Reactivity Series

1. Order of the Reactivity Series

• The reactivity series arranges metals and other elements (like carbon and hydrogen) in order of their tendency to lose electrons and form positive ions.
• The required order to be memorised for the syllabus:
  Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminium (Al) > Carbon (C) > Zinc (Zn) > Iron (Fe) > Hydrogen (H) > Copper (Cu) > Silver (Ag) > Gold (Au)
• Highest reactivity is at the top (potassium), lowest reactivity is at the bottom (gold).
• Position is determined by experimental evidence such as reactions with water, acids, oxygen, and displacement reactions.

2. Relative Reactivities in Terms of Positive Ion Formation

• Reactivity is linked to the ease with which a metal atom loses electrons to form positive ions (cations):
  • Highly reactive metals (K, Na, Ca) lose electrons very easily due to low ionisation energy.
  • Moderately reactive metals (Mg, Zn, Fe) lose electrons less easily.
  • Unreactive metals (Cu, Ag, Au) lose electrons with great difficulty, making them resistant to corrosion and oxidation.
• Displacement reactions show reactivity:
  • A more reactive metal will displace a less reactive metal from its aqueous salt solution.
  • Example:
    Zinc + Copper(II) sulfate → Zinc sulfate + Copper
    Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
    (Zinc is more reactive than copper, so zinc displaces copper.)

3. Reactions with Water and Acids

(a) Potassium, Sodium, and Calcium with Cold Water

• Potassium (K):
  • Reacts extremely vigorously with cold water.
  • Floats on surface, moves rapidly, melts into a ball.
  • Produces potassium hydroxide (KOH) solution and hydrogen gas.
  • Ignites hydrogen with a lilac flame due to high heat.
  • Equation:
    2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
• Sodium (Na):
  • Less vigorous than potassium but still highly reactive.
  • Floats, moves, melts into a ball, produces hydrogen gas.
  • May ignite hydrogen with a yellow-orange flame.
  • Equation:
    2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
• Calcium (Ca):
  • Reacts less violently than Na and K.
  • Sinks initially but hydrogen bubbles stick to surface and make it float.
  • Produces calcium hydroxide (Ca(OH)₂) which is only slightly soluble.
  • Equation:
    Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

(b) Magnesium with Steam

• Magnesium (Mg):
  • Very slow reaction with cold water.
  • Reacts rapidly with steam, producing magnesium oxide and hydrogen gas.
  • Equation:
    Mg(s) + H₂O(g) → MgO(s) + H₂(g)

(c) Magnesium, Zinc, Iron, Copper, Silver, and Gold with Dilute Hydrochloric Acid

• Magnesium (Mg):
  • Reacts rapidly, producing hydrogen gas and magnesium chloride.
  • Equation:
    Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Zinc (Zn):
  • Reacts moderately quickly with acid.
  • Equation:
    Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• Iron (Fe):
  • Reacts slowly with acid, more rapidly if heated.
  • Equation:
    Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
• Copper (Cu), Silver (Ag), Gold (Au):
  • No reaction with dilute hydrochloric acid because they are below hydrogen in the reactivity series.

4. Apparent Unreactivity of Aluminium

• Aluminium is high in the reactivity series but appears unreactive because its surface is coated with a thin, strong aluminium oxide (Al₂O₃) layer.
• This oxide layer is:
  • Hard and impermeable to water and oxygen.
  • Prevents further oxidation and corrosion.
• When oxide layer is removed (e.g., by scratching or using molten salts), aluminium reacts vigorously with acids and oxygen.

5. Deducing an Order of Reactivity from Experimental Results

• Steps:
  1. Perform displacement reactions between metals and aqueous solutions of other metals’ salts.
  2. Observe reaction speed (effervescence, heat, colour changes).
  3. Compare reaction vigour with acids or water.
  4. Rank metals from most to least reactive based on:
     • Speed of hydrogen production in acids/water.
     • Ability to displace other metals.
• Example data analysis:
  • Mg displaces Zn from ZnSO₄ → Mg is above Zn.
  • Zn displaces Cu from CuSO₄ → Zn is above Cu.
  • No displacement between Cu and MgSO₄ → Cu is below Mg.