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13.1. Period 3: Oxides and Chlorides

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1 13.1. Period 3: Oxides and Chlorides
IB Chem HL

2 Sites


4 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)

5 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

6 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.

7 Oxides Na2O MgO Al2O SiO P4O SO Cl2O P4O6 P4O SO 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.

8 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.

9 The acid-base properties are also linked to their structure.

10 Metals: Na, Mg Na2O(s) + H2O(l)  2Na+(aq) + 2OH-(aq)
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)

11 Lattice Structure of Ionic Compounds
Oppositely charged ions held in a regular 3-dimensional lattice by electrostatic attraction Cl- Chloride ion Na+ Sodium ion IONS HELD STRONGLY IN IONIC LATTICE

12 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

13 Silicon dioxide is giant covalent , containing single Si-O bonds
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)

14 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.

15 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

16 Chlorides NaCl MgCl AlCl SiCl PCl S2Cl 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.

17 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.


19 Chlorides of Period 3

20 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

21 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:

22 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)

23 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.

24 This explains why aqueous magnesium chloride is slightly acidic.
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.

25 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)

26 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.

27 At room temperature, solid aluminium chloride has an ionic lattice with a lot of covalent character.
At temperatures around °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.





32 Reactions of period 3 Reactions with water:
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

33 Aluminum The 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 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


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