Reactions of the Period 3 Elements with Water, Oxygen, and Chlorine
This section describes the chemical reactions exhibited by elements of Period 3 of the periodic table with water (H₂O), oxygen (O₂), and chlorine (Cl₂).
a) Reactions with Water
The reactivity of Period 3 elements with water varies significantly:
Sodium (Na):
A highly exothermic reaction occurs between sodium and cold water. This vigorous reaction produces hydrogen gas (H₂) and a colorless solution of sodium hydroxide (NaOH). The balanced chemical equation for this reaction is:
2Na + 2H₂O → 2NaOH + H₂
Magnesium (Mg):
Magnesium exhibits a minimal reaction with cold water. However, when a clean magnesium coil is introduced to hot steam (H₂O vapor), it ignites with a white flame, producing white magnesium oxide (MgO) and hydrogen gas. The equation for this reaction is:
Mg + H₂O (Steam) → MgO + H₂
Aluminum (Al):
Aluminum powder reacts slowly with steam to form hydrogen gas and aluminum oxide (Al₂O₃). The slow reaction rate stems from the formation of a protective aluminum oxide layer on the metal surface, which hinders further reaction. The balanced equation is:
2Al + 3H₂O (Steam) → Al₂O₃ + 3H₂
Phosphorus (P) and Sulfur (S):
These elements do not react with water under normal conditions.
Chlorine (Cl₂):
Chlorine exhibits a partial solubility in water, forming a greenish solution. A reversible reaction takes place, generating a mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl). The equation for this reaction is:
Cl₂ + H₂O ⇌ HCl + HOCl
Sunlight catalyzes the decomposition of hypochlorous acid, further producing hydrochloric acid and releasing oxygen gas. The overall reaction in sunlight can be represented as:
2Cl₂ + 2H₂O (Sunlight) → 4HCl + O₂
Argon (Ar):
This element exhibits no reaction with water.
b) Reactions with Oxygen
The interaction of Period 3 elements with oxygen showcases diverse reactivity:
Sodium (Na):
When ignited in oxygen, sodium burns vigorously with an orange flame, producing a white solid mixture of sodium oxide (Na₂O) and sodium peroxide (Na₂O₂). The equations for these reactions are:
- For sodium oxide:
Na + O₂ → 2Na₂O
- For sodium peroxide:
4Na + O₂ → 2Na₂O₂
Magnesium (Mg):
Magnesium burns intensely in oxygen with a white flame, forming white solid magnesium oxide (MgO). The balanced equation for this reaction is:
2Mg + O₂ → 2MgO
Silicon (Si):
Upon strong heating, silicon will react with oxygen to produce silicon dioxide (SiO₂). The equation for this reaction is:
Si + O₂ → SiO₂
Phosphorus (P):
White phosphorus ignites spontaneously in air, burning with a white flame and producing white smoke. This smoke is a mixture of phosphorus (III) oxide (P4O6) and phosphorus (V) oxide (P₄O₁₀). The proportions of these oxides depend on the available oxygen. Here are the balanced equations:
- For phosphorus (III) oxide (limited oxygen):
P₄ + 3O₂ → P4O6
-
- For phosphorus (V) oxide (excess oxygen):
P₄ + 5O₂ → P₄O₁₀
Sulfur (S):
Gentle heating of sulfur in air or oxygen results in a pale blue flame and the production of colorless sulfur dioxide gas (SO₂). The balanced equation for this reaction is:
S + O₂ → SO₂
Chlorine (Cl₂) and Argon (Ar):
Despite having several oxides, chlorine does not react directly with oxygen. Similarly, argon shows no reactivity with oxygen.
Table: Properties of the Oxides of Elements in Period 3
| Formula of Oxide | State | Conductivity | Structure | ΔfH0 at 298K, kJ mol⁻¹) | Effect on Water | Nature of Oxide | Color |
|---|---|---|---|---|---|---|---|
| Na₂O (Sodium Oxide) |
Solid | Good | Giant lattice of ions | -416 | Reacts to form NaOH (alkaline) | Basic (alkaline) | White |
| MgO (Magnesium oxide) |
Solid | Good | Giant lattice of ions | -602 | Reacts to form Mg(OH)₂ (alkaline) | Basic (weakly alkaline) | White |
| Al₂O₃ (Aluminium oxide) |
Solid | Good | Giant lattice of ions | -1676 | Does not react | Amphoteric | White |
| SiO₂ (silicon dioxide) |
Solid | Very poor | Giant covalent oxide | -910 | Reacts to form H₂SiO₃ (acid) | Acidic | White |
| P4O10
(Phosphorus pentoxide) |
Solid | Nil | Simple covalent oxide | P4O10 (-2984) |
P4O10 reacts to form H₃PO₄ (acid) | Acidic | white trigonal crystals |
| P4O6
(phosphorus trioxide) |
Solid | Nil | Simple covalent oxide | P4O6 (−1593.7) |
P4O10 reacts to form H₃PO₃ (acid) | Acidic | white monoclinic crystals |
| SO₂
(Sulfur dioxide) |
Gas | Nil | Simple covalent oxide | −296.9 | Reacts to form H₂SO₃ (acid) | Acidic | colorless gas |
| SO₃ (Sulfur trioxide) |
Solid | Nil | Simple covalent oxide | -395.8 | Reacts to form H₂SO₄ (acid) | Acidic | Colorless to white monoclinic crystalline solid which will fume in air |
| Cl₂O₇
(Dichlorine heptoxide) |
Liquid | Nil | Simple covalent oxide | Cl₂O₇ (251) |
Reacts to form HClO₄ (acid) | Acidic | colorless oily liquid |
| Cl₂O (Dichlorine monoxide) |
Gas | Nil | Simple covalent oxide | Cl₂O (75.7) |
Reacts with water to produce hypochlorous acid (HOCl)
|
Acidic | brownish-yellow gas |
Explanation of Key Concepts:
State of Oxide:
This refers to whether the oxide exists as a solid, liquid, or gas at room temperature.
Conductivity in Liquid Oxide:
This indicates whether the molten oxide can conduct electricity. Good conductors allow for the flow of charged particles.
Structure of Oxide:
This describes the arrangement of atoms and ions within the oxide. Common structures include giant ionic structures (e.g., Na₂O, MgO), simple molecule structures (e.g., SO₂), giant covalent structures (SiO₂), and covalent network structures (P₄O₆, P₄O₁₀).
Enthalpy Change of Formation:
This value represents the energy change that occurs when one mole of the oxide is formed from its constituent elements under standard conditions (298K and 1 atm pressure). A negative value indicates an exothermic reaction (energy is released).
Effect of Oxide Adding Acid or Alkali to Water:
This describes how the oxide reacts with acidic or basic solutions. Basic oxides react with acids to form salts and water. Acidic oxides react with bases to form salts and water. Amphoteric oxides can react with both acids and bases.
Nature of Oxide:
This classification indicates whether the oxide is acidic, basic, or amphoteric based on its reaction with water. Acidic oxides produce acidic solutions when dissolved in water. Basic oxides produce basic solutions when dissolved in water. Amphoteric oxides can exhibit both acidic and basic behavior depending on the reacting substance.
c) Reactions with Chlorine
The elements in Period 3 exhibit a range of reactivity towards chlorine (Cl₂):
Sodium (Na):
Sodium reacts vigorously with chlorine, burning with a bright orange flame. This reaction produces white solid sodium chloride (NaCl). The balanced equation for this reaction is:
2Na + Cl₂ → 2NaCl
Magnesium (Mg):
Magnesium reacts with chlorine, burning with its characteristic intense white flame. The product of this reaction is white magnesium chloride (MgCl₂). The equation is:
Mg + Cl₂ → MgCl₂
Aluminum (Al):
Aluminum reacts with chlorine, burning with a very bright white flame. This reaction produces aluminum chloride (AlCl₃). The equation is:
2Al + 3Cl₂ → 2AlCl₃
Silicon (Si):
When heated, silicon powder reacts with chlorine gas to produce colorless liquid silicon tetrachloride (SiCl₄). The equation for this reaction is:
Si + 2Cl₂ → SiCl₄
Phosphorus (P):
White phosphorus reacts spontaneously with chlorine, burning and producing phosphorus (III) chloride (PCl₃) as a white fume. Excess chlorine can further react with phosphorus (III) chloride to form phosphorus (V) chloride (PCl₅). The equations for these reactions are:
- For phosphorus (III) chloride:
P₄ + 6Cl₂ → 4PCl₃
- For phosphorus (V) chloride (with excess chlorine):
PCl₃ + Cl₂ → PCl₅
Sulfur (S):
Sulfur reacts with chlorine, producing disulfur dichloride (S₂Cl₂). This reaction is highly exothermic, releasing a significant amount of heat. The equation is:
2S + Cl₂ → S₂Cl₂
Argon (Ar):
Argon exhibits no reaction with chlorine.
Key Points to Remember:
- The reactivity of Period 3 elements with chlorine generally decreases moving from left to right across the period. This trend reflects the increasing strength of the elements’ bonds with their own electrons as you move towards noble gases.
- Chlorine readily gains an electron from most Period 3 elements, forming ionic chlorides (e.g., NaCl, MgCl₂).
- Some elements like silicon form covalent chlorides (e.g., SiCl₄).
