Relative Ease of Oxidation & Reduction (HL IB Chemistry)

• Depending on the position in a group, a metal (in Group 1 or Group 2) will be more or less likely to be oxidised
• When lithium reacts with water hydrogen gas and lithium hydroxide are formed

2Li (s) + 2H₂O (l) → 2LiOH (aq) + H₂ (g) 

• Lithium is oxidised from Li to Li⁺
  • The oxidation number changes from 0 to +1
• This reaction is vigorous, though down Group 1 the reaction becomes even more vigorous and violent
• Therefore down the group, the relative ease of oxidation increases
• The same pattern also occurs in Group 2
  • For example, magnesium reacts very slowly with water and calcium reacts much more quickly
    • Mg (s) + 2H₂O (l) → Mg(OH)₂ (s) + H₂ (g) = very slow reaction
    • Ca (s) + 2H₂O (l) → Ca(OH)₂ (s) + H₂ (g) = very quick reaction

Oxidising power of the Group 17 elements

• Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge

Ca (s) + Cl₂ (g) → Ca²⁺(Cl^-)₂ (s)

• Halogens are therefore oxidising agents:
• Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
• Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)
• The oxidising power of the halogens decreases going down the group (the halogens get less reactive)

The relative oxidising power of the halogens

F2	strongest oxidising agent most readily becomes reduced
Cl2
Br2
I2	weakest oxidising agent least readily becomes reduced

Reaction of the halogens with halide ions in displacement reactions

• A halogen displacement occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide
• The reactivity of Group 17 non-metals increases as you move up the group
• Out of the 3 halogens, chlorine, bromine and iodine, chlorine is the most reactive and iodine is the least reactive

Aqueous Solution Colour of Halogens

Halogen displacement reactions

Chlorine and bromine

• If you add chlorine solution to colourless potassium bromide solution, the solution becomes orange as bromine is formed
• Chlorine is above bromine in Group 17 so it is more reactive
• Chlorine will therefore displace bromine from an aqueous solution of a metal bromide

2KBr (aq) + Cl₂ (aq) → 2KCl (aq) + Br₂ (aq)

potassium bromide + chlorine → potassium chloride + bromine

Bromine and iodine

• Bromine is above iodine in Group 17 so it is more reactive
• Bromine will therefore displace iodine from an aqueous solution of a metal iodide

Br₂ (l) + 2NaI (aq) → 2NaBr (aq) + I₂ (aq)

bromine + sodium Iodide → sodium bromide + iodine

• Metals can be ranked in order of reactivity
  • This is called the reactivity series or just activity series
• Metals higher in reactivity can displace less reactive metals from their compounds in solutions or from their oxides
• An example of a metal displacement reaction occurs between magnesium and aqueous copper (II) sulfate solution

Mg (s) + CuSO₄ (aq) → MgSO₄  (aq) +  Cu (s)

• What are the redox processes going on here?
  • If we split the equation into half equations it’s easy to see which species is oxidised and which is reduced:

Mg   → Mg²⁺+ 2e^-         Loss of electrons  =  oxidation

Cu²⁺ + 2e^- → Cu            Gain of electrons  =  reduction

• The more reactive metal acts as a reducing agent
• This allows metals to be ranked from most reactive (strongest reducing agents) to least reactive:

 The metal activity series

The more reactive a metal is the better it is at pushing electrons onto less reactive metal ions. Magnesium is better at pushing electrons onto copper(II) ions than copper is at pushing electrons onto magnesium ions