3.2.4 Properties of Period 3 elements and their oxides

The reactions of the Period 3 elements with oxygen are considered. The pH of the solutions formed when the oxides react with water illustrates further trends in properties across this period. Explanations of these reactions offer opportunities to develop an in-depth understanding of how and why these reactions occur.

What is the difference between Sodium and Magnesium in its ionic configuration

• Sodium needs to lose one electron to achieve a stable electron configuration, whereas magnesium needs to lose two.

What does this ionic difference cause

This difference affects their reactivity

Which is more reactive and why?

Sodium is an alkali metal (Group 1) and readily loses one electron to form a positive ion, making it highly reactive, while magnesium, in Group 2, loses two electrons, making it less reactive compared to sodium. 

Explain Sodium’s Reaction with Cold Water:

-What is observed

-The reaction

-The pH of the product

• Sodium reacts vigorously with cold water, forming a molten ball that skims along the surface, accompanied by vigorous fizzing due to hydrogen gas production.

• 2Na + 2H₂​O → 2NaOH + H2​

• The solution will have a high pH due to the formation of NaOH, a strong base, and the release of hydrogen gas, which may be observed as bubbles.

Magnesium’s Reaction with Cold Water:

-What is observed

-The reaction

-The pH of the product

• Magnesium reacts very slowly with cold water, forming a small amount of magnesium hydroxide and hydrogen gas.

• Mg + 2H_2​O → Mg(OH)₂​ + H2​

• Magnesium hydroxide (Mg(OH)2​), which is only sparingly soluble in water, producing a weakly alkaline solution with a lower pH than sodium hydroxide.

Magnesium’s Reaction with Steam:

-What is observed

-The reaction

• The reaction is vigorous, and magnesium burns with a bright white flame, indicative of high energy release.

• Mg + H2​O (g) → MgO +H2

Reactions,observation of sodium with oxygen.

-Sodium reacts vigorously with oxygen, producing a bright yellow flame and forming a white solid, sodium oxide.

-4Na + O2​ → 2Na2​O

What is the pH of Na2O in water

Sodium oxide dissolves in water to form a strongly alkaline solution of sodium hydroxide (pH approx 13-14).

Reactions,observation of magnesium with oxygen.

• Magnesium reacts vigorously with oxygen when heated, producing a bright white light and forming magnesium oxide, a white solid.

• 2Mg + O₂ ​→ 2MgO

pH of MgO in Water

Magnesium oxide is sparingly soluble in water, producing a weakly alkaline solution of magnesium hydroxide pH approx 9-10.

Reactions,observation of Aluminium with oxygen.

• Aluminium reacts slowly with oxygen in powdered form to produce aluminium oxide, a white solid.
  

• 4Al + 3O₂​ → 2Al_2​O₃

pH of Al₂O₃ in Water

Aluminium oxide is amphoteric, meaning it can react with both acids and bases, but it is insoluble in water and does not affect pH directly.

Reactions,observation of Silicon with oxygen.

• Silicon reacts slowly with oxygen when finely divided and heated, forming silicon dioxide, a white solid (the main component of sand).
  

• Si + O₂​ → SiO_2​

pH of SiO₂​ in Water

Silicon dioxide is insoluble in water and does not affect pH, but it behaves as an acidic oxide, reacting with strong bases.

Reactions,observation of phosphorus with oxygen.

Reaction:

Phosphorus combusts spontaneously in air with a very bright white flame, forming phosphorus(V) oxide (white smoke).

Equation:

P₄​ + 5O₂​ → P_​4O₁₀

P_​4O₁₀ pH in water

Phosphorus(V) oxide dissolves in water to form phosphoric acid (H_3​PO4​), resulting in a strongly acidic solution pH approx 1-2.

Reactions,observation of Sulfur with oxygen.

• Sulfur burns with a blue flame in oxygen, producing misty fumes of sulfur dioxide (a pungent gas).

• S + O₂ ​→ SO₂​
  

Write the equation for the reaction and state the catalyst needed for the further oxidation of SO₂

• Sulfur dioxide can be further oxidized to sulfur trioxide in the presence of a vanadium(V) oxide catalyst:

• 2SO₂​ + O₂ ​→ 2SO₃
  

pH of SO₂​ and SO₃​ in Water

• SO2​ dissolves in water to form sulfurous acid (H2​SO3​), resulting in an acidic solution pH approx 3-4.

• SO3​ dissolves in water to form sulfuric acid (H2​SO4​), creating a highly acidic solution pH approx 1.

Explain the bonding and melting point of metal oxides: Na₂​O, MgO, and Al₂O₃

-Bonding: These oxides have giant ionic structures with strong electrostatic forces between the metal cations and oxygen anions.

Melting Points:

• Na₂​O: Has a relatively high melting point due to ionic bonds between Na⁺ and O²⁻ ions.

• MgO: Higher melting point than Na₂​O because Mg²⁺ ions have a higher charge density, creating a stronger attraction to O²⁻ ions.

• Al₂O₃​: Despite having Al³⁺ ions, it has a slightly lower melting point than expected due to some covalent character introduced by the high charge density of Al³⁺, which polarizes the O²⁻ electron cloud.

Structure and Bonding of Silicon Dioxide (SiO₂)

• Structure: Silicon dioxide has a giant covalent structure similar to diamond, with each silicon atom covalently bonded to oxygen atoms in a 3D lattice.

• Melting Point: High melting point due to the energy required to break the strong covalent bonds throughout the structure.

Explain the bonding and melting point of non-metal oxides: P₄​O_10​ and SO₃

-Bonding: These oxides are composed of simple molecular structures held together by weak intermolecular forces (van der Waals forces and dipole-dipole interactions).

Melting Points:

• P₄​O₁₀​: Moderate melting point as it has a larger molecular size and higher molecular mass than SO^₃​, resulting in stronger van der Waals forces.

• SO₃​: Lower melting point due to weaker van der Waals forces in its smaller molecular structure.

Sodium Oxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Dissolves and reacts with water to form sodium hydroxide, a strong alkali.

• Equation:

Na_2​O + H₂​O → 2NaOH (aq)

• pH: The resulting solution is strongly alkaline, with a pH around 14.

Magnesium Oxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Reacts sparingly with water to form magnesium hydroxide, a weak alkali due to its low solubility.

• Equation:

-MgO + H₂​O → Mg(OH)₂​ (aq)

• pH: The solution is weakly alkaline, with a pH around 9.

Aluminium Oxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Insoluble in water and does not react due to strong ionic bonds.

• Equation:

• pH: No effect on pH, but aluminium oxide is amphoteric and can react with both acids and bases in other conditions.

Silicon Dioxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Insoluble in water and unreactive due to strong covalent bonds in its giant lattice structure.

• Equation:

• pH: No effect on pH. However, silicon dioxide can react with bases as it behaves as an acidic oxide.

Phosphorus(V) Oxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Reacts vigorously with water to produce phosphoric acid.

• Equation:

P_4​O₁₀ ​+ 6H₂​O → 4H₃​PO_4​

• pH: The solution is strongly acidic, with a pH around 0.

Sulfur Dioxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Dissolves to form sulfurous acid, a weak acid because the reaction is reversible.

• Equation:

SO2​ + H2​O ↔ H2​SO3​

• pH: The resulting solution is weakly acidic, with a pH around 3.

Sulfur Trioxide with water:

• Reaction

• Equation

• pH

• Reaction with Water: Reacts vigorously to produce sulfuric acid.

Equation:

SO₃ + H₂O → H₂SO₄

• pH: The solution is strongly acidic, with a pH around 0.

What anion is formed by acidic oxides (Phosphorus(V) Oxide)

• Forms phosphoric acid (H₃​PO₄​), dissociating into phosphate ions (PO₄³⁻​)
  

What anion is formed by acidic oxides (Sulfur Dioxide)

• Forms sulfurous acid (H₂​SO³​), dissociating into sulfite ions (SO₃²⁻​)

What anion is formed by acidic oxides (Sulfur Trioxide)

• Forms sulfuric acid (H_2​SO⁴​), dissociating into sulfate ions (SO₄²⁻​)
  

Na2​O + HCl

MgO + H2​SO4

• Na₂​O + 2HCl → 2NaCl + H2​O

• MgO + H₂SO₄ → MgSO₄ + H2O

SiO2​ + NaOH

SO3​ + NaOH

SiO₂​ + 2NaOH → Na_2​SiO_3​ + H₂​O

SO_3​ + 2NaOH → Na₂​SO_4​ + H_2​O

Al2​O3 ​+ HCl

Al2​O3​ + NaOH

Al_2​O_3​ + 6HCl → 2AlCl₃ + 3H₂​O

Al₂​O₃​ + 2NaOH + 3H_2​O → 2NaAl(OH)_4​