Classwork Series and Exercises {Chemistry – SS2}: Oxides of Nitrogen

Chemistry SS 2 Week 6

Topic: Oxides of Nitrogen

Types of Nitrogen Oxides

Oxides of nitrogen provide a fascinating picture from the point of view of their varied structures and diverse chemical behaviour. They range from N₂O (oxidation state of nitrogen +1) through NO, N₂O₃, NO₂, N₂O₄ to N₂O₅ in which the oxidation state of nitrogen is from +2 to +5.

Structures of the Oxides of Nitrogen

 Various oxides of Nitrogen

Dinitrogen (I) oxide (N₂O)

Dinitrogen (I) oxide (N₂O) is prepared by heating ammonium nitrate. It is collected over hot water.

This gas is known as laughing gas because it causes uncontrollable fits of laughter when inhaled.

Physical Properties of Dinitrogen (I) Oxide

1. Dinitrogen (I) oxide (N₂O) is a colourless gas with a slightly sweet smell
2. The gas is slightly soluble in water
3. It is neutral to moist litmus paper
4. It is slightly denser than air
5. It is liquefied

Chemical Properties

1. Action of Heat: Dinitrogen (I) oxide decomposes into nitrogen and oxygen at 500oC and so, only a bright glowing splint, hot enough relights it

2N₂O_(g) —–> 2N_2(g) + O_2(g)

2. It easily oxidises the following strongly burning elements to their oxides: magnesium, carbon, sulphur and phosphorus

Mg_(s) + N₂O_(g) —-> MgO_(s) + N_2(g)

C_(s) + 2N₂O_(g) —-> CO_2(g) + 2N_2(g)

S_(s) + 2N₂O_(g) —-> SO_2(g) + 2N_2(g)

P_4(s) + 10N₂O_(g) —-> P₄O_10(s) + 10N_2(g)

Nitrogen (II) Oxide (NO)

Laboratory Preparation of Nitrogen (II) Oxide

In laboratory preparation, 50% trioxonitrate (V) acid is reacted with copper turnings to liberate nitrogen (II) oxide that is collected by downward delivery.

3Cu(s) + 8HNO₃(aq) → 3Cu(NO₃)₂(aq) + 4H₂O(l) + 2NO(g)

Also, Nitrogen (II) oxide is prepared by the catalytic oxidation of ammonia at 1100 K in the presence of platinum.

Physical Properties

• It is a colourless and poisonous gas
• It is neutral to litmus
• It is slightly denser than air
• It is almost insoluble in water

Chemical Properties

• Reaction with oxygen: When nitrogen (II) oxide is exposed to air, it reacts immediately with the oxygen to form reddish brown fumes of nitrogen (IV) oxide

2NO_(g) + O_2(g) —-> 2NO_2(g)

• Reaction with Iron (II) Tertraoxosulphate (VI): Nitrogen (II) oxide dissolves readily in iron (II) tetraoxosulphate (VI) solution forming a dark brown or black compound which decomposes again when heated

FeSO_4(aq) + NO_(g) <—-> FeSO₄.NO_(aq)

Note: These two above reactions are two main tests for nitrogen (II) oxide

• Reaction with Hot Metals: Nitrogen (II) oxide reacts with hot metals which reduce it to nitrogen e.g.

2Cu_(s) + 2NO_(g) —–> 2CuO_(aq) + N_2(g)

• As a Reducing Agent: It is a weak reducing agent. It reacts with acidified potassium tetraoxomanganate (VII) very slowly and decolorizes it.

3MnO^–_4(aq) + 4H⁺_(aq) + 5NO_(g) —-> 3Mn²⁺ + 5NO^–_3(aq) + 2H₂O_(l)

• Action of Heat: With high temperature, about 1000oC, nitrogen (II) oxide decomposes into equal volumes of nitrogen and oxygen

2NO_(g) —-> N_2(g) + O_2(g)

This gas also supports the combustion of strongly burning phosphorus or magnesium

P_4(g) + 10NO_(g) —-> P₄O_10(s) + 5N_2(g)

2Mg_(s) + 2NO_(g) —-> 2MgO_(s) + N_2(g)

Nitrogen (IV) Oxide

Laboratory Preparation of Nitrogen(IV) Oxide

Nitrogen (IV) oxide (nitrogen dioxide) is prepared in the laboratory by heating lead (II) trioxonitrate (V), Pb(NO₃)₂. The lead (II) trioxonitrate (V) crackles, melts and decomposes when heated strongly to produce nitrogen (IV) oxide, NO₂, lead(II) oxide, PbO, and oxygen, O₂.

2Pb(NO₃)₂(s) → 2PbO(s) + O₂(g) +4NO₂(g)

The lead (II) trioxonitrate (V) is heated in an inclined test tube held in place by a retort stand. A delivery tube is used to deliver the gaseous products (nitrogen (IV) oxide and oxygen) formed in the test tube into a U-tube immersed in a beaker containing a freezing mixture.

The nitrogen (IV) oxide is liquefied in the U-tube as a greenish liquid, while the oxygen travels out of the tube through a delivery tube connected to the other end of the U-tube.

The nitrogen (IV) oxide formed in the U-tube may be yellow in colour if it is pure nitrogen, while the lead (II) oxide residue in the test tube is reddish-brown when hot and yellow on cooling.
Although nitrogen (IV) oxide can be prepared by the thermal decomposition of the trioxonitrate (V) of other heavy metals, lead(II) trioxonitrate (V) is preferred because inhibitory water of crystallization is absent in its crystals.

It exists in equilibrium with N₂O₄ as:

Above 415 K it contains mainly NO₂ and at 250 K it contains mainly N₂O₄. By heating copper turnings with concentrated HNO₃

Physical Properties

• It is a reddish brown gas
• It has a strong pungent smell and acidic taste
• It is very poisonous and causes septic pneumonia
• It is much heavier than air and is very soluble in water producing a pale blue solution which is strongly acidic to litmus

Chemical Properties

• Action of Heat: At temperature below its boiling point a high proportion of nitrogen (IV) oxide exists in the form of a colourless dimer N2O4. As the temperature is raised, the dimer breaks up and the brown colour darkens. The temperature is about 150oC

N₂O_4(g) <—-> 2NO_2(g)

• Reaction with Water: Nitrogen (IV) oxide dissolves in water to give a faint blue solution containing a mixture of dioxonitrate (III) and trioxonitrate (V) acids

2NO_2(g) + H₂O_(l) —-> HNO_2(aq) + HNO_3(aq)

In excess oxygen, only trioxonitrate (V) acid is obtained, as the dioxonitrate (III) acid is oxidized to trioxonitrate (V) acid

2HNO_2(aq) + O_2(g) —-> 2HNO_3(aq)

• Reaction with Alkalis: Since the gas is a mixed acid anhydride, it gives the corresponding trioxonitrate (V) and dioxonitrate (III) salt

2NaOH_(aq) + 2NO_2(g) —-> NaNO_3(aq) + NaNO_2(aq) + H₂O_(l)

• Supporter of Combustion: Magnesium and phosphorus burn more brightly in nitrogen (IV) oxide thus showing that it supports combustion. This only happens if the material is hot enough to decompose the gas so that oxygen is released

2NO_2(aq) + 4Mg_(s) —-> 4MgO_(s) + N_2(g)

10NO_2(g) + 2P_4(s) —–> 4P₂O_5(g) + 5N_2(g)

Uses of Oxides of Nitrogen

Nitric oxide and nitrogen dioxide are important in the manufacture of nitric acid and nitrate fertilizers. In recent years, N₂O₄ has been used as an oxidizer for rocket fuels in missiles and space vehicles.