Reactions of the Period 3 Elements (Cambridge (CIE) A Level Chemistry)
Revision Note
Reactions of the Period 3 Elements
Reactions with oxygen & chlorine
Reaction of Period 3 elements with oxygen table
| Chemical equation | Reaction conditions | Reaction | Flame | Product |
|---|---|---|---|---|---|
Na | 4Na (s) + O2 (g) → 2Na2O (s) | Heat | Vigorous | Bright yellow | White solid |
Mg | 2Mg (s) + O2 (g) → 2MgO (s) | Heat | Vigorous | Bright white | White solid |
Al | 4Al (s) + 3O2 (g) → 2Al2O3 (s) | Powdered Al | Fast | Bright white | White powder |
Si | Si (s) + O2 (g) → SiO2 (s) | Powdered Si Heat strongly | Slow | Bright white sparkles | White powder |
P | 4P (s) + 5O2 (g) → P4O10 (s) | Heat | Vigorous | Yellow or white | White clouds |
S | S (s) + O2 (g) → SO2 (g) | Powdered S Heat | Gentle | Blue | Toxic fumes |
Reaction of Period 3 elements with chlorine table
| Chemical equation | Reaction conditions | Reaction |
|---|---|---|---|
Na | 2Na (s) + Cl2 (g) → 2NaCl (s) | Heat | Vigorous |
Mg | Mg (s) + Cl2 (g) → MgCl2 (s) | Heat | Vigorous |
Al | 2Al (s) + 3Cl2 (g) → Al2Cl6 (s) | Heat | Vigorous |
Si | Si (s) + 2Cl2 (g) → SiCl4 (l) | Heat | Slow |
P | 2P (s) + 5Cl2 (g) → 2PCl5 (l) | Heat Excess chlorine | Slow |
Reaction of sodium with water
Sodium reacts vigorously with cold water:
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
The sodium melts into a ball and moves across the water surface until it disappears
Hydrogen gas is given off
The solution formed is strongly alkaline (pH 14) due to the sodium hydroxide which is formed
The reaction of sodium with cold water
Sodium fizzes while vigorously reacting with water to form sodium hydroxide
Reaction of magnesium with water
Magnesium reacts extremely slowly with cold water:
Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)
The solution formed is weakly alkaline (pH 11) as the formed magnesium hydroxide is only slightly soluble
When magnesium is heated, it reacts vigorously with steam (water) to make magnesium oxide and hydrogen gas:
Mg (s) + H2O (g) → MgO (s) + H2 (g)
Examiner Tips and Tricks
Phosphorus exists as an allotrope. A common allotrope is P4 and is known as white phosphorus. It can also exist as red phosphorus.
White phosphorus reacts spontaneously with excess oxygen to produce phosphorus(V) oxide via the following reaction.
P4 (s) + 5O2 (g) → P4O10 (s)
P4 (s) + 5O2 (g) → 2P2O5 (s)
Oxidation Number of Period 3 Oxides & Chlorides
Chlorine is more electronegative than the other Period 3 elements
Therefore, the other Period 3 elements will generally have positive oxidation states in their chlorides, while the chlorine has a negative oxidation state of -1
Oxygen is more electronegative than any of the Period 3 elements
Therefore, the Period 3 elements will have positive oxidation states in their oxides, while the oxygen has a negative oxidation state of -2
The Pauling scale of electronegativity
The Pauling scale shows that chlorine has a higher electronegativity than the other Period 3 elements and oxygen has a higher electronegativity than all of the Period 3 elements
Oxidation numbers of Period 3 elements in their oxides
The normal Oxidation Number Rules apply to the Period 3 oxides
Table of formulae of the Period 3 oxides & the elements oxidation numbers
Period 3 element | Na | Mg | Al | Si | P | S |
|---|---|---|---|---|---|---|
Formula of oxide | Na2O | MgO | Al2O3 | SiO2 | P4O10 | SO2 SO3 |
Oxidation number of the Period 3 element | +1 | +2 | +3 | +4 | +5 | +4 +6 |
Examiner Tips and Tricks
SiO2 is not written in the specification as expected knowledge but you should be able to apply the oxidation number rules to this and other chemicals
Worked Example
Deducing oxidation numbers of Period 3 elements in their oxides
State the oxidation number of the bold atoms in these compounds or ions.
Na2O
Al2O3
P4O10
Answer
Na2O
1 O atom = –2
The overall charge of the compound = 0
2 Na atoms = +2
Oxidation number of 1 Na atom = (+2) / 2 = +1
Al2O3
3 O atoms = 3 x (–2) = –6
The overall charge of the compound = 0
2 Al atoms = +6
Oxidation number of 1 Al atom = (+6) / 2 = +3
P4O10
10 O atoms = 10 x (–2) = –20
The overall charge of the compound = 0
4 P atoms = +20
Oxidation number of 1 Al atom = (+20) / 4 = +5
Oxidation numbers of Period 3 elements in their chlorides
The normal Oxidation Number Rules apply to the Period 3 chlorides
Table of formulae of the Period 3 chlorides & the elements oxidation numbers
Period 3 element | Na | Mg | Al | Si | P |
|---|---|---|---|---|---|
Formula of chloride | NaCl | MgCl2 | AlCl3 | SiCl4 | PCl5 |
Oxidation number of the Period 3 element | +1 | +2 | +3 | +4 | +5 |
Worked Example
Deducing oxidation numbers of Period 3 elements in their chlorides
State the oxidation number of the bold atoms in these compounds or ions.
Na2O
Al2O3
P4O10
Answer
MgCl2
2 Cl atoms = 2 x (–1) = –2
The overall charge of the compound = 0
Oxidation number of 1 Mg atom = +2
Or, the oxidation number of a Group 2 metal is +2
AlCl3
3 Cl atoms = 3 x (–1) = –3
The overall charge of the compound = 0
Oxidation number of 1 Al atom = +3
PCl5
5 Cl atoms = 5 x (–1) = –5
The overall charge of the compound = 0
Oxidation number of 1 P atom = +5
Reaction of Period 3 Oxides & Water
Not all Period 3 oxides react with or are soluble in water
Reaction of Period 3 oxides with water table
Oxide | Chemical equation | pH | Comments |
|---|---|---|---|
Na2O | Na2O (s) + H2O (l) → 2NaOH (aq) | 12 - 14 (Strongly alkaline) | - |
MgO | MgO (s) + H2O (l) → Mg(OH)2 (aq) | 8 - 10 (Weakly alkaline) | - |
Al2O3 | No reaction | - | Al2O3 is insoluble in water |
SiO2 | No reaction | - | SiO2 is insoluble in water |
P4O10 | P4O10 (s) + 6H2O (l) → 4H3PO4 (aq) | 3 - 4 (Weakly acidic) | Vigorous reaction |
SO2 SO3 | SO2 (s) + H2O (l) → H2SO3 (aq) SO3 (s) + H2O (l) → H2SO4 (aq) | 1 - 2 (Strongly acidic) | - |
Examiner Tips and Tricks
Since aluminium oxide does not react or dissolve in water, the oxide layer protects the aluminium metal from corrosion
The reaction of silicon(IV) oxide is not required knowledge
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