Chemistry - 3.2.4: Properties of Period 3 Elements and their Oxides

Reaction of Na with water

2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

Reaction of Mg with cold water

Mg (s) + 2H₂O (l) → Mg(OH)₂ (aq) + H₂ (g)

How can you test whether a substance is ionic / contains ions? (2)

1. Heat until molten

2. It conducts electricity

What oxide does Na form?

Na₂O

What oxide does Mg form?

MgO

What oxide does Al form?

Al₂O₃

What oxide does Si form?

SiO₂

What oxide does P form?

P₄O₁₀

What oxide does S form?

SO₂ and SO₃

What period 3 elements react with cold water?

Na, Mg and Cl

Reaction of Mg with steam

Mg (s) + H₂O (g) → MgO (s) + H₂(g)

Reaction of Na with oxygen

2Na (s) + 0.5O₂ (g) → Na₂O (s)

Reaction of Mg with oxygen

Mg(s) + 0.5O₂ (g) → MgO (s)

Reaction of Al with oxygen

4Al (s) + 3O₂ (g) → 2Al₂O₃ (s)

What happens when Na reacts with oxygen?

It burns with a yellow flame to form a white powder of Na₂O

What happens when Mg reacts with oxygen? (2)

It burns with a white flame to form a white powder of MgO

What happens when Al reacts with oxygen?

It burns brightly to produce a white powder of Al₂O₃

Reaction of Si with oxygen

Si (s) + O₂ (g) → SiO₂ (s)

Reaction of P with oxygen

4P (s) + 5O₂ (g)→ P₄O₁₀ (s)

Reaction of S with oxygen

S (s) + O₂ (g) → SO₂ (g)

What happens when S reacts with oxygen?

It burns with a blue flame and produces fumes to form colourless SO₂ gas (and some SO₃)

Why can a thin layer of aluminium oxide protect aluminium from corrosion in (moist) air? (2)

1. It is inert

2. It is insoluble in water

How does electronegativity link to the type of bonding in the period 3 oxides?

If big difference in electronegativity → Ionic bonding

If small difference in electronegativity → Covalent bonding

Red phosphorus vs white phosphorus

Red phosphorus must be heated before it will react with oxygen - white phosphorus spontaneously ignites in air and the white smoke of phosphorous pentoxide is given off

Allotrope

The same element with different arrangements of atoms

2 allotropes of phosphorus

Red and white phosphorus

Trend in melting points of period 3 oxides

Increases from Na to Mg: greater charge, stronger ES FoA

Decreases Mg to Al: Al has some covalent character

Decreases Al to Si: Si has covalent bonds, weaker than ionic

Decreases Si to P: P has Van der Waal forces to break

Decreases to S: All have Van der Waal forces - melting point decreases as the number of electrons decreases

Why does Al₂O₃ have some covalent character?

The Al³⁺ ion is small but has a large positive charge (high charge density), so can distort the electron cloud of O^2-

Is Na₂O₃ an acid or base? Give its reaction with water and the pH of the resultant solution.

Base, pH of ~14

Na₂O (s) + H₂O (l) → 2Na⁺ (aq) + 2OH^- (aq)

Is MgO an acid or base? Give its reaction with water and the pH of the resultant solution.

Base, pH of ~9

MgO (s) + H₂O (l) → Mg(OH)₂ (s) ⇌ Mg²⁺ (aq) + O^2- (aq)

What period 3 oxides are insoluble in water?

Al₂O₃ and SiO₂

What period 3 oxides do not react with water?

Al₂O₃ and SiO₂

Is P₄O₁₀ an acid or base? Give its reaction with water and the pH of the resultant solution.

Acid, pH of ~1

P₄O₁₀ (s) + 6H₂O (l) → 4H₃PO₄ (aq)

H₃PO₄ (aq) ⇌ H⁺ (aq) + H₂PO₄^- (aq)

Is SO₂ an acid or base? Give its reaction with water and the pH of the resultant solution.

Acid, pH of ~2-3

SO₂ (s) + H₂O (l) → H₂SO₃ (aq)

H₂SO₃ (aq) ⇌ H⁺ (aq) + HSO₃^- (aq)

Is SO₃ an acid or base? Give its reaction with water and the pH of the resultant solution.

Acid, pH of ~0-1

SO₃ (s) + H₂O (l) → H₂SO₄ (aq) → H⁺ (aq) + HSO₄^- (aq)

Order the period 3 oxides according to their pH of the solution formed after reacting with water (descending)

1. Na₂O (14)

2. MgO (9)

3. Al₂O₃ / SiO₂ (7)

4. SO₂ (2-3)

5. P₄O₁₀ (1-2)

6. SO₃ (0-1)

Why does Na₂O have that pH?

O^2- ion is a very strong base so readily reacts with water to form OH^- ions

Why does MgO have that pH?

It forms O^2- ions but is less soluble in water than Na₂O

Why does Al₂O₃ have that pH?

It has some covalent character, so the bonding is too strong for the ions to separate

Why does SiO₂ have that pH?

It is a giant macromolecule so water does not affect it

Why does P₄O₁₀ have that pH?

It is a covalent molecule so can react with water to form acid solutions

Why does SO₂ have that pH?

It is a covalent molecule so can react with water to form acid solutions

Why does SO₃ have that pH?

It is a covalent molecule so can react with water to form acid solutions

Why is Al₂O₃ described as amphoteric?

It reacts with both acids and alkalis

What forms when Na₂O₃ reacts in an acid-base reaction?

A sodium salt and water

What forms when MgO reacts in an acid-base reaction?

A magnesium salt and water

What forms when Al₂O₃ reacts in an acid-base reaction?

When acting as a base: an aluminium salt and water

When acting as an acid: an aluminate salt [MAl(OH)₄] and water

What forms when SiO₂ reacts in an acid-base reaction?

A silicate salt [M₂(SiO₃)] and water

What forms when P₄O₁₀ reacts in an acid-base reaction?

In 3 stages, it forms MH₂PO₄ and water, M₂HPO₄ and water. then M₃PO₄ and water (assuming M has +1 charge)

What forms when SO₂ reacts in an acid-base reaction?

In 2 stages, first MHSO₃ and water, then M₂SO₃ and water (assuming M has +1 charge)

How can you determine the melting point of a sample? (3)

1. Put sample in melting point apparatus

2. Heat it slowly to establish the melting point range

3. If the melting point is close to the data book value / melts sharply, the sample is pure. If it has a lower melting point / melts over a broad range of temperatures, the sample is impure.

How can you distinguish between two different solid period 3 oxides? [e.g. Na and P oxides] (3)

1. React each sample with water

2. Add litmus paper

3. Litmus: turns blue with alkali solution and turns red with acidic solution

Why does Na2O form an alkaline solution when it reacts with water? (2)

1. Na2O contains O2- ions

2. These O2- ions react with water to form OH- ions

Give 2 equations that show the amphoteric nature of Al2O3

Al2O3 + 6HCl → 2Al3+ + 6Cl- + 3H2O

Al2O3 + 6H+ → 2Al3+ + 3H2O

Why can’t Al₂O₃ / SiO₂ dissolve in water?

Strong covalent bonds - water cannot supply enough energy to break lattice

Reaction of SiO2 with NaOH

2NaOH + SiO2 → Na2SiO3 + H2O