Acid-Base Behavior of the Oxides

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When examining the oxides of elements in Period 3 of the periodic table (from sodium to chlorine), it is essential to understand how they react with water, acids, and bases. This discussion will help you grasp the trends and specific reactions involved.

Trend in Acid-Base Behavior

There is a distinct trend in the acid-base behavior of Period 3 oxides:

• Oxides on the left-hand side are strongly basic.
• Oxides on the right-hand side are strongly acidic.
• In the middle, we have an amphoteric oxide (aluminium oxide), which exhibits both acidic and basic properties.

Reactions of Oxides with Water, Acids, and Bases

Sodium Oxide (Na₂O)

Basic Nature:

• Sodium oxide is strongly basic because it contains the oxide ion (O²⁻), which readily combines with hydrogen ions.

Reaction with Water:

• Sodium oxide reacts exothermically with water, producing sodium hydroxide solution with a pH around 14.
  \[ \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \]

Reaction with Acids:

• Sodium oxide reacts with dilute hydrochloric acid to form sodium chloride and water.
  \[ \text{Na}_2\text{O} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} \]

Magnesium Oxide (MgO)

Basic Nature:

• Magnesium oxide is a basic oxide but less strongly basic than sodium oxide due to stronger ionic attractions.

Reaction with Water:

• Magnesium oxide reacts slightly with water, producing a weakly alkaline solution with pH around 9.
  \[ \text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \]

Reaction with Acids:

• Magnesium oxide reacts with warm dilute hydrochloric acid to form magnesium chloride.
  \[ \text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O} \]

Aluminium Oxide (Al₂O₃)

Amphoteric Nature:

• Aluminium oxide exhibits both acidic and basic properties.

Reaction with Water:

• Aluminium oxide does not react with water due to strong ionic bonds in the solid lattice.

Reaction with Acids:

• It reacts with hot dilute hydrochloric acid to form aluminium chloride.
  \[ \text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} \]

Reaction with Bases:

• Aluminium oxide reacts with hot, concentrated sodium hydroxide to form sodium tetrahydroxoaluminate.
  \[ \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4 \]

Phosphorus (V) Oxide (P₄O₁₀)

Acidic Nature:

• Phosphorus (V) oxide reacts violently with water to form phosphoric acid.
  \[ \text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4 \]

Reaction with Bases:

• Phosphoric acid and Phosphorus (V) Oxide react with sodium hydroxide to form various sodium phosphates.
  \[ \text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O} \]
  \[ 2\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_2\text{HPO}_4 + 2\text{H}_2\text{O} \]
  \[ 3\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_3\text{PO}_4 + 3\text{H}_2\text{O} \]
  \[ 12\text{NaOH} + \text{P}_4\text{O}_{10} \rightarrow 4\text{Na}_3\text{PO}_4 + 6\text{H}_2\text{O} \]

Sulphur Oxides (SO₂ and SO₃)

Sulphur Dioxide (SO₂):

• Reacts with water to form sulphurous acid.
  \[ \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \]
• Reacts with bases to form sulfites and bisulfites.
  \[ \text{SO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_3 + \text{H}_2\text{O} \]
  \[ \text{Na}_2\text{SO}_3 + \text{H}_2\text{O} + \text{SO}_2 \rightarrow 2\text{NaHSO}_3 \]
  \[ \text{CaO} + \text{SO}_2 \rightarrow \text{CaSO}_3 \]

Sulphur Trioxide (SO₃):

• Reacts violently with water to form sulphuric acid.
  \[ \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \]
• Reacts with bases to form sulfates.
  \[ 2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} \]
  \[ \text{CaO} + \text{SO}_3 \rightarrow \text{CaSO}_4 \]

Chlorine Oxides (Cl₂O and Cl₂O₇)

Chlorine (VII) Oxide (Cl₂O₇):

• Reacts with water to form perchloric acid.
  \[ \text{Cl}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{HClO}_4 \]
• Reacts with bases to form perchlorates.
  \[ \text{NaOH} + \text{HClO}_4 \rightarrow \text{NaClO}_4 + \text{H}_2\text{O} \]
  \[ 2\text{NaOH} + \text{Cl}_2\text{O}_7 \rightarrow 2\text{NaClO}_4 + \text{H}_2\text{O} \]

Chlorine (I) Oxide (Cl₂O):

• Reacts with water to form hypochlorous acid.
  \[ \text{Cl}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{HOCl} \]
• Hypochlorous acid reacts with bases to form hypochlorites.
  \[ \text{NaOH} + \text{HOCl} \rightarrow \text{NaOCl} + \text{H}_2\text{O} \]
• Reacts with bases to form hypochlorites.
  \[ 2\text{NaOH} + \text{Cl}_2\text{O} \rightarrow 2\text{NaOCl} + \text{H}_2\text{O} \]

Understanding these reactions helps in grasping the general trends and specific chemical behavior of Period 3 oxides, making it easier to predict their properties and reactions in various chemical contexts.