8.1 - 8.4 + 22.1 - 22.3 Periodicity

Question 1

What structure and bonding do each of the elements in period 3 have?

Answer 1

Na, Mg, Al - Giant metallic
Si - Giant covalent
P, S, Cl - Simple molecular
Ar - Simple atomic

Question 2

Explain the melting points across period 3

Answer 2

Increases Na to Al:
- Charge on positive ion and the number of delocalised electrons increases
- So metallic bond gets stronger

Si has the highest:
- Macromolecular structure has lots of strong covalent bonds to break

Decrease P to Ar:
- M.p determined by the size of Van der Waals forces
- S8 > P4 > Cl2 > Ar

Question 3

Explain the trend in atomic radius across period 3

Answer 3

Decreases across period because:
- Nuclear charge increases but no. of energy levels stays the same
- This means atomic radius gets smaller

Question 4

Explain the trend in first ionisation energy across period 3

Answer 4

Generally increases across period because:
- Nuclear charge increases but shielding stays the same
Exceptions:
- Decrease Mg to Al because Al outer electron is further away and more shielded in 3p orbital
- Decrease P to S because S has an electron pair repelling each other

Question 5

Describe the reaction of sodium (Na) with oxygen

Answer 5

• Burns vigorously; yellow flame
• Very hot
  4Na + O₂ → 2Na₂O

Question 6

Describe the reaction of magnesium (Mg) with oxygen

Answer 6

• Burns vigorously; white flame
• Ash formed
  2Mg + O₂ → 2MgO

Question 7

Describe the reaction of aluminium (Al) with oxygen

Answer 7

• Burns vigorously
• Often has protective oxide layer
• White flame
  4Al + 3O₂ → 2Al₂O₃

Question 8

Describe the reaction of silicon (Si) with oxygen

Answer 8

• Slow reaction that requires heat
  Si + O₂ → SiO₂

Question 9

Describe the reaction of phosphorus (P) with oxygen

Answer 9

• Red phosphorous must be heated
• White phosphorous reacts spontaneously
  P₄ + 5O₂ → P₄O₁₀

Question 10

Describe the reaction of sulfur (S) with oxygen

Answer 10

• Melts easily
• Produces a pungent choking gas
• Burns with a blue flame
  S + O₂ → SO₂
  2S + 3O₂ → 2SO₃

Question 11

Describe the reaction of sodium (Na) with water

Answer 11

• Reacts vigorously
• Fizzes rapidly
• Floats
• Melts due to heat
• Forms strongly alkaline solution
  2Na + 2H₂O → 2NaOH + H₂

Question 12

Describe the reaction of magnesium (Mg) with water

Answer 12

• Reacts slowly
• Sparingly soluble
• Forms weakly alkaline solution
  Mg + 2H₂O → Mg(OH)₂ + H₂

Question 13

Describe the reaction of magnesium (Mg) with steam

Answer 13

• More vigorous
• White light
  Mg + H₂O → MgO + H₂

Question 14

Describe the reaction of sodium oxide (Na₂O) with water

Answer 14

• Dissolves exothermically
• Solution formed = pH 13
• Oxide ion reacts with water and gains a H
  Na₂O + H₂O → 2NaOH

Question 15

Describe the reaction of magnesium oxide (MgO) with water

Answer 15

• Sparingly soluble
• Solution formed = pH 9
  MgO + H₂O → Mg(OH)₂

Question 16

Describe the reaction of aluminium oxide (Al₂O₃) with water

Answer 16

• Insoluble
• Solution formed = pH 7

Question 17

Describe the reaction of silicon oxide (SiO₂) with water

Answer 17

• Insoluble
• Solution formed = pH 7

Question 18

Describe the reaction of phosphorus oxide (P₄O₁₀) with water

Answer 18

• Soluble
• Solution formed = pH 1
• P₄O₁₀ disosciates fully and is triporotic so [H⁺] is increased and therefore the pH decreases
  P₄O₁₀ + 6H₂O → 4H₃PO₄

Question 19

Describe the reaction of sulphur dioxide (SO₂) with water

Answer 19

• Reasonably soluble
• Solution formed = pH 3
  SO₂ + H₂O → H₂SO₃

Question 20

Describe the reaction of sulphur trioxide (SO₃) with water

Answer 20

• Soluble
• Violent reaction
• Solution formed = pH 0
  SO₃ + H₂O → H₂SO₄

Question 21

What is the acidic/base nature of basic metal oxides?

Answer 21

Will react with acids

Question 22

Describe the reaction between sodium oxide and sulphuric acid

Answer 22

Na₂O + H₂SO₄ → Na₂SO₄ + H₂O

Question 23

Describe the reaction of magnesium oxide (MgO) and sulphuric acid

Answer 23

MgO + H₂SO₄ → MgSO₄ + H₂O

Question 24

What is the acidic/base nature of amphoteric metal oxides?

Answer 24

Will react with acids and bases

Question 25

Describe the reaction of aluminium oxide and HCl

Answer 25

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Al2O3 + 6H+ → 2Al3+ + 3H2O

Question 26

Describe the reaction of aluminium oxide and sodium hydroxide

Answer 26

Al₂O₃ + 2NaOH → 2NaAl(OH)₄ + 3H₂O

Al2O3 + 2OH− → 2Al(OH) +3H2O

Question 27

What is the acidic/base nature of non-metal oxides?

Answer 27

Will react with bases

Question 28

Describe the reaction of the non-metal oxide silicon oxide (SiO₂) and sodium hydroxide

Answer 28

• SiO₂ acts as a weak acid with strong bases
  SiO₂ + 2NaOH → 2Na₂SiO₃ + H₂O

Question 29

Phosphorus oxide (P₄O₁₀) becomes phosphoric acid (H₃PO₄) when dissolved in water. Describe the reaction of H₃PO₄ with sodium hydroxide.

Answer 29

Loses 3 protons so the reaction occurs in 3 stages:
H₃PO₄ + NaOH → NaH₂PO₄ + H₂O
NaH₂PO₄ + NaOH → Na₂HPO₄ + H₂O
Na₂HPO₄ + NaOH → Na₃PO₄ + H₂O

Overall:
H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O
or
P4O10 + 12NaOH → 4Na3PO4 + 6H2O

Question 30

Describe the reaction of the non-metal oxide sulphur dioxide (SO₂) and sodium hydroxide

Answer 30

SO₂ + NaOH → NaHSO₃
and then…
NaHSO₃ + NaOH → Na₂SO₃ + H₂O

Question 31

Describe the structure of the period 3 oxides

Answer 31

Na20, MgO, Al2O3 = Ionic
SiO2 = Macromolecular
P4O6, P4O10, SO2, SO3, Cl2O, Cl2O7 = Simple molecular

Question 32

What type of bonding do period 3 oxides Na -> Al have?

Answer 32

• Ionic
• Contain 02- ions
• O2- ions accept protons to form OH-

Question 33

What type of bonding is present in Aluminium oxide (Al₂O₃)?

Answer 33

Ionic

Question 34

Explain why the Van der Waals forces in liquid argon are very weak

Answer 34

• Argon particles are single atoms with electrons closer to the nucleus
• Cannot easily be polarised