Transcript Periodicity in Period 3

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The period 3 elements
The eight period 3 elements are found in the third row of the
periodic table.
1
2
3
4
5
6
7
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Physical properties of period 3
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What properties?
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Reactions with water
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Reactions with water: summary
Element Description
Equation
Mg
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
slow with cold water; Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g)
vigorous with steam Mg(s) + H2O(g) → MgO(s) + H2(g)
Al
no reaction
–
Si
no reaction
–
P
no reaction
–
S
no reaction
–
Cl
dissolves to form
chlorine water
Cl2(aq) + H2O(l)
Ar
no reaction
–
Na
vigorous
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HClO(aq) + HCl(aq)
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Reactions with water: redox
The reactions with water are all redox reactions.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
0
+1 -2
+1 -2 +1
0
Mg(s) + H2O(g) → MgO(s) + H2(g)
0
+1 -2
Cl2(aq) + H2O(l)
0
+1 -2
+2 -2
0
HClO(aq) + HCl(aq)
+1 +1 -2
+1 -1
The metals are oxidised and their oxidation state increases.
The hydrogen is reduced and its oxidation state decreases.
The chlorine is both oxidized and reduced.
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Reactions with oxygen
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Reactions with oxygen: summary
Element Description
Equation
Na
burns vigorously with a yellow
flame
4Na(s) + O2(g) → 2Na2O(s)
Mg
burns vigorously with a bright
white flame
2Mg(s) + O2(g) → 2MgO(s)
Al
burns vigorously with a bright
white flame
4Al(s) + 3O2(g) → 2Al2O3(s)
Si
burns with a bright white
flame and white smoke
Si(s) + O2(g) → SiO2(s)
P
burns spontaneously with a
bright white flame and smoke
4P(s) + 5O2(g) → P4O10(s)
S
burns with a blue flame
S(s) + O2(g) → SO2(g)
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Reactions with oxygen: redox
The reactions of the period 3 elements with oxygen are redox
reactions. In each reaction, the oxidation state of the
elements increases and the oxidation state of the oxygen
decreases.
For example, when sodium is burned in oxygen the oxidation
state of the sodium increases from 0 to +1 (oxidation), while
the oxidation state of the oxygen decreases from 0 to -2
(reduction).
4Na(s) + O2(g) → 2Na2O(s)
0
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0
+1 -2
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What’s the oxidation number?
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Reactions: summary
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Properties of period 3 oxides
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Reaction of period 3 oxides with water
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Reaction of sulfur oxides with water
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Period 3 oxides and water: summary
Oxide Bonding
Ions present
after reaction
Type of solution
pH
Na2O ionic
Na+(aq), OH-(aq)
strongly alkaline
13–14
MgO
ionic
Mg2+(aq), OH-(aq)
moderately alkaline 10
Al2O3
ionic/covalent – (insoluble)
–
7
SiO2
covalent
– (insoluble)
–
7
P4O10 covalent
H+(aq), H2PO4-(aq) strongly acidic
0–1
SO2
covalent
H+(aq), HSO3-(aq)
weakly acidic
2–3
SO3
covalent
H+(aq), HSO4-(aq)
strongly acidic
0–1
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Alkaline oxides
The reaction of period 3 oxides with water can be explained
by examining their bonding and structure.
Sodium and magnesium oxides are ionic compounds. They
both contain the oxide ion (O2-), which is a very strong base.
It reacts readily with water to produce hydroxide ions (OH-)
and a strongly alkaline solution.
Na2O(s) + H2O(l) → 2Na+(aq) + 2OH-(aq)
MgO(s) + H2O(l) → Mg2+(aq) + 2OH-(aq)
Magnesium oxide produces a less alkaline solution than
sodium oxide due to its lower solubility.
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Insoluble oxides
Aluminium oxide is an ionic compound but the bonds between
aluminium and oxygen ions display some covalent character.
This is because the difference in electronegativity
between aluminium (1.6) and oxygen (3.4) is less than for
sodium (0.9) and magnesium (1.3).
Silicon dioxide is a giant
macromolecule, with its
atoms covalently bonded
together.
Both these oxides are
insoluble in water.
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Acidic oxides
Phosphorus oxides and sulfur oxides are covalent molecules
and react with water to form acidic solutions:
P4O10(s) + 6H2O(l) → 4H3PO4(aq)
4H+(aq) + 4H2PO4-(aq)
SO2(g) + H2O(l) → H2SO3(aq)
H+(aq) + HSO3-(aq)
SO3(s) + H2O(l) → H2SO4(aq)
H+(aq) + HSO4-(aq)
The general trend is that oxides change from alkaline to
acidic across period 3.
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pH of period 3 oxide solutions
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Period 3 oxides and water: true or false?
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Reaction with acids: Na2O, MgO
Both sodium oxide and magnesium oxide react with acids to
produce salt and water. For example:
Sodium oxide reacts with hydrochloric acid to produce sodium
chloride:
Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l)
Magnesium oxide reacts with sulfuric acid to produce
magnesium sulfate:
MgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l)
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Reaction with acids and bases: Al2O3
Aluminium oxide reacts with both acids and bases. It is an
amphoteric oxide. For example:
Aluminium oxide reacts with sulfuric acid to produce
aluminium sulfate:
Al2O3(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2O(l)
Aluminium oxide reacts with hot concentrated sodium
hydroxide to produce sodium aluminate:
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
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Reaction with bases: SiO2
Silicon dioxide reacts as a weak acid with strong bases.
For example, it reacts with hot concentrated sodium
hydroxide to produce sodium silicate:
SiO2(s) + 2NaOH(aq) → Na2SiO3(aq) + H2O(l)
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Reaction with bases: P4O10
The reaction of phosphorus(V) oxide with alkalis is really the
reaction of phosphoric(V) acid, formed in a reaction with water.
Phosphoric(V) acid is tribasic – it can donate 3H+ ions – so the
reaction with sodium hydroxide has three steps:
H3PO4(aq) + NaOH(aq) → NaH2PO4(aq) + H2O(l)
NaH2PO4(aq) + NaOH(aq) → Na2HPO4(aq) + H2O(l)
Na2HPO4(aq) + NaOH(aq) → Na3PO4(aq) + H2O(l)
The overall reaction is:
3NaOH(aq) + H3PO4(aq) → Na3PO4(aq) + 3H2O(l)
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Reaction with bases: SO2
Sulphur dioxide reacts with sodium hydroxide to first
produce sodium hydrogensulfate(IV):
SO2(aq) + NaOH(aq) → NaHSO3(aq)
A further reaction occurs to produce sodium sulfate(IV):
NaHSO3(aq) + NaOH(aq) → Na2SO3(aq) + H2O(l)
The overall pattern is that the basic metal oxides react with
acids to form salts and the acidic non-metal oxides react
with bases to form salts.
The oxides of elements in the middle of the period are
amphoteric and show both acidic and basic properties.
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Reaction with acids and bases
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Glossary
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What’s the keyword?
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Period 3 trends: summary
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Multiple-choice quiz
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