Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Nitrogen
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Nitrogen
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 2
Theme: Chemistry And Environment
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state the generalproperties of group VA elements explain the laboratorypreparation of nitrogen explain the industrialpreparation of nitrogen from liquidair list the propertiesof nitrogen out line the uses of nitrogen list the oxides of nitrogen explain the nitrogen cycle. Explain the Huber process for the preparation of ammonia state the uses of ammonia
Group VA elements, also known as the pnictogens, include Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi).
Electronic Configuration: All elements in Group VA have 5 valence electrons, with a general outer electronic configuration of ns2np
3. This configuration explains their common oxidation states and chemical behaviour.
Metallic Character: A distinct trend exists down the group. Nitrogen and Phosphorus are non-metals. Arsenic is a metalloid. Antimony is a metalloid, but often exhibits more metallic properties. Bismuth is a metal. This transition from non-metal to metal down the group is due to increasing atomic size and decreasing ionization energy.
Physical State: Nitrogen is a diatomic gas (N2) at room temperature due to weak intermolecular forces. Phosphorus exists as various allotropes (e.g., white P4, red P, black P), typically solid. Arsenic, Antimony, and Bismuth are solid metals/metalloids.
Oxidation States: Group VA elements typically exhibit oxidation states from -3 to +
5. Nitrogen, due to its small size and high electronegativity, commonly shows -3 (in nitrides, ammonia), -2, -1, 0 (in N2), +1, +2, +3, +4, and +5 (in nitrates). Heavier elements tend to favour the +3 and +5 states more due to the inert pair effect.
Covalency: Nitrogen can form a maximum of four covalent bonds (e.g., in NH4+) because it lacks d-orbitals in its valence shell for expansion. Heavier elements can expand their octet due to the presence of empty d-orbitals.
Electronegativity: Electronegativity decreases down the group. Nitrogen is the most electronegative, making its hydrides (e.g., ammonia) capable of hydrogen bonding, which influences their properties. Nitrogen gas can be prepared in the laboratory by heating an aqueous solution of ammonium nitrite (NH4NO2).
However, ammonium nitrite is unstable, so it is usually prepared in situ by mixing solutions of sodium nitrite (NaNO2) and ammonium chloride (NH4Cl).
Reaction: NH4Cl(aq) + NaNO2(aq) → NH4NO2(aq) + NaCl(aq) Then, upon heating: NH4NO2(aq) → N2(g) + 2H2O(l)
Overall Equation: NH4Cl(aq) + NaNO2(aq) $\xrightarrow{\text{heat}}$ N2(g) + NaCl(aq) + 2H2O(l)
Procedure: A solution containing approximately equal amounts of sodium nitrite and ammonium chloride is placed in a round-bottom flask. The flask is set up for gentle heating, usually with a Bunsen burner. The nitrogen gas produced is collected by the downward displacement of water (or over water) because it is sparingly soluble in water and denser than air (though collection over water is more common).
Purification: The gas may contain impurities like nitric oxide (NO) and ammonia (NH3). These can be removed by passing the gas through: Dilute H2SO4 (to absorb NH3) Potassium dichromate (K2Cr2O7) acidified with H2SO4 (to oxidize NO to NO2 and then remove it). Safety
Note: Heating should be gentle and controlled. Overheating can lead to the formation of ammonium nitrate (NH4NO3), which is explosive. Industrially, nitrogen is primarily obtained by the fractional distillation of liquid air. Air is a mixture mainly composed of nitrogen (approx. 78%), oxygen (approx. 21%), and argon (approx. 0.9%).
Process Steps: Purification of Air: Atmospheric air is first filtered to remove dust particles. Then, it is compressed and cooled to remove carbon dioxide (which solidifies at low temperatures) and water vapour (which condenses).
Liquefaction of Air: The purified air is then further compressed and allowed to expand rapidly. This rapid expansion causes a significant drop in temperature (Joule-Thomson effect), cooling the air to extremely low temperatures until it liquefies. This liquid air is primarily a mixture of liquid nitrogen and liquid oxygen.
Fractional Distillation: The liquid air is then fed into a fractionating column. Liquid nitrogen has a lower boiling point (-196 °C) than liquid oxygen (-183 °C). As the liquid air slowly warms up in the column, nitrogen boils off first as a gas and rises to the top of the column, where it is collected. Liquid oxygen, having a higher boiling point, remains in the lower part of the column and is collected separately. Argon, with a boiling point of -186 °C, separates between nitrogen and oxygen. This process yields high-purity nitrogen, oxygen, and argon, which are all commercially valuable.
Physical Properties: State: Colourless, odourless, tasteless gas at room temperature.
Diatomic Molecule: Exists as N2 molecules, with a strong triple covalent bond (N≡N).
Solubility: Sparingly soluble in water (less soluble than oxygen).
Density: Slightly less dense than air (relative molecular mass of N2 = 28, average molecular mass of air = 29).
Melting and Boiling Points: Very low melting point (-210 °C) and boiling point (-196 °C) due to weak Van der Waals forces between N2 molecules.
Non-flammable: Does not burn and does not support combustion.
Chemical Properties: Inertness: Nitrogen is largely unreactive at room temperature due to the high dissociation energy of the N≡N triple bond (945 kJ/mol). This makes it kinetically inert.
Reactivity at High Temperatures: At elevated temperatures, or under specific conditions (e.g., lightning, high pressure, catalysts), nitrogen becomes reactive.
Reaction with highly reactive metals: Reacts with very reactive metals like lithium and magnesium to form nitrides. 6Li(s) + N2(g) $\xrightarrow{\text{heat}}$ 2Li3N(s) (Lithium nitride) 3Mg(s) + N2(g) $\xrightarrow{\text{heat}}$ Mg3N2(s) (Magnesium nitride)
Reaction with hydrogen (Haber Process): N2(g) + 3H2(g) $\rightleftharpoons$ 2NH3(g) (Requires high temperature, pressure, and catalyst) Reaction with oxygen (in lightning or internal combustion engines): N2(g) + O2(g) $\xrightarrow{\text{high temp/lightning}}$ 2NO(g) (Nitric oxide)
Formation of Nitrides: Generally, nitrides are formed by direct combination with active metals or by reaction of ammonia with metals or their oxides.
Agriculture and Food Security in Nigeria: The most critical application is the role of nitrogen in fertilizers. Nitrogen, as a component of ammonia produced via the Haber process, is converted into fertilizers like urea (widely used by Nigerian farmers). Increased availability of nitrogenous fertilizers directly translates to higher crop yields (maize, rice, cassava, millet) and improved food security for the nation. Without adequate nitrogen, plants cannot synthesize proteins and nucleic acids, leading to stunted growth. Students can relate this to local farms or markets where they see fertilizers being sold or applied. Environmental Impact and Pollution in Urban Nigeria: The oxides of nitrogen (NOx), particularly NO and NO2, are significant air pollutants. These are formed during high-temperature combustion processes, such as in vehicle engines in busy Nigerian cities (e.g., Lagos, Port Harcourt) or industrial facilities. NOx contributes to smog formation, respiratory problems, and acid rain, which can damage buildings and harm aquatic ecosystems. Understanding the formation and properties of NOx helps students appreciate the environmental challenges and the need for emission controls.
Food Preservation and Industry: Nitrogen gas is widely used in Nigeria's food and beverage industry for modified atmosphere packaging (MAP). Many locally produced snacks (e.g., potato chips, biscuits), packaged groundnuts, and even bottled beverages (like some palm wine or juices) are flushed with nitrogen to displace oxygen. This prevents oxidation, rancidity, and microbial growth, thereby extending the shelf life of products and reducing food waste, which is economically beneficial for local businesses. Liquid nitrogen is also used for rapid freezing in industrial food processing. ---