Na & Mg with water (AQA A Level Chemistry): Revision Note

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 much 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