3.2.4 Properties of Period 3 elements and their oxides

Question 1

Why is sodium more reactive than magnesium?

Answer 1

Sodium loses one electron to form Na⁺, while magnesium loses two to form Mg²⁺.
It takes less energy to remove one electron than two, so sodium loses electrons more readily and is more reactive.

Question 2

Describe sodium’s reaction with cold water.
(give an equation)

Answer 2

• Reacts Vigorously, producing a molten sodium ball that floats on the surface of water.
• Fizzing - hydrogen produced.
• Strongly alkaline solution formed
  2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Question 3

Describe magnesium’s reaction with cold water.
(give an equation)

Answer 3

• Mg reacts slowly
• Forming a weakly alkaline solution of Mg(OH)₂, which is only sparingly soluble.
  Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

Question 4

How does magnesium react differently with steam?
(Give an equation)

Answer 4

Magnesium reacts faster & more vigorously with steam
Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Question 5

What is the general trend in oxidation state across Period 3 oxides?

Answer 5

The oxidation state of the element in its oxide increases from left to right across the period.

Question 6

What is the formula, observation and oxidation state of sodium in sodium oxide?

Answer 6

Formula: Na₂O
Observe a yellow flame
Oxidation state: +1

Question 7

How does magnesium react with oxygen?
(Give an equation)

Answer 7

Burns with a bright white flame to form white MgO powder.
Equation:
2Mg(s) + O₂(g) → 2MgO(s)

Question 8

What is aluminium’s oxide formula and how does it react with oxygen?
(Give an equation)

Answer 8

Al₂O₃; aluminium powder burns brightly when heated in oxygen.
Equation:
4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Question 9

What oxide does silicon form and how?
(Give an equation)

Answer 9

Silicon dioxide (SiO₂) is formed when silicon is strongly heated in oxygen.
Equation:
Si(s) + O₂(g) → SiO₂(s)

Question 10

Describe phosphorus’ reaction with oxygen.

Answer 10

White phosphorus spontaneously combusts in air, forming white smoke of phosphorus pentoxide (P₄O₁₀).
Equation:
4P(s) + 5O₂(g) → P₄O₁₀(s)

Question 11

What happens when sulfur burns in oxygen?
(Give two equations)

Answer 11

It burns with a blue flame forming SO₂. With a catalyst and high temperatures, SO₃ gas is also formed.
Equations:
S(s) + O₂(g) → SO₂(g)
S(s) + 1.5O₂(g) → SO₃(g)

Question 12

What type of bonding and structure do Na₂O and MgO have?

Answer 12

Ionic bonding between ions with a giant ionic lattice structures, due to large electronegativity differences between metal and oxygen atoms = strong ionic bonds

Question 13

What is unique about the bonding in Al₂O₃?

Answer 13

Al₂O₃ is ionic but has covalent character due to the small, highly charged Al³⁺ ion distorting the electron cloud of O²⁻.
Giving a partial covalent bond between Al and O

Question 14

What structure does SiO₂ have?

Answer 14

SiO₂ forms a macromolecular structure
with strong covalent bonds extending in all directions.

Question 15

What type of structures do P₄O₁₀, SO₂ and SO₃ have?

Answer 15

They are simple molecular compounds with molecules held together by weak intermolecular forces.

Question 16

Why do Na₂O, MgO, and Al₂O₃ have high melting points?
(above 2000 degrees)

Answer 16

Their lattice structures are held together by strong electrostatic forces between oppositely charged ions.

Question 17

Why does MgO have a higher melting point than Na₂O?

Answer 17

Mg²⁺ is smaller and has higher charge density than Na⁺, creating stronger attractions in the MgO lattice.

Question 18

Why does Al₂O₃ have a lower melting point than MgO?

Answer 18

Al₂O₃ has a lower melting point because the ionic bonding in Al₂O₃ has some covalent character
So electrostatic forced between ions are weaker

Question 19

Why does SiO₂ have a high melting point?
(lower than Al₂O₃)

Answer 19

It contains many strong covalent bonds in its macromolecular structure that require a lot of energy to break.

Question 20

Why do P₄O₁₀, SO₂ and SO₃ have low melting points?

Answer 20

They are simple molecular and have only weak intermolecular forces to overcome when melting.
(vdw forces between all and permanent dipole-dipole between P₄O₁₀ and SO₂).

Question 21

Why is P₄O₁₀’s melting point higher than SO₃’s?

Answer 21

P₄O₁₀ has larger molecules with** more electrons**, leading to stronger van der waals forces.

Question 22

What is the general trend in oxides solutions across Period 3?

Answer 22

Solutions become less alkaline and more acidic from left to right across the period

Question 23

How do Na₂O and MgO react with water?
(Give two equations & rough pH)

Answer 23

They dissolve and form alkaline solutions due to oxide ions reacting with water to form OH⁻.

Na₂O + H₂O → 2NaOH (pH 12–14)

MgO + H₂O → Mg(OH)₂ (pH ≈10)

Question 24

Why don’t Al₂O₃ and SiO₂ react with water?

Answer 24

The bonding in Al₂O₃ is partially ionic and covalent so it is insoluble
SiO₂ has a giant covalent structure, also making it insoluble.

Question 25

What happens when P₄O₁₀ reacts with water? (Give an equation)

Answer 25

Phosphorus pentoxide reacts **vigorously** to form phosphoric (V) acid, producing an acidic solution - **pH 1–2**. **P₄O₁₀ + 6H₂O → 4H₃PO₄**

Question 26

How do SO₂ and SO₃ react with water? (Give two equations)

Answer 26

They dissolve and form acidic solutions: **SO₂ + H₂O → H₂SO₃** (sulfurous acid, **pH 2–3**) **SO₃ + H₂O → H₂SO₄** (sulfuric acid, **pH 0–1**)

Question 27

Which Period 3 oxides are **alkaline**?

Answer 27

Sodium oxide (Na₂O) Magnesium oxide (MgO)

Question 28

Which Period 3 oxides are **acidic**?

Answer 28

Silicon dioxide (SiO₂) Phosphorus pentoxide (P₄O₁₀) Sulfur dioxide (SO₂), and sulfur trioxide (SO₃)

Question 29

Which Period 3 oxide is amphoteric and what does that mean?

Answer 29

Aluminium oxide (Al₂O₃) is amphoteric - it reacts with both acids and bases.

Question 30

Write the balanced equation for sodium oxide reacting with sulfuric acid. (include state symbols)

Answer 30

Na₂O(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l)

Question 31

How does aluminium oxide react with hydrochloric acid?

Answer 31

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)

Question 32

How does aluminium oxide react with hot, concentrated sodium hydroxide?

Answer 32

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

Question 33

How does silicon dioxide react with sodium hydroxide to form a silicate salt?

Answer 33

SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)

Question 34

What is the full reaction pathway for phosphorus pentoxide reacting with NaOH? (Give the two equations)

Answer 34

1. P₄O₁₀ + 6H₂O → 4H₃PO₄ 2. H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O

Question 35

What is the overall combined/fully neutralised equation for the reaction of P₄O₁₀ with NaOH?

Answer 35

P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O

Question 36

How does sulfur dioxide react with sodium hydroxide? (Give an equation)

Answer 36

SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)

Question 37

How does sulfur trioxide react with sodium hydroxide? (Give an equation)

Answer 37

SO₃(g) + 2NaOH(aq) → Na₂SO₄(aq) + H₂O(l)