During the Olympics in Rio de Janeiro in 2016 the water in some swimming and diving pools turned green - Leaving Cert Chemistry - Question f - 2018

To determine whether chlorine was oxidised or reduced in the reaction, we will assign oxidation numbers to the relevant species in the equation:

1. Oxidation Numbers Calculation:

   • For sodium hypochlorite (NaOCl):
     • Na: +1
     • Cl: +1
     • O: -2
     • Thus, the oxidation state of Cl in NaOCl is +1.
   • For the products:
     • In NaCl:
       • Na: +1
       • Cl: -1
       • Therefore, the oxidation state of Cl in NaCl is -1.
   • In water (H2O):
     • H: +1
     • O: -2
     • Thus, the oxidation states do not directly involve Cl.
   • For oxygen (O2):
     • O: 0 (as it is in elemental form).

2. Identifying Changes in Oxidation States:

   • Initial state of Cl: +1 (in NaOCl)
   • Final state of Cl: -1 (in NaCl)

3. Conclusion:

   • Since the oxidation state of chlorine decreased from +1 to -1, we conclude that chlorine was reduced. Reduction corresponds with a gain of electrons, which is reflected in the decrease in oxidation state.