AQA A Level Chemistry复习笔记6.1.1 Na & Mg with water

Sodium & Magnesium

• Both sodium, Na, and magnesium, Mg, are metals and are found in Group 1 and Group 2 of the periodic table respectively
• Both have high melting points, but magnesium has a higher melting point than sodium
  • This is because of the 2+ charge of magnesium, meaning that it is has a higher charge density

   

• Both are silvery metals
  • Sodium is quite a soft, silvery metal which tarnishes quickly in air
  • Magnesium is harder than sodium and you will often see it as magnesium ribbon

   

Reactions with water

• Despite their similarities, sodium and magnesium will react with water quite differently:
• Sodium with cold water:

2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)

• • This is a very vigorous, exothermic reaction
  • The sodium floats on the surface of the water fizzing rapidly and melting as a result of the heat produced during the reaction
  • The colourless sodium hydroxide formed will have a pH of around 13-14, so a very alkaline solution is formed
  • The oxidation state of the sodium changes from 0 in its elemental state, to +1 in the sodium hydroxide
• Magnesium with cold water:

Mg (s) + 2H2O (l)  → Mg(OH)2 (aq) + H2 (g)

• • This is an extremely slow reaction - only a very small number of bubbles will form on the magnesium ribbon
  • The magnesium hydroxide formed will have a pH of around 10 - it is less alkaline than sodium hydroxide because magnesium hydroxide is only partially soluble
  • This is the key component in 'milk of magnesia'
  • The oxidation state of the magnesium changes from 0 in the elemental state, to +2 in the magnesium hydroxide

   

• Heated magnesium with steam:

Mg (s) + H2O (g)  → MgO (s) + H2 (g)

• • This reaction is must faster than with cold water
  • The magnesium burns with a bright, white flame
  • The products of this reaction are different - magnesium oxide is produced instead of magnesium hydroxide
  • The oxidation state of the magnesium changes from 0 in its elemental state, to +2 in the magnesium oxide

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