Changes in Oxidation Numbers Redox reactions

Rusting of Iron Nails

Oxidation and Reduction in Terms of Changes in Oxidation Numbers
Redox reactions – oxidation number of all elements change.
Rusting of iron, combustion, displacement of metal from its salt solution, displacement of halogen from its halide solution and electrolysis are redox reaction.
-10 …. -3  -2  -1  0  +1  +2  +3  …  +10
<———-  Reduction || Oxidation ———->

• H₂ (g) + CuO (s) –> H₂O (l) + Cu (s)
  Hydrogen: 0 –> +1 (Oxidised to water & Hydrogen is a reducing agent)
  Copper oxide: +2 –> 0 (Reduced to copper & Copper oxide is a oxidising agent)

• 2Zn (s) + O₂ (g) –> 2ZnO (s)
  Zinc: 0 –> +2 (Oxidised to zinc ion & Zinc is a reducing agent)
  Oxygen: 0 –> -2 (Reduced to oxide ion & Oxygen is an oxidising agent)

• 2Mg (s) + CO₂ (g) –> 2MgO (s) + C (s)
  Magnesium: 0 –> +2 (Oxidised to magnesium ion & Magnesium is a reducing agent)
  Carbon dioxide: +4 –> 0 (Reduced to carbon & Carbon dioxide is an oxidising agent)

• Br₂ (l) + 2HI (aq) –> 2HBr (aq) + I₂ (s)
  Hydroiodic acid / Hydrogen iodide: -1 –> 0 (Oxidised to iodine & Hydroiodic acid is a reducing agent)
  Bromine: 0 –> -1 (Reduced to hydrobromic acid & Bromine is a oxidising agent)

Non-redox reactions – oxidation number of all elements remain unchanged.
Precipitation, decomposition and neutralisation are not redox reaction (non-redox reaction)
Precipitation:

• AgNO₃ (aq) + NaCl (aq) –> AgCl (s) + NaNO₃ (aq)
  +1 +5 3(-2)      +1  -1              +1  -1        +1 +5  3(-2)

No change in the oxidation numbers.
Decomposition:

• ZnCO₃ (s) –> ZnO (s) + CO₂ (g)
  +2 +4  3(-2)    +2 -2       +4  2(-2)

No change in the oxidation numbers.
Neutralisation:

• NaOH (aq) + HCl (aq) –> NaCl (aq) + H₂O (l)
  +1 -2 +1          +1 -1             +1  -1             2(+1)  -2

No change in the oxidation numbers.