A LevelChemistryCIERevision Notes9. The Periodic Table: Chemical Periodicity9.2 Periodicity of Chemical Properties of the Elements in Period 3Reactions of the Period 3 Elements

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Reactions of the Period 3 Elements (CIE A Level Chemistry)

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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

The Periodic Table - Sodium Reaction with Water, downloadable AS & A Level Chemistry revision notes

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 Tip

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

Hydrocarbons Pauling Scale of Elements, downloadable AS & A Level Chemistry revision notes

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

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 Tip

  • 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.

  1. Na2O
  2. Al2O3
  3.  P4O10

Answer

  1. 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
  2. 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
  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

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.

  1. Na2O
  2. Al2O3
  3.  P4O10

Answer

  1. 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
  2. AlCl3
    • 3 Cl atoms = 3 x (–1) = –3
    • The overall charge of the compound = 0
    • Oxidation number of 1 Al atom = +3
  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 Tip

  • 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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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.