22.2 The oxides in elements of period 3

Question 1

Describe the melting points of the metal oxides eg sodium, magnesium and aluminium

Answer 1

. High melting points
They form giant ionic lattices where the bonding expands through the compound

Question 2

Why is the melting point of Al2O3 lower than MgO

Answer 2

Because aluminium forms a very small ion with a large positive charge.

This means it has some covalent character as well as ionic as the Al3+ ion can approach easily to the O2- and distort its electron cloud.

Ionic bonding is stronger than covalent due to the larger difference in electronegativity of the elements.

Question 3

Describe the overall trend in melting point of period 3 oxides

Answer 3

Melting point decreases overall, however MgO has the highest melting point as it is ionic so has strong electrostatic forces between Mg2+ and O2-. It is a smaller molecule also so has a higher charge density than Na2O

Question 4

How can you predict the ionic character of a bond

Answer 4

.Look at the differences in electronegativity- the bigger the difference the more ionic it is

Question 5

Describe the structure of silicon oxide

Why does it have a high melting point

Answer 5

it is a giant covalent (macromolecular) structure, and this bonding extends throughout the structure

These covalent bonds are very strong and require lots of energy to overcome these forces and melt it

Question 6

Describe the structure of the non metal oxides:

Phosphorus oxide (P4O10)
and sulfur

Why do they have low melting points

Answer 6

They are molecular

They both exist as separate covalently bonded molecules.
The phosphorus oxides are solids

The intermolecular forces are weak van der walls and dipole-dipole forces, so don’t require a lot of energy to overcome them

Question 7

Name and show the chemical formula of the oxides for the period 3 elements

Sodium oxide
Magnesium oxide
Aluminium oxide

Answer 7

Na2O

MgO

Al2O3

Question 8

Name and show the chemical formula of the oxides for the period 3 elements

Silicon

Phosphorus

and the two sulfur

Answer 8

SiO2

P4O10

SO2 and SO3

Question 9

What is the structure of phosphoric acid

Answer 9

H3PO4 is the formula

               O
              || H- - -O- - - P ---- O-----H
               |
               O
               |
               H

Question 10

Describe the reaction of the basic oxides with water

sodium oxide

Answer 10

Sodium oxide reacts with water to give sodium hydroxide solution which is strongly alkaline

Na2O + H2O(l) –> 2NaOH

And the PH is 14

NaOH is 2Na+(aq) + 2OH-(aq) and it is the OH- that makes it alkaline

Question 11

How does the basic oxide magnesium oxide react with water

Answer 11

It reacts with water to give a magnesium hydroxide solution which is sparingly soluble and produces a somewhat alkaline solution

MgO(s) + H2O(l) –> Mg(OH)2(s)
->
<- (reversible) Mg2+(aq) + 2OH-

PH of solution is 9

Question 12

What are the insoluble oxides

Answer 12

Aluminium oxide and silicon dioxide

Question 13

Describe the acidic oxides

Answer 13

The non metal elements on the right side of the periodic table make acidic solutions usually

Phosphorus and sulfur

They are covalent molecules and react with water to form acidic solutionas they release H+ ions

Question 14

How does phosphorus pentoxide react with water

Write the equation

What happens when the phosphuric acid ionises

Answer 14

It reacts violently to produce an acidic solution of phosphoric (v) acid which ionises so the solution is acidic

P4O10(s) + 6H20(l) –> 4H3PO4(aq)

H3PO4 –>(reversible) H+(aq) + H2PO4-

Question 15

How does sulfur dioxide react with water

Answer 15

Sulfur dioxide is fairly soluble in water and reacts to give an acidic solution of sulfuric(IV) acid (sufurous acid)
This partially dissociates to produce H+ ions which make the solution acidic

SO2(g) + H2O(l) –> H2SO3(aq)

H2SO3(aq) –> H+(aq) + HSO3-(aq)
<–

Question 16

How does sulfur trioxide react with water

Answer 16

. It reacts violently with water to produce sulfuric acid

SO3(g) + H2O(l) –> H2SO4(aq)

H2SO4(aq) –> H+(aq) + HSO4-(aq)
<–

Question 17

Why do the sodium and magnesium oxides make alkaline solution with water

Why is magnesiums solution with water less alkaline than sodiums

Answer 17

. They are both composed of ions so contain the O2- ion which is a very strong base as it attracts protons
. So it readily reacts with water to make hydroxide ions

Because it is less soluble so less OH- ions dissolve in solution

Question 18

Why are aluminium oxide and silicon dioxide insoluble in water

Answer 18

Aluminium oxide is ionic but the bonding in the ionic lattice is too strong to separate the ions.
Partly because of the covalent bonds it has

Silicon dioxide is a giant macromolecule and water will not affect this type of structure

Question 19

Whats the general trend of the oxides PH with water across the period

Answer 19

They go from alkaline to acidic