3.2.4 Period 3 elements and their oxides

Question 1

Write an equation for the following period 3 elements and their reactions with O2 (g):
a) Na
b) Mg
c) Al
d) Si
e) P
f) S
Include the colour of flame and a description of the product.

Answer 1

a) 4Na + O2 → 2Na2O (s)
bright yellow flame (white solid)

b) 2Mg + O2 → 2MgO (s)
bright white flame (white solid)

c) 4Al + 3O2 → 2Al2O3 (s)
bright white flame (white powder)

d) Si + O2 → SiO2 (s)
bright white flame (white powder)

e) 4P + 5O2 → P4O10 (s)
yellow/white flame (white clouds)

f) S + O2 → SO2 (g)
blue flame (toxic fumes)

Question 2

Write an equation for the reaction of sodium/magnesium and water.

Answer 2

2Na + 2H2O → 2NaOH + H2

Mg + H2O → MgO + H2
Mg + H2O → 2Mg(OH)2 + H2
(water is steam in order to produce mg hydroxide)

Question 3

How do the following period 3 oxides react with water?
Suggest the pH of each solution formed.
a) Na2O
b) MgO
c) Al2O3
d) SiO2
e) P4O10
f) SO2
g) SO3

Answer 3

a) Na2O + H2O → 2NaOH (pH 14)

b) MgO + H2O → Mg(OH)2 (pH 10)

c) does not react - insoluble in water

d) does not react - insoluble in water

e) P4O10 + 6H2O → 4H3PO4 (pH 2)

f) SO2 + H2O →H2SO3 (pH 1)

g) SO3 + H2O → H2SO4 (pH 1)

Question 4

Draw each of the following acids and show equations for each of their dissociations in aqueous solution:
a) H3PO4
b) H2SO3
c) H2SO4

Answer 4

a) H3PO4 → H2PO4- + H+
b) H2SO3 → HSO3- + H+
c) H2SO4 →HSO4- + H+

Question 5

What is the pattern of the reactions of period 3 oxides and water?

Answer 5

Ionic oxides = basic
Simple covalent oxides = acidic

Question 6

Explain why Na2O forms an alkaline solution when it reacts with water.

Answer 6

O2- + H2O → 2OH-
- Na2O contains O2- ions which react with the water to form OH- ions

Question 7

Describe how bonding changes across the period 3 oxides.

Answer 7

Na2O → MgO → Al2O3 → SiO2
Ionic bonding → slightly covalent → giant covalent

→ P4O10 → SO3 → SO2
→ simple molecular

Question 8

What is the general trend in melting points of period 3 oxides?
What is the exception to this rule?

Answer 8

L to R: melting points decrease across the period
- General change from ionic to covalent character

Exception from Na2O→MgO, where m.p increases
- Mg2+ has smaller ionic radius, larger nuclear charge and similar shielding compared to Na+
- Mg2+ has a larger ionic charge as well, stronger attraction between oxygen and magnesium ion

Question 9

Which period 3 oxides react as bases and how?

Answer 9

Na2O + 2HCl → 2NaCl + H2O
MgO + H2SO4 → MgSO4 + H2O
Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O

Question 10

Which oxides react as acids and how?

Answer 10

Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
SiO2 + 2NaOH → Na2SiO3 + H2O
P4O10 + 12NaOH → 4Na3PO4 + 6H2O
SO2 + 2NaOH → Na2SO3 + H2O
SO3 + 2NaOH →Na2SO4 + H2O

Question 11

What is special about aluminium oxide?

Answer 11

Amphoteric
- reacts as both an acid and base

Question 12

Write an ionic equation for the reaction of phosphorus oxide and NaOH.

Answer 12

P4O10 + 12OH- → 4PO4^3- + 6H2O