Unit 5 Section 2 Period 3 Elements

Reactions with Water

Sodium:
Reacts vigorously with cold water, forming a molten ball on the surface, fizzing and producing H2 gas:
2Na(s) + 2H2O(l) >>> 2NaOH(aq) + H2(g)
Forms a strongly alkaline solution of pH 12 - 14

Magnesium:
Reacts slowly with cold water. Forms a weakly alkaline solution of pH 9 - 10:
Mg(s) + 2H2O(l) >>> Mg(OH)2(aq) + H2(g)
Weakly alkaline because magnesium is not very soluble in water, relatively few hydroxides produced.
Reacts faster with steam (when there is more energy), to form magnesium oxide.

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Reactions with Oxygen:

Period 3 elements are usually oxidised to their highest oxidation states - the same as their group number.

Sulfur is an exception to this - it forms SO2, in which it's only got a +4 oxidation state (a high temperature and a catalyst are needed to make SO3).

The more reactive metals (Na, Mg) and the non-metals (P, S) react readily in air while Al and Si react slowly.

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2Na(s) + 1/2O2(g) >>> Na2O(s)

Element: Na

Oxide: Na2O - Sodium Oxide

Reaction in air: Vigorous

Flame: Yellow

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Mg(s) + 1/2O2(g) >>> MgO(s)

Element: Mg

Oxide: MgO - Magnesium Oxide

Reaction in air: Vigorous

Flame: Brilliant white

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2Al(s) + 1.1/2O2(g) >>> Al2O3(s)

Element: Al

Oxide: Al2O3 - Aluminium Oxide

Reaction in air: Slow

Flame: N/A

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Si(s) + O2(g) >>> SiO2(s)

Element: Si

Oxide: SiO2 - Silicon Dioxide

Reaction in air: Slow

Flame: N/A

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P4(s) + 5O2(g) >>> P4O10(s)

Element: P

Oxide: P4O10

Reaction in air: Spontanously combusts

Flame: Brilliant white

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S(s) + O2(g) >>> SO2(g)

Element: S

Oxide: SO2 - Sulfur Dioxide

Reaction in air: Burns steadily

Flame: Blue

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