Chemistry

Laboratory preparation of nitrogen gas

Principle: Nitrogen is prepared by heating ammonium nitrite, which is formed in situ by warming a concentrated solution of ammonium chloride and sodium nitrite taken in equal amounts (1 : 1 by mass).

Chemicals required: Ammonium chloride (NH₄Cl), sodium nitrite (NaNO₂), water.

Procedure:

1. Dissolve equal amounts of ammonium chloride and sodium nitrite in a little water and place the concentrated solution in a round-bottom flask.
2. Fit the flask with a cork carrying a delivery tube whose other end dips under water in a trough (nitrogen is almost insoluble in water).
3. Heat the flask gently. Remove the burner if the reaction becomes too vigorous, since it is exothermic once started.
4. Allow the first bubbles to escape to expel air from the apparatus.
5. Collect nitrogen by downward displacement of water in an inverted gas jar filled with water. Cover the full jar with a glass plate and take it out.

Chemical reaction:

$\text{NH}_4\text{Cl} + \text{NaNO}_2 \xrightarrow{\Delta} \text{NH}_4\text{NO}_2 + \text{NaCl}$

$\text{NH}_4\text{NO}_2 \xrightarrow{\Delta} \text{N}_2\uparrow + 2\text{H}_2\text{O}$

Overall:

$\text{NH}_4\text{Cl} + \text{NaNO}_2 \xrightarrow{\Delta} \text{N}_2\uparrow + \text{NaCl} + 2\text{H}_2\text{O}$

Diagram:

   Round-bottom flask
   (NH4Cl + NaNO2 solution)
        ___
       |   |
      /     \         delivery tube
     |       |________
     | soln  |        \
      \_____/          \      gas jar
         ^              \    ___|___
         |               \  |       |
      burner              \ | N2    |
                           \|  gas  |
      ~~~~~~~~~~~~~~~~~~~~~~|~~~~~~~|~~~~  water trough

Labels: round-bottom flask with NH₄Cl + NaNO₂ solution, burner (heat), cork, delivery tube, water trough, inverted gas jar, nitrogen gas.

Precautions:

1. Heat gently; the reaction can become violent.
2. The apparatus must be airtight.
3. Let the first bubbles escape to remove air.
4. Remove the delivery tube from the water before stopping heating to prevent suck-back.

Differences between ionic and covalent bonds

Explanation with examples

Ionic bond: Sodium (Na) has one electron in its outermost shell and chlorine (Cl) needs one electron to complete its octet. Na transfers its valence electron to Cl, forming Na⁺ and Cl⁻. The strong electrostatic attraction between these oppositely charged ions gives the ionic bond in sodium chloride (NaCl).

Covalent bond: Each hydrogen atom has one electron and needs one more to complete its duplet. Two hydrogen atoms share one pair of electrons between them, forming a single covalent bond in the H₂ molecule. Similarly, in CH₄ carbon shares four electron pairs with four hydrogen atoms.

a) Chemical bonding and types of hydrogen bonding

Chemical bonding: The attractive force that holds two or more atoms, ions or molecules together in a stable aggregate is called chemical bonding. It arises due to the tendency of atoms to attain a stable noble gas electronic configuration (duplet or octet) by losing, gaining or sharing electrons. The main types are ionic (electrovalent), covalent, coordinate (dative) and metallic bonding. A special weak bond, the hydrogen bond, also exists between molecules.

Hydrogen bonding: When hydrogen is covalently bonded to a highly electronegative atom (F, O or N), the bond becomes highly polar. The H atom carries a partial positive charge (δ+) and the electronegative atom carries a partial negative charge (δ−). The electrostatic attraction between the δ+ hydrogen of one molecule and the δ− atom of another molecule is called a hydrogen bond. It is a weak bond (about 2–10 kcal/mol), much weaker than a covalent bond. It is of two types:

1. Intermolecular hydrogen bonding: Hydrogen bonding that occurs between two different molecules of the same or different substances is called intermolecular hydrogen bonding. It increases the boiling point, melting point, viscosity and solubility in water.

Examples:

• Water (H₂O): Each water molecule is bonded to four neighbouring water molecules through hydrogen bonds (two through its H atoms and two through the lone pairs of its O atom). This gives water an unusually high boiling point (100 °C) and high surface tension.
• Hydrogen fluoride (HF): HF molecules associate to form a zig-zag chain, HF⋯HF⋯HF, raising its boiling point above that of HCl.
• Ethanol (C₂H₅OH) and ammonia (NH₃) also show intermolecular hydrogen bonding.

2. Intramolecular hydrogen bonding: Hydrogen bonding that occurs within the same molecule, between the H atom of one group and the electronegative atom of another group present in the same molecule, is called intramolecular hydrogen bonding. It decreases the boiling point and solubility in water, and increases volatility.

Examples:

• o-Nitrophenol: The –OH hydrogen is bonded to the oxygen of the –NO₂ group present in the same molecule (chelation). Because of this, o-nitrophenol is more volatile and less soluble in water than p-nitrophenol, which forms intermolecular hydrogen bonds.
• Salicylaldehyde (o-hydroxybenzaldehyde) and o-fluorophenol also show intramolecular hydrogen bonding.

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b) Periodic law and classification of elements based on electronic configuration

Periodic law: The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. It was given by Moseley (1913) and is the basis of the modern periodic table.

Classification of elements based on electronic configurations: On the basis of the orbital in which the last (differentiating) electron enters, elements are classified into four blocks:

1. s-block elements: Elements in which the last electron enters the s-orbital of the outermost shell. These belong to Group IA (alkali metals) and Group IIA (alkaline earth metals). General valence shell configuration: ns¹⁻². They are soft, highly reactive metals with low ionisation energy; e.g. Na (3s¹), Mg (3s²).

2. p-block elements: Elements in which the last electron enters the p-orbital of the outermost shell. These belong to Groups IIIA to VIIIA (13 to 18). General configuration: ns²np¹⁻⁶. They include metals, non-metals and metalloids; e.g. Al (3s²3p¹), Cl (3s²3p⁵).

3. d-block elements (transition elements): Elements in which the last electron enters the d-orbital of the penultimate (second outermost) shell. These belong to Groups IB to VIIIB (3 to 12). General configuration: (n–1)d¹⁻¹⁰ ns¹⁻². They are hard metals with variable oxidation states, form coloured compounds and show catalytic activity; e.g. Fe ([Ar]3d⁶4s²), Cu ([Ar]3d¹⁰4s¹).

4. f-block elements (inner transition elements): Elements in which the last electron enters the f-orbital of the antepenultimate (third outermost) shell. These belong to two series: lanthanides (4f series, Ce–Lu) and actinides (5f series, Th–Lr). General configuration: (n–2)f¹⁻¹⁴ (n–1)d⁰⁻¹ ns². They are heavy metals with similar properties; e.g. Ce ([Xe]4f¹5d¹6s²), U ([Rn]5f³6d¹7s²).

Copper is mainly extracted from copper pyrites (chalcopyrite), CuFeS₂, and also from cuprite (Cu₂O), copper glance (Cu₂S) and malachite. The extraction involves the following steps.

1. Concentration of ore (froth flotation process) The powdered ore is mixed with water, a little pine oil (frother) and sodium ethyl xanthate (collector) in a tank, and air is blown through it. Sulphide ore particles become water-repellent and stick to the air bubbles, which rise to the surface as froth. The froth is skimmed off, while gangue (earthy impurities) settles at the bottom. Thus the ore is concentrated to about 20–25% copper.

2. Roasting The concentrated ore is heated strongly in a reverberatory furnace in the presence of excess air. Sulphide ores are converted to oxides, and volatile impurities like S, As, Sb are removed as their oxides.

$2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$ $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$

3. Smelting The roasted ore is mixed with silica (SiO₂) and coke and heated in a blast furnace in the absence of air. Iron sulphide is oxidized to FeO, which combines with silica to form a fusible slag of iron silicate (FeSiO₃). The molten mass separates into two layers — lighter slag on top and heavier molten matte (Cu₂S + FeS) at the bottom. The slag is removed and the matte is collected.

$FeO + SiO_2 \rightarrow FeSiO_3 \text{ (slag)}$

4. Bessemerisation (conversion) The molten matte is transferred to a Bessemer converter and a blast of air is passed through it. FeS is oxidised to FeO (removed as slag) and Cu₂S is partly converted to Cu₂O. The Cu₂O then reacts with the remaining Cu₂S to give blister copper (about 98% pure, so called because escaping SO₂ makes its surface blistered).

$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$ $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$

5. Refining (electrolytic refining) Blister copper is made the anode and a thin sheet of pure copper the cathode, dipped in acidified copper sulphate solution (CuSO₄ + dilute H₂SO₄). On passing current, pure copper is deposited on the cathode, while impurities (Ag, Au, Pt) collect as anode mud. This gives 99.9% pure copper.

Uses of copper

1. Used for making electrical wires, cables and coils because of its high electrical conductivity.
2. Used in making alloys — brass (Cu + Zn), bronze (Cu + Sn) and German silver (Cu + Zn + Ni).
3. Used for making cooking utensils, water pipes and boilers due to its high thermal conductivity and resistance to corrosion.
4. Used in electroplating, in making coins, statues and ornaments.
5. Used in making electrical motors, generators, transformers and telephone cables.