13.1. Period 3: Oxides and Chlorides
IB Chem HL
http://www.chemguide.co.uk/inorganic/period3/chlorides.h
tml
Sites
• http://martinbrakkeibchem.escuelacampoalegre
.wikispaces.net/file/view/T13D01+-+10.31.11++13.1+HL+Trends+Across+Period+3.pdf
• http://ibchem.com/IB/ibnotes/brief/perhl.htm#third
• http://scienceaid.co.uk/chemistry/inorganic/perio
d3.html
• http://www.chemicalelements.com/show/melting
point.html
WHAT ARE OXIDES??
Reaction of period 3 elements with oxygen
• Sodium: Burns with a yellow flame
4Na(s) + O2(g) => 2Na2O(s)
• Magnesium: Burns with a brilliant white
flame.
2Mg (s) + O2 (g) => 2MgO (s)
• Aluminium: The aluminium needs to be
finely divided.
4Al(s) + 3O2(g) “=> 2Al2O3 (s)
• Silicon:
Si(s) + O2(g) => SiO2 (s)
• Phosphorus:
4P(s) + 5O2(g) => P4O10 (s)
• Sulphur: Burns with a brilliant blue flame.
S(s) + O2(g) “=> SO2(g)
• Chlorine does not react directly with
oxygen: Cl2O7 , Cl2O
• These slides explain the relationship
between the physical properties of the
oxides of Period 3 elements (sodium to
chlorine) and their structures.
• Argon is obviously omitted because it
doesn't form an oxide.
Oxides
Na2O
MgO
Al2O3
SiO2
P4O10
P4O6
SO3
SO2
Cl2O7
Cl2O
P4O6
Those oxides in the top row are known as the highest
oxides(ox#) of the various elements.
.
The oxides of Na,Mg and Al are ionic: high
MP and electrical conductivity when
molten.
• Silicon dioxide has a diamond like
macromolecular structure with high BP.
• The other oxides all have a small
difference in EM between the elements
resulting in simple covalent molecular
structures with low MP and BP.
• The acid-base properties are also linked to
their structure.
WHY??
Metals: Na, Mg
– ionic solids
– Basic oxides
– react with water to make alkaline solution
Na2O(s) + H2O(l)  2Na+(aq) + 2OH-(aq)
MgO(s) + H2O(l)  Mg(OH)2(aq)
Lattice Structure of Ionic Compounds
Oppositely charged ions held in a regular
3-dimensional lattice by electrostatic attraction
ClChloride ion
Na+
Sodium ion
IONS HELD STRONGLY IN IONIC LATTICE
• Metalloids: Al
– “giant covalent lattices”
– high melting/boiling points
– insoluble in water
– Al2O3 is amphoteric (acts as an acid or base)
As a base:
Al2O3(s) + 6HCl(aq)  2AlCl3(aq) + 3H2O(l)
As an acid:
Al2O3(s) + 2NaOH(aq)  NaAl(OH)4(aq)
sodium aluminate
• Silicon dioxide is giant covalent , containing
single Si-O bonds.It behaves as a weak acid.
• A molecular structure would contain Si=O
bonds; these would be much weaker than
two of the Si-O single bonds found in the
silica lattice. SiO2 is insoluble in water but
reacts with fused NaOH:
SiO2(s) + 2NaOH(l) => Na2SiO3(l) + H2O(g)
SILICON DIOXIDE
MELTING POINT
VERY HIGH
many covalent bonds must be broken to separate the atoms
STRENGTH
STRONG
each silicon atom is joined to four oxygen atoms
each oxygen atom are joined to two silicon atoms
ELECTRICAL
NON-CONDUCTOR - no mobile electrons
Silica, which is found in sand, has a similar
structure to diamond. It is also hard and has
a high melting point, but contains silicon
and oxygen atoms, instead of carbon atoms.
The fact that it is a semi-conductor makes it
immensely useful in the electronics industry:
most transistors are made of silica.
• The oxides of P,S and Cl are all strongly
acidic:
SO2(g) + H2O(l) => H2SO3 sulfurous acid
P4O10 (s) + 6 H2O(l) =>4 H3PO4(aq) phosphoric acid
Cl2O7(l) + H2O(l) => 2HClO4(aq) perchloric acid
Chlorides
NaCl
MgCl2
AlCl3
SiCl4
PCl5
S2Cl2
PCl3
• There are three chlorides of sulphur, but the
only one mentioned is S2Cl2.
• As you will see , aluminium chloride exists in
some circumstances as a dimer, Al2Cl6.
Electronegativity increases as you go across
the period and, by the time you get to
aluminium, there isn't enough
electronegativity difference between
aluminium and chlorine for there to be a
simple ionic bond.
http://www.chemguide.co.uk/inorganic/period3/chlorides.html
CHLORIDES
Chlorides of Period 3
NaCl and MgCl2
Are ionic solids-conduct electricity when
molten and have high MP.
Both dissolve easily in water.
NaCl- aqueous solution is neutral
MgCl- aqueous solution is weakly acidic
Hydration and Hydrolysis
• Before considering the reactions that
occur with oxides and chlorides in water,
we must first consider the effect of two
processes, hydration and hydrolysis.
Video:
http://www.wou.edu/las/physci/ch412/hydrolysis.htm
Hydration and hydrolysis
When sodium chloride dissolves in water, the sodium and
chloride ions and the polar water molecules are strongly
attracted to one another by ion-dipole interactions. The
solvent molecules (water in this case) surround the ions
removing them from the crystal and forming the solution.
As the dissolving process proceeds, the individual ions are
removed from the solid surface becoming completely
separate, hydrated species in the solution.
Na+(aq) + 6H2O → [Na(H2O)6]2+(aq)
Hydrolysis
• Metal ions in aqueous solution behave as Lewis
acids. The positive charge on the metal ion
draws electron density from the O-H bond in the
water. This increases the bond's polarity making
it easier to break. When the O-H bond breaks,
an aqueous proton is released producing an
acidic solution.
• If the radius of the ion is small and ionic charge
large, hydrolysis also occurs. Hydrolysis is a
reaction in which the O-H bonds in water are
broken.
For example:
[Mg(H2O)6]2+(aq) + H2O (l) → [Mg(OH)(H20)5]+(aq) + H3O+(l)
This explains why aqueous magnesium
chloride is slightly acidic.
• All the other chlorides including aluminum
chloride react vigorously with water to
produce acidic solutions of hydrochloric
acid together with fumes of hydrogen
chloride.
2AlCl3(g) + 3H2O(l) => Al2O3(s)+6HCl(aq)
SiCl4(l) + 4H2O(l) => Si(OH)4(aq) + 4HCl(aq)
PCl3(l) + 3H2O(l) => H3PO3(aq) + 3HCl(aq)
Cl2(aq) + H2O(l) => HCl(aq) + HClO(aq)
Aluminum Chloride
• Aluminium chloride is complicated by the way its
structure changes as temperature increases.
• In the solid state at 0ºC there is considerable
evidence that it is ionic, but at room temperature
it seems to be covalent. At higher temperatures
it sublimes as a dimer with the formula Al2Cl6.
• At room temperature, solid aluminium chloride
has an ionic lattice with a lot of covalent
character.
• At temperatures around 180 - 190°C (depending
on the pressure), aluminium chloride coverts to
a molecular form, Al2Cl6. This causes it to melt
or vaporise because there are now only
comparatively weak intermolecular attractions.
• As the temperature increases a bit more, it
increasingly breaks up into simple AlCl3
molecules.
Reactions of period 3
•
•
http://ibchem.com/IB/ibnotes/brief/per-hl.htm#third
http://www.chemguide.co.uk/inorganic/period3/elementsreact.html#top
Reactions with water:
Sodium has a very exothermic reaction with cold water
producing hydrogen and a colourless solution of sodium
hydroxide.
2 Na(s) + H2O => 2 NaOH + H2
Magnesium has a very slight reaction with cold water, but
burns in steam
Aluminum
The reaction is relatively slow because of the existing
strong aluminium oxide layer on the metal, and the buildup of even more oxide during the reaction.
Al + H2O => Al2O3 + H2
Silicon
There is a fair amount of disagreement in the books and on
the web about what silicon does with water or steam.
Phosphorus and Sulphur
These have no reaction with water