Nitrogen

Occurrence of Nitrogen

Nitrogen occurs both as a free element and in combined form:

Free Nitrogen

Most nitrogen exists as nitrogen gas (N₂) in the atmosphere. When you breathe in air, about 78% of what enters your lungs is nitrogen. The remaining 22% includes oxygen (21%), carbon dioxide, water vapor, and noble gases.

Unlike oxygen which you need for respiration, nitrogen passes through your lungs unchanged. Your body cannot use nitrogen gas directly, even though you breathe it constantly.

Combined Nitrogen

Nitrogen combines with other elements to form important compounds found in:

• Proteins in living things: All plants and animals need nitrogen to make proteins, DNA, and other vital molecules
• Soil: Nitrates (NO₃⁻) and ammonium (NH₄⁺) compounds provide nitrogen for plant growth
• Fertilizers: Farmers buy urea, ammonium sulfate, and NPK fertilizers containing nitrogen
• Explosives: TNT, dynamite, and gunpowder contain nitrogen compounds
• Minerals: Sodium nitrate (Chile saltpeter) and potassium nitrate (saltpeter) occur naturally

Laboratory Preparation of Nitrogen

You can prepare nitrogen in the school laboratory using ammonium nitrite (NH₄NO₂). However, solid ammonium nitrite is unstable and dangerous to heat directly because it can explode.

Safe Method Using Two Solutions

Instead of heating pure ammonium nitrite, we mix two solutions that react to form it:

Solution 1: Sodium nitrite (NaNO₂)
Solution 2: Ammonium chloride (NH₄Cl)

When heated together, these solutions react:

NaNO₂(aq) + NH₄Cl(aq) → NH₄NO₂(aq) + NaCl(aq)

The ammonium nitrite formed immediately decomposes when warmed:

NH₄NO₂(aq) → N₂(g) + 2H₂O(l)

Laboratory Setup

1. Mix sodium nitrite solution and ammonium chloride solution in a round-bottom flask
2. Gently heat the mixture using a water bath (not direct flame)
3. Collect the nitrogen gas produced over water using a gas jar
4. The gas bubbles through water and displaces it, filling the jar

Safety note: Never heat solid ammonium nitrite directly. Always use the two-solution method. Wear safety goggles and work in a well-ventilated lab.

Impurities in Laboratory Nitrogen

The nitrogen collected may contain small amounts of:

• Ammonia (NH₃) – removed by passing through dilute sulfuric acid
• Water vapor – removed by passing through calcium chloride
• Oxygen – removed by passing over heated copper

Alternative Laboratory Method: From Air

Since air is 78% nitrogen, you can obtain nitrogen by removing oxygen and other gases from air:

Method:

1. Pass air over heated copper turnings at 400°C
2. Copper reacts with oxygen, removing it: 2Cu + O₂ → 2CuO
3. The remaining gas is mostly nitrogen (with small amounts of noble gases)
4. Remove carbon dioxide by passing through sodium hydroxide solution
5. Dry the nitrogen by passing through calcium chloride

This method gives nitrogen containing about 1% argon and other noble gases, which don’t affect most experiments.

Industrial Production of Nitrogen

Industries need large amounts of pure nitrogen for making fertilizers, preserving food, and manufacturing chemicals. They obtain nitrogen by fractional distillation of liquid air.

Fractional Distillation Process

Step 1: Purify the air

• Filter out dust and solid particles
• Remove carbon dioxide by passing through sodium hydroxide
• Remove water vapor by passing through concentrated sulfuric acid

Step 2: Liquefy the air

• Compress the clean air to about 200 atmospheres pressure
• Cool the compressed air using expansion chambers
• Air becomes liquid at -200°C

Step 3: Separate by distillation

• Slowly warm the liquid air in a fractionating column
• Different gases boil at different temperatures
• Nitrogen boils at -196°C and comes off first
• Argon boils at -186°C and comes off next
• Oxygen boils at -183°C and remains as liquid longer

This process is similar to separating petrol from crude oil at a refinery, but much colder. Companies in Nigeria import liquid nitrogen produced this way for industrial use.

Why Nitrogen is Unreactive

Nitrogen gas exists as N₂ molecules. The two nitrogen atoms share three pairs of electrons, forming a triple bond (N≡N). This triple bond is one of the strongest bonds in chemistry.

Breaking this bond requires a lot of energy (945 kJ/mol). That’s why nitrogen doesn’t easily react with other substances at room temperature. You can mix nitrogen with oxygen in air for years, and they won’t react unless lightning strikes or you heat them above 3000°C.

This unreactivity makes nitrogen useful for:

• Preserving food in packages (prevents spoilage)
• Filling light bulbs (prevents the filament from burning)
• Providing inert atmosphere in chemical plants
• Inflating aircraft tires (doesn’t react with hot brakes)

The Nitrogen Cycle in Nature

Although nitrogen gas is unreactive, living things need nitrogen in combined forms. Nature cycles nitrogen through these steps:

1. Nitrogen fixation: Lightning and soil bacteria (Rhizobium) convert N₂ into nitrates
2. Absorption: Plant roots absorb nitrates from soil and make proteins
3. Consumption: Animals eat plants and use plant proteins to build animal proteins
4. Decomposition: When plants and animals die, bacteria break down proteins, releasing ammonia
5. Nitrification: Other bacteria convert ammonia to nitrites, then nitrates
6. Denitrification: Some bacteria convert nitrates back to N₂ gas, returning it to air

This cycle keeps nitrogen moving between air, soil, plants, and animals. Without it, soil would lose all nitrogen and plants couldn’t grow.

Comparison Table: Laboratory vs Industrial Nitrogen Production

Common Exam Mistakes

WAEC Chief Examiners report these frequent errors about nitrogen:

1. Wrong percentage: Writing that nitrogen is 21% of air (that’s oxygen) or 88% of air. The correct value is 78% by volume.
2. Dangerous method: Describing direct heating of solid ammonium nitrite. Examiners expect you to mention using NaNO₂ and NH₄Cl solutions instead for safety.
3. Incomplete equations: Writing the equation without balancing or forgetting the state symbols. Always show (s), (l), (g), or (aq).
4. Confusing distillation methods: Mixing up fractional distillation of air (for nitrogen) with fractional distillation of crude oil. These are different processes at different temperatures.
5. Wrong boiling points: Stating nitrogen boils at -183°C (that’s oxygen). Nitrogen boils at -196°C.
6. Confusing “occurrence” with “uses”: When asked where nitrogen occurs, students list its uses instead. Occurrence means where it is found naturally.
7. Triple bond notation: Writing N=N (double bond) instead of N≡N (triple bond). The triple bond is crucial for explaining nitrogen’s unreactivity.
8. Poor command words understanding: Not knowing the difference between “state” (list briefly), “describe” (give detailed account), and “explain” (give reasons why).

Practice Questions

Multiple Choice Questions

1. What percentage of the atmosphere is nitrogen?
a) 21%
b) 65%
c) 78% ✓
d) 88%

2. In the laboratory preparation of nitrogen, which TWO solutions are mixed?
a) Sodium chloride and ammonium nitrite
b) Sodium nitrite and ammonium chloride ✓
c) Sodium hydroxide and ammonium sulfate
d) Sodium carbonate and ammonium nitrate

3. The boiling point of nitrogen is:
a) -210°C
b) -196°C ✓
c) -183°C
d) -100°C

4. Why is nitrogen gas unreactive at room temperature?
a) It has a weak single bond
b) It exists as single atoms
c) It has a very strong triple bond ✓
d) It is highly electronegative

Essay Questions

1. (a) State the percentage composition of nitrogen in the atmosphere. (1 mark)
(b) Describe how nitrogen is prepared in the laboratory from two solutions. Include relevant equations. (6 marks)
(c) Give TWO reasons why the laboratory method described in (b) is safer than directly heating solid ammonium nitrite. (3 marks)

Examiner’s Tip: For part (b), describe the mixing of NaNO₂ and NH₄Cl solutions, write both equations, and mention gentle heating. For 6 marks, provide detailed steps.

2. (a) Explain the industrial production of nitrogen by fractional distillation of liquid air. (8 marks)
(b) State TWO uses of the unreactive nature of nitrogen. (2 marks)

Examiner’s Tip: “Explain” means give detailed account with reasons. Cover air purification, liquefaction (pressure and cooling), and separation by different boiling points. Include at least one temperature value.

3. (a) Draw the structural formula of nitrogen molecule showing all bonding electrons. (2 marks)
(b) Explain why nitrogen is unreactive at room temperature. (3 marks)
(c) Name THREE important compounds of nitrogen found in nature. (3 marks)
(d) Apart from free nitrogen in air, state TWO places where combined nitrogen occurs. (2 marks)

Examiner’s Tip: For part (a), show N≡N with three pairs of dots or lines between the atoms. For part (b), mention the triple bond and high bond energy (945 kJ/mol).

Memory Aids

Remember nitrogen’s percentage: “Lucky 78”
In WAEC exams, you’re “lucky” if you remember nitrogen is 78% of air (the most commonly tested percentage).

Mnemonic for Lab Preparation: “SAND”

• Sodium nitrite
• Ammonium chloride
• Nitrogen produced
• Don’t heat pure ammonium nitrite (dangerous!)

Remember the equations in order:
Step 1: Mix the solutions (NaNO₂ + NH₄Cl → NH₄NO₂ + NaCl)
Step 2: Heat decomposes it (NH₄NO₂ → N₂ + 2H₂O)

Boiling points from coldest to warmest: “Now All Oxygen”

• Nitrogen: -196°C (coldest, boils first)
• Argon: -186°C (middle)
• Oxygen: -183°C (warmest, boils last)

Triple bond memory: Nitrogen is “NOT EASY” to break (N≡N triple bond = NOT EASY = unreactive)

Related Topics

• Physical Properties of Nitrogen – Learn about nitrogen’s characteristics and behavior
• Chemical Properties of Nitrogen – Understand how nitrogen reacts with other elements
• Uses of Nitrogen – Discover practical applications of nitrogen in industry and daily life
• Oxides of Nitrogen – Study nitrogen compounds like NO, NO₂, N₂O
• Oxygen – Compare nitrogen with the second most abundant gas in air