Unit 19: Unit 19: Nitrogen and Sulphur — Short Questions

Nitrogen is obtained by the fractional distillation of air
(a) Fractional Distillation
First of all, carbon dioxide present in air is removed.

• The air is subjected to about 200 atmospheric pressure.
• This compressed air is then cooled and allowed to pass through a spiral jet and the air suffers sudden expansion. This process of compression and expansion is repeated again and again till the air is liquified. (b) Distillation of Liquid air The liquid air is then fractionally distilled. Since the boiling point of nitrogen is less than oxygen, nitrogen evaporates first leaving behind the oxygen. The separated nitrogen is reliquified and stored.

By heating methane in limited oxygen: Hydrogen is produced by heating methane in the limited amount of oxygen that is not enough to completely oxidize methane to carbon dioxide and water. With less oxygen available, the reaction products contain primarily hydrogen and carbon monoxide and a relatively small amount of carbon dioxide. The carbon monoxide obtained is then reacted with water to form carbon dioxide and more hydrogen gas.
CH₄₍g₎ + H₂O₍g₎ ⟶ CO₍g₎ + 3H₂₍g₎
CO₍g₎ + H₂O₍g₎ ⟶ CO₂₍g₎ + H₂₍g₎

The clean and dry gases (SO₂ and O₂) are passed over vanadium (V) oxide catalyst at 450°C and 2-3 atmospheric pressure through a contact chamber. Although the reaction is reversible yet under these conditions 98% SO₂ gas is converted to SO₃.
2SO₂₍g₎ + O₂₍g₎ ⟶ 2SO₃₍g₎ (at 450°C, 2-3 atmosphere, with V₂O₅ catalyst)

Difference in behavior between CO and CO₂ comes from their chemical bonding and ability to react with water
Carbon monoxide – Neutral oxide
CO has C ≡ Oxygen structure having triple bond between carbon and oxygen. It has less tendency to react because of strong triple.
CO₂ – Acidic oxide

• Its structure is line as.
• O=C =O (Linear molecule)
• It reacts with water due to its structure to form carbonic acid CO₂ + H₂O ⟶ H₂CO₃

Magnesium and calcium differ with each other towards their reactions with water due to their position in the reactivity series. Calcium is present above magnesium in reactivity series so it is more reactive with water then magnesium.
Reaction of Magnesium with water

• Magnesium reacts very slowly because of oxide layer on its surface. in cold state.
• Mg reacts to form magnesium oxide and hydrogen gas with steam. Mg₍s₎ + H₂O₍g₎ (steam) ⟶ MgO₍s₎ + H₂₍g₎ ↑ Reaction of calcium with water Calcium reacts readily to form calcium hydroxide in cold state. Ca₍s₎ + 2H₂O₍f₎ ⟶ Ca(OH)₂₍aq₎ + H₂₍g₎ ↑

Reactivities of metals
Base on their reactions with water and acids, the metals can be arranged in decreasing order of their reactivity. Such an arrangement is called reactivity series of metals.
According to this reactivity series, metals above the series are more reactive metals and metals below the hydrogen in series are less reactive metals.
Reactivity series: The order of reactivity of some of the metals.
K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu

Nitrogen oxides (NO and NO₂) are primary pollutants but in the atmosphere they react form secondary pollutants.
Examples
(i) Nitric acid
2NO + H₂O ⟶ HNO₃ + HNO₂
(ii) Sulphuric acid oxides of nitrogen catalyse the production of sulphuric acid which is also a major secondary pollutant.
2SO₂ + O₂ ⟶ 2SO₃ (Oxides of Nitrogen catalyst)
SO₃ + H₂O ⟶ H₂SO₃
(iii)Photochemical – smog components
NO₂ reacts with volatile organic components (VOCs) and sunlight to form ozone O₃ and peroxyacetyl nitrates (PANs). Both of these are components of Photochemical smog.

Ammonia is produced industrially by German Chemist F.J Haber Therefore, the process is called Haber process after the name of the discoverers.
Reaction: In this process a mixture of nitrogen and hydrogen in the ratio of 1:3 by volume is heated at 400 – 500°C under 200 atmospheric pressure and in the presence of catalyst Fe / Al₂O₃ to give ammonia.
N₂₍g₎ + 3H₂₍g₎ ⟶ 2NH₃₍g₎ (at 200atm, 400-500°C, Fe/Al₂O₃ catalyst)

The mixture from the reaction contains 35% ammonia by volume.
Liquefaction of ammonia
This mixture is cooled by the refrigeration coil where ammonia gas changes to liquid ammonia at −33.4°C and is removed from mixture.
The other components of the mixture, hydrogen and nitrogen gases are then recycle back into the reaction chamber.

[Diagram shown with boxes for Nitrogen, Hydrogen, Nitrogen + Hydrogen (1:3 ratio), 400–500°C 200atm Iron catalyst, Unreacted gases recycled, Gases are cooled and ammonia turns to liquid, Liquid Ammonia]

Source of nitrogen One of the raw materials used for the production of ammonia is nitrogen gas. This gas is abundantly found 78% by volume in air so, it is obtained by the fractional distillation of air. After removing CO₂: the compressed air (200 atm) is allowed to pass through a spiral jet and expansion of sudden nature decreases its temperature. This process is repeated again and again till air is liquified. From this liquid air nitrogen is collected at −196°C by fractional distillation.

About 13% of total nitrogen fixation in the environment is contributed by the Haber process. This process utilize nitrogen to produce ammonia gas.
N₂ + 3H₂ ⟶ 2NH₃ (at Fe/Al₂O₃, 200atm, 400-500°C)

Role of catalyst in Haber process: The catalyst used in Haber process is Fe / Al₂O₃.
It increases the production of more ammonia without affecting the position of equilibrium. The catalyst allows the process to occur more efficiently although it is a reversible reaction, and ammonia is produced with lower number of moles than the reactants.
N₂ + 3H₂ ⟶ 2NH₃ (at Fe/Al₂O₃, 400-500°C, 200atm)

Ammonia liquefaction The mixture obtained from catalytic chamber contains 35% of ammonia
N₂ + 3H₂ ⟶ 2NH₃ (at Fe/Al₂O₃, 400-500°C, 200atm)
This mixture is cooled by the refrigeration coils where ammonia gas changes to liquid at −33.4°C and is removed from the mixture.

King of chemicals Sulphuric acid is called the "king of chemicals" because it is used in almost all industries including fertilizers, plastics, explosive, paints, textiles, leather tanning and metallurgy.
Its demand is often considered an indicator of a country's industrial strength.
It is involved directly or indirectly in the manufacturing of many essential products.

The key chemical reaction in contact process is the catalytic oxidation of SO₂ to SO₃ in the presence of oxygen.
2SO₂ + O₂ ⟶ 2SO₃ (at V₂O₅, 450°C, 2-3 atm)
This reaction is exothermic and reversible and require optimum conditions for maximum yield.

Production of SO₂ Sulphur dioxide gas is produced either by burning elemental sulphur in air or roasting of sulphur ore, iron pyrite in excess of air.
S₍s₎ + O₂₍g₎ ⟶ SO₂₍g₎
4FeS₂₍s₎ + 11O₂₍g₎ ⟶ 2Fe₂O₃₍s₎ + 8SO₂₍g₎

Purification of SO₂ gas Sulphur dioxide produced is contact process by burning sulphur or iron Pyrite in excess of oxygen is then passed through purifying chamber. For this purpose, coke filter is used to remove traces of suspended particulate matter from the SO₂ gas. This purification step is essential to prevent catalyst poisoning.

Drying of moist gases in contact process: The moist gases (SO₂ and O₂) in contact process are dried by passing through a drying tower in which cove H₂SO₄ is being sprayed which acts as a dehydrating agent.

Sulphuric trioxide is absorbed in concentrated sulphuric acid (98%) in the absorption tower to form oleum (H₂S₂O₇).
H₂SO₄ + SO₃ ⟶ H₂S₂O₇ (Oleum)
Oleum is a fuming liquid and safer way to store and transport SO₃.

Concentrated sulphuric acid is produced by carefully adding water to oleum. In this way desired concentration of sulphuric acid is produced.
H₂S₂O₇ + H₂O ⟶ 2H₂SO₄

Oxides Binary compounds of elements with oxygen are called oxides. Oxygen shows an oxidation state of –2 in these oxides.
Examples: CaO, CO₂, SO₃, Na₂O, CuO and Al₂O₃
They can be classified into

• Acidic oxide, Basic oxide, Neutral oxide. Amphoteric oxide

Acidic SO₂, CO₂
Basic: Na₂O, CaO, CuO
Amphoteric: ZnO, Al₂O₃
Neutral: NO, CO

Basic oxides When metals react with oxygen at high temperature basic oxides are produced.
Examples: Na₂O, CaO etc.
4Na + O₂ ⟶ 2Na₂O
2Ca + O₂ ⟶ 2CaO
Basic oxides when combine with water alkalis (bases) are produced.
Na₂O + H₂O ⟶ 2NaOH

Amphoteric oxides The oxides which can react with acids as well as with bases, displaying both acidic and basic properties are called amphoteric oxide.
Examples: Al₂O₃, ZnO

Water is an amphoteric substance. It can act as both an acid and a base, depending on the other substance it reacts with.
Example
H₂O + HCl ⟶ H₃O⁺ + Cl⁻
(acting as base)
H₂O + NH₃ ⟶ NH₄⁺ + OH⁻
(acting as an acid)

Some general properties of metals are:
(i) Mostly metals occur in earth crust in the form of their oxides, hydroxides, carbonates and sulphides.
(ii) Metals have a tendency to lose electrons and form cations.
(iii)Usually they form ionic bonds with other elements.
(iv) They are good conductors of heat and electricity.

Most of the elements present in the first and second groups of the periodic table react vigorously with cold water producing their respective hydroxides and hydrogen gas.
Examples:
2Na + 2H₂O ⟶ 2NaOH + H₂
2K + 2H₂O ⟶ 2KOH + H₂

Reactive metals like Li, Na, K and Ca react violently with steam and the reaction can be dangerous because these reactions are highly exothermic.

Be and Al react with steam at high temperatures (around 700°C to give their respective oxides and hydrogen).
Be + H₂O ⟶ BeO + H₂
Mg, iron and Zinc have a moderate reaction with steam producing their respective oxides and hydrogen gas.
Mg + H₂O ⟶ MgO + H₂

Fame test Metals react with oxygen to give metal oxide. They can be identified by the characteristics flame of their oxide.
Examples:
4Na + O₂ ⟶ 2Na₂O
2Na + O₂ ⟶ Na₂O₂
Sodium burns in air to produce yellow flame.
Mg → Produces Intense white flame
Ca → Produces White flame with a tinge of red colour
Sr → Produces crimson red flame
Ba → Produces a pale green colour of flame.
This is called flame test.

Reactivity Series Based on their reactions with water and acids, the metals can be arranged in decreasing order of their reactivities. Such an arrangement is called reactivity series. Highly reactive>moderately reactive > less > unreactive.

The metals at the top of the series are powerful reducing agent since they are easily oxidized. However, the reducing ability of metals decrease going down the series.

Primary pollutants in the atmosphere are those harmful substance which are directly emitted into the atmosphere from natural or human – made sources.
Examples: Oxides of nitrogen and sulphur etc.
It also includes various hydrocarbons.
Secondary pollutants
Pollutants which are not emitted directly but are produced when primary pollutants react with each other
Examples: O₃, PAN, H₂SO₄ sulphuric acid and nitric acid.

Sources of oxides of nitrogen NOₓ: Oxides of nitrogen NOₓ are produced.

• By electrical discharges during lightning
• By combustion of fossil fuels
• By agricultural activities and use of fertilizers
• By use of automobiles

Ultraviolet radiations present in sunlight interacts with oxides of nitrogen and VOCs through a complex series of chemical reactions to produce secondary pollutants like ozone, aldehydes and proxy acetyl nitrates (PAN).

Oxides of nitrogen are responsible for the formation of pollutant called acid rain. Especially NOₓ reacts with water and other chemicals present in air, to produce vapours of nitric acid and nitrous acid.
2NO₂ + H₂O ⟶ HNO₃ + HNO₂
These acidic vapours then mix with water vapours present in air and fall to earth as acid rain.

See Q.1 from theory.

Oxides of nitrogen (NOₓ) act as catalyst to convert, a primary pollutant, sulphur dioxide present in air, to sulphuric acid which is a major component of acid rain.
2SO₂₍g₎ + O₂₍g₎ ⟶ 2SO₃₍g₎ (Oxides of nitrogen catalyst)
SO₃₍g₎ + H₂O₍g₎ ⟶ H₂SO₄₍g₎

Burning of fossil fuels in car engines: The formation of nitrogen oxides (NO and NO₂) collectively called NOₓ in car engines are produced due to the combustion reaction of air – fuel mixture at high temperature.
Source of nitrogen oxides: Air contains 78% nitrogen (N₂) and 21% oxygen. During combustion in car engines, both these gases combine to give nitric oxide at high temperature is cylinders.
N₂ + O₂ ⟶ 2NO (high temp)
This nitric oxides (NO) can then be oxidized in air to nitrogen dioxide.
2NO + O₂ ⟶ 2NO₂
These are the primary pollutants contributing to acid rain, smog and respiratory problems.

Metals present at the top of the reactivity series like Li, Na, K etc. have more tendency to lose electrons and form cations. They have low standard reduction potential as it is increasing from top to bottom, making these top metals more reactive. Metals at the bottom of reactivity series like Cu, Ag and Au etc are least reactive or inert due to high standard reduction potential. They have least tendency to lose electron and form cation. So, that is why they are less reactive metals.

Sulphur in fossil fuels SO₂ (sulphur dioxide) is produced when fossil fuels containing sulphur are burnt. Coal, petrol diesel and natural gas often contain Sulphur impurities. When Sulphur in the fuel burns in oxygen (air) it forms sulphur dioxide.
S + O₂ ⟶ SO₂
The reaction is highly exothermic
In this way SO₂ which is a primary pollutant is produced form fossil fuels.

See Q.2 from theory

See Q.5 from theory

See Q.6 from theory

See Q.9 from theory

See Q.12 from theory