Na, Mg, and Period 3 Elements Reactions

• Trends in reactions with water pertain to Na and Mg.

Na and Mg Reactivity with Water

• Sodium reacts with cold water, producing fizzing on the surface.
• The equation is: 2 Na (s) + 2 H2O (l) → 2 NaOH (aq) + H2 (g).
• Magnesium reacts slowly with cold water to form the hydroxide.
• Magnesium reacts more readily with steam to form the oxide
• The equation is: Mg (s) + H2O (g) → MgO (s) + H2 (g)

Reactions of Period 3 Elements with Oxygen

• All Period 3 elements react with oxygen, forming oxides.
• Sodium burns with a yellow flame, yielding a white solid.
• Mg, Al, Si, and P burn with a white flame, producing white solid smoke.
• Sulfur burns with a blue flame, forming an acidic choking gas.
• Equations include:
• 4 Na (s) + O2 (g) → 2 Na2O (s)
• 2Mg (s) + O2 (g) → 2MgO (s)
• 4Al + 3O2 (g) → 2Al2O3 (s)
• Si + O2 (g) → SiO2 (s)
• 4P + 5O2 (g) → P4O10 (s)
• S + O2 (g) → SO2 (g)
• Sodium is stored under oil, and phosphorus under water, to prevent contact with air and subsequent reaction.

Properties of Period 3 Oxides

• The understanding of the physical properties of the oxides of elements Na to S and their structure and bonding is important

Types of Oxides

• Ionic oxides (Na2O, MgO, Al2O3) are characterized by high melting points.
• They form ionic giant lattice structures with strong attractive forces.
• Ionic nature arises from the large electronegativity difference between the metal and oxygen.
• Increased cation charge strengthens ionic forces, increasing lattice enthalpies in the progression from Na to Al, thus elevating melting points.
• Al2O3 is ionic but exhibits some covalent characteristics
• Less of a differece in electronegativity or the small aluminum ion with a high charge can get close to the oxide and distort it

Macromolecular Oxides

• SiO2 is macromolecular with many strong covalent bonds.
• Breaking these bonds requires high energy, leading to very high melting and boiling points.

Simple Molecular Oxides

• P4O10 (s), SO2 (g) feature simple molecules with weak intermolecular forces (van der Waals and permanent dipoles), resulting in lower melting points.
• Their covalent nature is due to the small electronegativity difference between the non-metal and oxygen atoms.
• P4O10, a larger molecule with more electrons than SO2, experiences larger van der Waals forces, leading to a higher melting point.

Experiment to Prove Ionic Compounds

• To confirm the presence of ions, melt the solids and demonstrate their electrical conductivity.

Aluminum Oxide Corrosion Protection

• Aluminium is protected from corrosion in moist air by a thin layer of aluminium oxide.
• The high lattice strength of aluminium oxide and its insolubility in water make this layer impermeable to air and water.

Reactions of Oxides with Water

• Metal ionic oxides react with water to form alkaline hydroxides.
• Na2O (s) + H2O (l) → 2Na+ (aq) + 2OH (aq) with a pH of 13 being a vigorous exothermic reaction
• The ionic oxides act as bases, with oxide ions accepting protons to form hydroxide ions.
• MgO (s) + H2O (l) → Mg(OH)2 (s) pH 9, Mg(OH)2 is slightly soluble due to its stronger lattice, resulting in fewer free OH- ions and a lower pH.
• Non-metal covalent oxides react with water to form acids.
• P4O10 (s) + 6 H2O (l) → 4 H3PO4(aq) pH 0 (vigorous exothermic reaction)
• SO2 (g) + H2O (l) → H2SO3 (aq) pH 3 (weak acid)
• SO3 (g) + H2O (l) → H2SO4 (aq) pH 0
• SO2 + H2O → H+ + HSO3¯
• SO3 + H2O → H+ + HSO4

Oxide Behavior Trend

• Ionic metal oxides demonstrate basic behavior while non-metal covalent oxides show acidic behavior.
• Aluminum oxide exhibits amphoteric behavior, acting as both a base and an acid.

Displayed Formulae of Acids

• Diagrams show H2SO4, H2SO3 and H3PO4.

Acid-Base Reactions with Period 3 Oxides

• Basic oxides react with acids to produce salts.
• Na2O (s) + 2 HCl (aq) → 2NaCl (aq) + H2O (I)
• Na2O (s) + H2SO4 (aq) → Na2SO4 (aq) + H2O (1)
• MgO (s) + 2 HCl (aq) → MgCl2 (aq) + H2O (1)
• Ionically these reactions can be represented as:
• Na2O (s) + 2H+ (aq) → 2Na+ (aq) + H2O (I)
• MgO (s) + 2 H+ (aq) → Mg2+ (aq) + H2O (I)

Amphoteric Nature of Aluminum Oxide

• Aluminum oxide acts as both an acid and an alkali.

Reactions as a Base

• Al2O3 (s)+ 3H2SO4 (aq)→ Al2(SO4)3 (aq) + 3H2O (I)
• Al2O3 + 6HCI → 2AICI3 + 3H2O
• Or ionically Al2O3 + 6H+ → 2AI3+ + 3H2O

Aluminum Oxide as an Acid

• Al2O3 (s)+ 2NaOH (aq) + 3H2O (l) → 2NaAl(OH)4 (aq)
• Al2O3 (s)+ 2OH (aq) + 3H2O (l) → 2Al(OH)4¯ (aq)

Reactions of Acidic Oxides with Bases

• Acidic oxides such as P4O10 and SO react with bases to form salts.
• P4O10 (s) + 12 NaOH (aq) → 4Na3PO4(aq) + 6 H2O (I) or P4O10 + 6 Na2O → 4Na3PO4
• SO2 (g) + 2NaOH (aq) → Na2SO3 (aq) + H2O (I)
• SO3 (g) + 2NaOH(aq) → Na2SO4 (aq) + H2O (1)
• Ionically, reactions are:
• P4O10 (s) + 12 OH- (aq) → 4PO4 3-(aq) + 6 H2O (1)
• SO2 (g) + 2OH (aq) → SO32- (aq) + H2O (I)
• SO3 (g) + 2OH (aq) → SO42- (aq) + H2O (I)

Silicon Dioxide (SiO2)

• SiO2 possesses a giant covalent structure with strong bonds.
• The giant covalent structure of SiO2 prevents it from dissolving or reacting with water and weak alkali solutions.
• It will, however, react with very concentrated NaOH
• 2NaOH (I) + SiO2 (s)→ Na2SiO3 (aq) + H2O and si still classified as an acidic oxide