3)_2+2NO_2\uparrow+2H_2O$Cu+4HNO₃(conc.)→ Cu(NO₃)₂+2NO₂↑+2H₂O.

Answer: (c) Hydrogen.

Only very dilute $HNO_3$HNO₃ with the active metals magnesium and manganese behaves as a normal acid: $Mg+2HNO_3(very\ dil.)\rightarrow Mg(NO_3)_2+H_2\uparrow$Mg+2HNO₃(very dil.)→ Mg(NO₃)₂+H₂↑.

Answer: (a) An all-glass (quickfit) apparatus is used because hot $HNO_3$HNO₃ vapour attacks and corrodes rubber and cork; R correctly explains A.

$KNO_3+H_2SO_4(conc.)\xrightarrow{\Delta}KHSO_4+HNO_3$KNO₃+H₂SO₄(conc.)→ (Δ)KHSO₄+HNO₃

The temperature is kept below $200^\circ C$200° C because at higher temperatures nitric acid decomposes ($4HNO_3\rightarrow 4NO_2+O_2+2H_2O$4HNO₃→ 4NO₂+O₂+2H₂O), and hot $KHSO_4$KHSO₄ could form $K_2SO_4$K₂SO₄, which would solidify and clog the apparatus.

(i) Add a few copper turnings and warm. Reddish-brown fumes of nitrogen dioxide are given off and the solution turns blue-green: $Cu+4HNO_3\rightarrow Cu(NO_3)_2+2NO_2\uparrow+2H_2O$Cu+4HNO₃→ Cu(NO₃)₂+2NO₂↑+2H₂O.

(ii) $NaHCO_3+HNO_3\rightarrow NaNO_3+H_2O+CO_2\uparrow$NaHCO₃+HNO₃→ NaNO₃+H₂O+CO₂↑ (brisk effervescence).

Test for nitrate ion ($NO_3^-$NO₃^-):

1. To the salt solution add freshly prepared iron(II) sulphate ($FeSO_4$FeSO₄) solution and mix.
2. Then carefully add concentrated sulphuric acid down the side of the tilted test tube so that it forms a separate lower layer.

Observation: A brown ring appears at the junction of the two liquids, confirming a nitrate. The brown ring is the complex $[Fe(H_2O)_5NO]SO_4$[Fe(H₂O)₅NO]SO₄ formed as nitric acid liberated by $H_2SO_4$H₂SO₄ oxidises $Fe^{2+}$Fe^2+ and the resulting $NO$NO combines with $FeSO_4$FeSO₄.

(i) Acidic reaction with a carbonate (dilute acid):
$Na_2CO_3+2HNO_3\rightarrow 2NaNO_3+H_2O+CO_2\uparrow$Na₂CO₃+2HNO₃→ 2NaNO₃+H₂O+CO₂↑

Here it behaves as a typical acid, liberating carbon dioxide.

(ii) Oxidising action on carbon (hot concentrated acid):
$C+4HNO_3(conc.)\rightarrow CO_2\uparrow+4NO_2\uparrow+2H_2O$C+4HNO₃(conc.)→ CO₂↑+4NO₂↑+2H₂O

Carbon is oxidised to $CO_2$CO₂ while nitric acid is reduced to brown $NO_2$NO₂, showing its oxidising nature.

Nitric acid is a powerful oxidising agent. Any hydrogen that would normally be liberated is immediately oxidised to water by the acid, while the nitric acid itself is reduced to oxides of nitrogen ($NO$NO or $NO_2$NO₂) rather than releasing $H_2$H₂.

With dilute nitric acid, copper gives colourless nitric oxide (which turns brown in air):
$3Cu+8HNO_3(dil.)\rightarrow 3Cu(NO_3)_2+2NO\uparrow+4H_2O$3Cu+8HNO₃(dil.)→ 3Cu(NO₃)₂+2NO↑+4H₂O

No hydrogen is evolved. (Only very dilute acid with magnesium or manganese is the exception, giving $H_2$H₂.)

In the Ostwald process nitric acid is manufactured by the catalytic oxidation of ammonia.

Stage 1 - catalytic oxidation of ammonia (platinum-rhodium gauze, $\approx800^\circ C$≈800° C):
$4NH_3+5O_2\xrightarrow{Pt}4NO+6H_2O$4NH₃+5O₂→ (Pt)4NO+6H₂O

Stage 2 - oxidation of nitric oxide (on cooling, in air):
$2NO+O_2\rightarrow 2NO_2$2NO+O₂→ 2NO₂

Stage 3 - absorption in water (with excess air):
$4NO_2+O_2+2H_2O\rightarrow 4HNO_3$4NO₂+O₂+2H₂O→ 4HNO₃

The dilute acid obtained is concentrated by distillation to about $68\%$68%.

(a)(i) Copper with concentrated acid:
$Cu+4HNO_3(conc.)\rightarrow Cu(NO_3)_2+2NO_2\uparrow+2H_2O$Cu+4HNO₃(conc.)→ Cu(NO₃)₂+2NO₂↑+2H₂O

(a)(ii) Sulphur with hot concentrated acid (oxidised to sulphuric acid):
$S+6HNO_3(conc.)\rightarrow H_2SO_4+6NO_2\uparrow+2H_2O$S+6HNO₃(conc.)→ H₂SO₄+6NO₂↑+2H₂O

(b) Uses of nitric acid:

1. Manufacture of nitrogenous fertilisers (e.g. ammonium nitrate, calcium nitrate).
2. Manufacture of explosives such as TNT, nitroglycerine and gun-cotton.

(c) Passivity of iron: Concentrated nitric acid oxidises the surface of iron, forming a thin, adherent, unreactive layer of iron oxide ($Fe_3O_4$Fe₃O₄) that coats the metal and prevents further reaction. The iron is said to become passive.

(i) The brown ring test confirms the nitrate ion ($NO_3^-$NO₃^-).

(ii) Action of heat on lead nitrate:
$2Pb(NO_3)_2\xrightarrow{\Delta}2PbO+4NO_2\uparrow+O_2\uparrow$2Pb(NO₃)₂→ (Δ)2PbO+4NO₂↑+O₂↑

(iii) The brown gas is nitrogen dioxide ($NO_2$NO₂); the gas that relights a glowing splint is oxygen ($O_2$O₂).

(iv) It is a thermal decomposition reaction. (The yellow residue is lead(II) oxide, $PbO$PbO.)