[3.2.4] Properties of Period 3 Elements & their Oxides

Question 1

Describe the conditions/observations and the equation for the reaction between Na and water.

Answer 1

• Sodium reacts with cold water. It fizzes around on the surface.
• 2 Na (s) + 2 H₂O (l) -> 2 NaOH (aq) + H₂ (g)

Question 2

Describe the conditions/observations and the equation for the reaction between Na and oxygen.

Answer 2

• Sodium urns with a yellow flame to produce a white solid.
• 4Na (s) + O₂ (g) -> 2Na₂O (s)

Question 3

Describe the conditions/observations and the equation for the reaction between Mg and water.

Answer 3

• Magnesium reacts very slowly with cold water to form the
  hydroxide.
  -2 Mg (s) + 2 H₂O (l) -> 2 MgOH (aq) + H₂ (g)

Question 4

Describe the conditions/observations and the equation for the reaction between Mg and steam.

Answer 4

• Magnesium reacts readily with steam to form the oxide.
• Mg (s) + H₂O (g) -> MgO (s) + H₂ (g)

Question 5

Describe the conditions/observations and the equation for the reaction between Mg and oxygen.

Answer 5

• Magnesium burns with a white flame to give a white solid smoke.
• 2Mg (s) + O₂ (g) -> 2MgO (s)

Question 6

Describe the conditions/observations and the equation for the reaction between Al and oxygen.

Answer 6

• Magnesium burns with a white flame to give a white solid smoke.
• 4Al (s) + 3O₂ (g) -> 2Al₂O₃ (s)

Question 7

Describe the conditions/observations and the equation for the reaction between Si and oxygen.

Answer 7

• Silicone burns with a white flame to give a white solid smoke.
• Si (s) + O₂ (g) -> SiO₂ (s)

Question 8

Describe the conditions/observations and the equation for the reaction between P and oxygen.

Answer 8

• Phosphorus burns with a white flame to give a white solid smoke.
• 4P (s) + 5O₂ (g) -> P₄O₁₀ (s)

Question 9

Describe the conditions/observations and the equation for the reaction between S and oxygen.

Answer 9

• Sulfur burns with a blue flame to form an acidic choking gas.
• S (s) + O₂ (g) -> SO₂ (g)

Question 10

Describe and explain the melting points of the oxides of the elements Na-S.

Answer 10

MACROMOLECULAR OXIDES

• SiO₂ is macromolecular.
  
  • It has many very strong covalent bonds between atoms.
  • High energy is needed to break many strong covalent bonds therefore, it has a very high melting and boiling point.

IONIC OXIDES

• The metal oxides (Na₂O, MgO, Al₂O₃) are ionic.
• They have giant ionic lattice structures: strong forces of attraction between oppositely charged ions.
  
  • Therefore, they have high melting points.
    
    • The increased charge on the cation makes the ionic forces stronger going from Na to Al so leading to increasing melting points.

SIMPLE MOLECULAR OXIDES

• P₄O₁₀ and SO₂ are simple molecular with weak intermolecular forces between molecules (van der waals + permanent dipoles) so have lower melting points.
• P₄O₁₀ is a molecule containing 4P’s and 10 O’s. As it is a bigger molecule and has more electrons than SO₂, it will have large vdW between molecules and thus a higher melting point.

Question 11

Describe the equation for the reaction between Na₂O and water. What is the pH of the resulting solution?

Answer 11

• Na₂O (s) + H₂O (I) -> 2Na⁺ (aq) + 2OH⁻ (aq).
• pH 13 - this is a vigorous exothermic reaction.

Question 12

Describe the equation for the reaction between MgO and water. What is the pH of the resulting solution?

Answer 12

• MgO (s) + H₂O (I) -> Mg(OH)₂ (s).
• pH 9 - Mg(OH)₂ is only slightly soluble in water as its lattice is stronger so fewer free OH⁻ ions are produced and so lower pH.

Question 13

Describe and explain the reaction between Al₂O₃ and water and SiO₂ and water. What is the pH of the resulting solutions?

Answer 13

• Al₂O₃ and SiO₂ do not dissolve in water because of the high strength of the Al₂O₃ ionic lattice and the SiO₂ macromolecular structure.
• Therefore, they give a neutral pH 7.

Question 14

Describe the equation for the reaction between P₄O₁₀ and water. What is the pH of the resulting solution?

Draw the structure of the acid and describe the ionic equation that shows the formation of the anion when P₄O₁₀ reacts with water.

Answer 14

• P₄O₁₀ (s) + 6H₂O (I) -> 4H₃PO₄ (aq)
• pH 0 - this is a vigorous exothermic reaction.
• P₄O₁₀ + H₂O -> H⁺ + HPO₄⁻

Question 15

Describe the equation for the reaction between SO₂ and water. What is the pH of the resulting solution?

Draw the structure of the acid and describe the ionic equation that shows the formation of the anion when SO₂ reacts with water.

Answer 15

• SO₂ (g) + H₂O (I) -> H₂SO₃ (aq)
• pH 3 (weak acid).
• SO₂ + H₂O -> H⁺ + HSO₃⁻

Question 16

Describe the equation for the reaction between SO₃ and water. What is the pH of the resulting solution?

Draw the structure of the acid and describe the ionic equation that shows the formation of the anion when SO₃ reacts with water.

Answer 16

• SO₃ (g) + H₂O (I) -> H₂SO₄ (aq)
• pH 0 .
• SO₃ + H₂O -> H⁺ + HSO₄⁻