Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Sulphur
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Sulphur
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 3
Theme: Chemistry And Environment
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state the general propertiesof group VIA elements write and draw the electronconfiguration of sulphur explain the meaning of allotropy identify the allotropes of sulphur state the uses of sulphur name some commoncompounds of sulphur determine the oxidationstates of sulphur in its majorcompounds describe the in dustrialpreparation of H2SO4 by the contact process state the uses of H2SO4
Group VIA elements, also known as Chalcogens, include Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te), Polonium (Po), and Livermorium (Lv).
Electronic Configuration: All elements in Group VIA have six valence electrons. Their general outer electronic configuration is ns2np
4. For example, Oxygen is 2s22p4, and Sulphur is 3s23p
4. Valency: Due to having six valence electrons, these elements typically gain two electrons to achieve a stable octet, exhibiting a common valency of -
2. They can also exhibit positive oxidation states, especially sulphur, selenium, and tellurium, by sharing electrons.
Physical State: At room temperature, Oxygen is a gas, while Sulphur and Selenium are solids. Tellurium and Polonium are also solids.
Metallic Character: Metallic character increases down the group. Oxygen and Sulphur are non-metals. Selenium and Tellurium are metalloids (exhibiting properties of both metals and non-metals). Polonium is a metal.
Electronegativity: Electronegativity decreases down the group. Oxygen is the second most electronegative element after fluorine.
Ionization Energy: Ionization energy generally decreases down the group as atomic size increases and the outermost electrons are further from the nucleus.
Atomic Size: Atomic size increases down the group as new electron shells are added. Sulphur has an atomic number of
1
6. This means a neutral sulphur atom has 16 protons and 16 electrons. Electron Configuration (Spectroscopic Notation): 1s2 2s2 2p6 3s2 3p4 Electron Configuration (Noble Gas Notation): [Ne] 3s2 3p4 Orbital Diagram: ``` 1s 2s 2p 3s 3p ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑ ``` Explanation: The first two electrons fill the 1s orbital. The next two fill the 2s orbital. The next six electrons fill the 2p orbitals (one electron in each, then pairing up). The next two electrons fill the 3s orbital. The last four electrons fill the 3p orbitals (one electron in each of the three 3p orbitals, then the fourth electron pairs up in one of the 3p orbitals, leaving two unpaired electrons).
Valence Electrons: Sulphur has 6 valence electrons (2 in 3s and 4 in 3p). Allotropy is the property of some chemical elements to exist in two or more different forms (allotropes) in the same physical state. These allotropes differ in their physical properties (e.g., density, melting point, colour, crystal structure) due to different arrangements of atoms, but they exhibit similar chemical properties. Examples include Carbon (diamond, graphite, fullerenes) and Oxygen (O2, O3). Sulphur exhibits several allotropic forms, primarily: Rhombic Sulphur (α-sulphur): Appearance: Pale yellow, opaque, brittle solid.
Crystal System: Orthorhombic.
Shape: Octahedral crystals (looks like two pyramids joined at their bases).
Density: 2.07 g/cm
3. Melting Point: 113°
C. Solubility: Insoluble in water, soluble in carbon disulphide (CS2).
Stability: Stable below 95.6°C (transition temperature). It is the most stable allotrope at room temperature.
Molecular Structure: Composed of S8 rings (crown-shaped).
Preparation: Obtainable by slow evaporation of a solution of sulphur in carbon disulphide. Monoclinic Sulphur (β-sulphur): Appearance: Transparent, needle-like, amber-coloured crystals.
Crystal System: Monoclinic.
Shape: Long, needle-like prisms.
Density: 1.96 g/cm
3. Melting Point: 119°
C. Solubility: Insoluble in water, soluble in carbon disulphide (CS2).
Stability: Stable above 95.6°
C. It slowly changes to rhombic sulphur below this temperature.
Molecular Structure: Also composed of S8 rings, but packed differently in the crystal lattice.
Preparation: Obtained by melting rhombic sulphur and allowing it to cool slowly, or by cooling molten sulphur above 95.6°
C. Plastic Sulphur (γ-sulphur) / Amorphous Sulphur: Appearance: Dark brown, rubber-like, elastic, amorphous mass.
Crystal System: Amorphous (no definite crystal structure).
Density: ~1.92 g/cm
3. Melting Point: Does not have a sharp melting point. Softens over a range.
Solubility: Insoluble in carbon disulphide.
Stability: Unstable at room temperature, slowly reverts to rhombic sulphur over time.
Molecular Structure: Consists of long, helical chains of sulphur atoms (S_n_ where n is large).
Preparation: Prepared by pouring molten sulphur (heated to around 200°C - 300°C) into cold water. The rapid cooling prevents the formation of an ordered crystal lattice.
Transition Temperature: The transition temperature between rhombic and monoclinic sulphur is 95.6°C. At this temperature, both forms can coexist in equilibrium. Below 95.6°C, rhombic is stable; above it, monoclinic is stable.
Sulphur (S₈):
Sulphur is an element in its uncombined state.
Oxidation state of S = 0
Hydrogen Sulphide (H₂S):
Let the oxidation state of S be x.
Oxidation state of H = +1.
(2 × +1) + x = 0
2 + x = 0
x = -2
Sulphur Dioxide (SO₂):
Let the oxidation state of S be x.
Oxidation state of O = -2.
x + (2 × -2) = 0
x - 4 = 0
x = +4
Sulphur Trioxide (SO₃):
Let the oxidation state of S be x.
Oxidation state of O = -2.
x + (3 × -2) = 0
x - 6 = 0
x = +6
Sulphuric Acid (H₂SO₄):
Let the oxidation state of S be x.
Oxidation state of H = +
1.
Oxidation state of O = -2.
(2 × +1) + x + (4 × -2) = 0
2 + x - 8 = 0
x - 6 = 0
x = +6
Sulphite ion (SO₃²⁻):
Let the oxidation state of S be x.
Oxidation state of O = -2.
x + (3 × -2) = -2 (sum equals the charge of the ion)
x - 6 = -2
x = -2 + 6
x = +4
Thiosulphate ion (S₂O₃²⁻):
Let the oxidation state of S be x. (
Note: This is an average oxidation state, as the two sulphur atoms are not equivalent).
Oxidation state of O = -2.
(2 × x) + (3 × -2) = -2
2x - 6 = -2
2x = -2 + 6
2x = 4
x = +2
2.8 Industrial Preparation of H₂SO₄ by the Contact Process
The Contact Process is the primary industrial method for manufacturing sulphuric acid. It is a multi-step process involving specific conditions for optimal yield.
Raw Materials:
Sulphur (S) or sulphide ores (e.g., iron pyrites, FeS₂)
Air (source of oxygen)
Water
Steps:
Production of Sulphur Dioxide (SO₂):
Sulphur dioxide is produced by burning elemental sulphur in excess air:
S(s) + O₂(g) → SO₂(g)
Alternatively, it can be obtained by roasting sulphide ores (e.g., iron pyrites):
4FeS₂(s) + 11O₂(g) → 2Fe₂O₃(s) + 8SO₂(g)
Purification of Sulphur Dioxide:
The SO₂ gas must be purified to remove dust, arsenic compounds, and moisture, as impurities can poison the catalyst.
This involves:
Dust precipitators: To remove dust particles.
Cooling pipes: To cool the gas.
Washing tower: SO₂ is washed with water to remove soluble impurities.
Drying tower: SO₂ is dried with concentrated H₂SO₄ to remove moisture.
Arsenic purifiers: To remove arsenic impurities (which can poison the catalyst).
Catalytic Oxidation of Sulphur Dioxide to Sulphur Trioxide (SO₃):
Purified SO₂ and excess oxygen (from air) are passed over a vanadium(V) oxide (V₂O₅) catalyst.
Conditions:
Catalyst: Vanadium(V) oxide (V₂O₅). Platinum (Pt) can also be used, but it's more expensive and easily poisoned.
Temperature: Optimised at 450°C. Higher temperatures shift equilibrium backwards (exothermic reaction), lower temperatures reduce reaction rate.
Pressure: 1 to 2 atmospheres (atm). Although higher pressure favours product formation, 1-2 atm is sufficient for a good yield and is less costly.
Reaction: This is a reversible, exothermic reaction.
2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ; ΔH = -197 kJ/mol
Absorption of Sulphur Trioxide (SO₃):
Sulphur trioxide is NOT directly absorbed in water because the reaction (SO₃ + H₂O → H₂SO₄) is highly exothermic, producing a fine mist of sulphuric acid that is difficult to condense and hazardous.
Instead, SO₃ is absorbed in concentrated sulphuric acid (98%) to form oleum (fuming sulphuric acid, H₂S₂O₇):
SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) (oleum)
Dilution of Oleum:
The oleum is then carefully diluted with the calculated amount of water to produce concentrated sulphuric acid (98%):
H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)
Agriculture and Food Security: Sulphur is essential for plant nutrition, and its compounds (e.g., ammonium sulphate, superphosphate) are critical components of fertilizers. In Nigeria, where agriculture is a cornerstone of the economy, understanding sulphur's role in soil enrichment directly impacts crop yields (e.g., maize, rice, cassava) and food security. Sulphur-based pesticides also protect crops from diseases and pests.
Transportation and Industry: The vulcanization of rubber using sulphur is fundamental to the manufacturing of vehicle tyres, conveyor belts, and other durable rubber products widely used in Nigeria.
Furthermore, sulphuric acid, produced from sulphur, is the electrolyte in lead-acid batteries that power millions of vehicles, generators, and solar inverter systems across the country. Its use in petroleum refining is also crucial for Nigeria's oil and gas sector.
Environmental Awareness and Health: While sulphur compounds are vital, sulphur dioxide (SO2) is a significant air pollutant, particularly from industrial emissions and the combustion of fossil fuels (e.g., vehicles, power plants). This lesson provides a foundation for students to understand the sources, effects (e.g., acid rain, respiratory issues), and control measures for such pollutants, fostering environmental literacy crucial for sustainable development in Nigeria.