Mada za sehemu hiiPeriodic ClassficationMada 4
- Periodicity
- Periodic Trends in Physical Properties
- Periodic Trends in Chemical Properties
- Diagonal Relationship
Chemical properties along the period depend on the transition of elements from strongly metallic to non-metallic. Sodium (Na) and Magnesium (Mg) are strongly metallic, while Aluminum (Al) is weakly metallic. Silicon (Si), Phosphorus (P), Sulphur (S), and Chlorine (Cl) are non-metallic elements. Thus, metallic properties decrease across the period from the left side of the periodic table to the right.
The hydrides of Period 3 elements include: NaH, MgH₂, AlH₃, SiH₄, PH₃, H₂S, and HCl. Sodium hydride is strongly ionic, Magnesium hydride is largely ionic but has some covalent character, while Aluminium hydride is purely covalent.
Reaction of hydrides with water
The hydrides of sodium, magnesium, and aluminum are basic in nature. They react with water to release hydrogen gas and form metal hydroxides.
- Sodium Hydride (NaH). Sodium hydride reacts with water to form sodium hydroxide and hydrogen gas. NaH + H₂O → NaOH + H₂↑
- Magnesium Hydride (MgH₂). Magnesium hydride reacts with water to form magnesium hydroxide and hydrogen gas. MgH₂ + 2H₂O → Mg(OH)₂ + 2H₂↑
- Aluminum Hydride (AlH₃). Aluminum hydride reacts with water to form aluminum hydroxide and hydrogen gas. 2AlH₃ + 6H₂O → 2Al(OH)₃ + 3H₂↑
- Silicon Hydride (SiH₄). Silicon hydride reacts with water in an alkaline medium to form silicates and hydrogen gas. This reaction is catalyzed. SiH₄ + 4H₂O → Si(OH)₄ + 4H₂↑
- Phosphine (PH₃). Phosphine is a non-polar covalent compound and does not react with water due to a small difference in electronegativity between Hydrogen and Phosphorus.
- Hydrogen Sulphide (H₂S). Hydrogen sulfide is a polar covalent compound and hydrolyzes in water to form an acidic solution. H₂S + H₂O → H₃O⁺ + HS⁻
- Hydrogen Chloride (HCl). Hydrogen chloride is also a polar covalent compound and hydrolyzes in water to form hydrochloric acid. HCl + H₂O → H₃O⁺ + Cl⁻
Thus, hydrides of metallic elements form alkaline solutions, while hydrides of non-metals form acidic solutions.
The chlorides of Period 3 elements include: NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₃, PCl₅, S₂Cl₂. Sodium and Magnesium chlorides are ionic salts, while the others (AlCl₃, SiCl₄, etc.) are covalent. As metallic character decreases across the period, ionic character of the chlorides decreases as well.
Reaction of chlorides with water
When ionic chlorides are added to water, there is an immediate attraction of polar water molecules to the ions in the chlorides. Some chlorides dissolve to form free ions, while others undergo hydrolysis.
- Sodium Chloride (NaCl). Sodium chloride is an ionic compound that does not hydrolyze in water. NaCl → Na⁺ + Cl⁻ (no reaction with water)
- Magnesium Chloride (MgCl₂). Magnesium chloride is ionic, but its hydrated crystals undergo hydrolysis when heated to produce hydrogen chloride and magnesium hydroxide. MgCl₂ + 6H₂O (heated) → Mg(OH)₂ + 2HCl
- Aluminum Chloride (AlCl₃). Aluminum chloride hydrolyzes easily in water to produce an acidic solution due to the high polarizing power of Al³⁺ ions. AlCl₃ + 6H₂O → [Al(H₂O)₆]³⁺ + 3Cl⁻ This leads to the formation of hydroxonium ions, which makes the solution acidic.
- Silicon Chloride (SiCl₄). Silicon chloride hydrolyzes completely in water to form silicic acid. SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl
- Phosphorus Chlorides (PCl₃ and PCl₅). Phosphorus chlorides like PCl₃ and PCl₅ hydrolyze in water to form phosphoric acid and hydrochloric acid. PCl₃ + 3H₂O → H₃PO₃ + 3HCl PCl₅ + 4H₂O → H₃PO₄ + 5HCl
- Sulfur Chlorides (S₂Cl₂). Sulfur chlorides hydrolyze in water to produce sulfuric acid. S₂Cl₂ + 2H₂O → H₂SO₄ + 2HCl
As metallic character decreases across the period, ionic nature of the chlorides decreases, while the extent of hydrolysis increases.
The hydroxides in Period 3 include: NaOH, Mg(OH)₂, Al(OH)₃, Si(OH)₂, P(OH)₃, SO₂(OH)₂, ClO₃(OH), and Cl(OH). The hydroxides from silicon to chlorine are not true hydroxides, but they are oxy-acids.
Acidic and basic characters
Sodium and Magnesium hydroxides are strong bases, while the hydroxides of non-metals are acidic. Aluminum hydroxide is amphoteric, meaning it can behave both as an acid and a base.
- Sodium Hydroxide (NaOH). Sodium hydroxide is a strong base that dissociates completely in water. NaOH → Na⁺ + OH⁻
- Magnesium Hydroxide (Mg(OH)₂). Magnesium hydroxide is a weak base and slightly soluble in water. Mg(OH)₂ (s) → Mg²⁺ + 2OH⁻ (aq)
- Aluminum Hydroxide (Al(OH)₃). Aluminum hydroxide is amphoteric and can react with both acids and bases. Al(OH)₃ + 3H⁺ → [Al(H₂O)₆]³⁺ (acidic reaction) Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (basic reaction)
- Silicon Hydroxide (Si(OH)₂). Silicon hydroxide is an oxy-acid and behaves as a weak acid. Si(OH)₂ + H₂O → SiO(OH)₂ + H⁺
Oxides of sodium and magnesium are basic, while aluminum oxide is amphoteric. The oxides of the remaining elements (Si, P, S, Cl) are acidic.
Reaction of oxides with water
Oxides of sodium and magnesium form hydroxides with water due to their strong metallic nature. Oxides of non-metals like phosphorus, sulfur, and chlorine react with water to form acidic solutions.
- Sodium Oxide (Na₂O). Sodium oxide reacts with water to form sodium hydroxide. Na₂O + H₂O → 2NaOH
- Magnesium Oxide (MgO). Magnesium oxide reacts with water to form magnesium hydroxide. MgO + H₂O → Mg(OH)₂
- Silicon Dioxide (SiO₂). Silicon dioxide does not react with water, but it is acidic. SiO₂ + H₂O → No Reaction
- Sulfur Trioxide (SO₃). Sulfur trioxide reacts with water to form sulfuric acid. SO₃ + H₂O → H₂SO₄
- Phosphorus Pentoxide (P₄O₁₀). Phosphorus pentoxide reacts with water to form phosphoric acid. P₄O₁₀ + 6H₂O → 4H₃PO₄
- Chlorine Trioxide (Cl₂O₃). Chlorine trioxide reacts with water to form hydrochloric acid. Cl₂O₃ + 3H₂O → 2HCl + 3O₂
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