13.1. Period 3: Oxides and Chlorides

IB Chem HL

http://www.chemguide.co.uk/inorganic/period3/chlorides.html

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/per-hl.htm#third

• http://scienceaid.co.uk/chemistry/inorganic/period3.html

• http://www.chemicalelements.com/show/meltingpoint.html

WHAT ARE OXIDES??

Reaction of period 3 elements with oxygen• Sodium: Burns with a yellow flame4Na(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.

OxidesNa2O MgO Al2O3 SiO2 P4O10 SO3 Cl2O7 P4O6

P4O6 SO2 Cl2O

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

Cl-Chloride ion

Na+

Sodium ion

Oppositely charged ions held in a regular3-dimensional lattice by electrostatic attraction

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 DIOXIDEMELTING POINT VERY HIGH

many covalent bonds must be broken to separate the atoms

STRENGTH STRONGeach 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 acidP4O10 (s) + 6 H2O(l) =>4 H3PO4(aq) phosphoric acidCl2O7(l) + H2O(l) => 2HClO4(aq) perchloric acid

ChloridesNaCl 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 MgCl2Are ionic solids-conduct electricity when

molten and have high MP.

Both dissolve easily in water.

NaCl- aqueous solution is neutralMgCl- 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

AluminumThe reaction is relatively slow because of the existing

strong aluminium oxide layer on the metal, and the build-up of even more oxide during the reaction.

Al + H2O => Al2O3 + H2

SiliconThere is a fair amount of disagreement in the books and on

the web about what silicon does with water or steam.

Phosphorus and SulphurThese have no reaction with water