Q.Thiosulphate reacts differently with iodine and bromine in the reactions given below:
2S2O3^2- + I2 → S4O6^2- + 2I^-
S2O3^2- + 2Br2 + 5H2O → 2SO4^2- + 2Br^- + 10 H^+
Which of the following statements justifies the above dual behaviour of thiosulphate?

The key concept here is Oxidation State Stability and the relative oxidising strengths of halogens.

1. In both reactions, thiosulphate ($S_2O_3^{2-}$) acts as a reducing agent, meaning its sulphur atoms are oxidised. The average oxidation state of sulphur in $S_2O_3^{2-}$ is $+2$.
2. In the reaction with iodine ($I_2$), sulphur is oxidised from $+2$ to an average oxidation state of $+2.5$ in the tetrathionate ion ($S_4O_6^{2-}$). Iodine ($I_2$) is reduced to $I^-$.
3. In the reaction with bromine ($Br_2$), sulphur is oxidised from $+2$ to a much higher oxidation state of $+6$ in the sulphate ion ($SO_4^{2-}$). Bromine ($Br_2$) is reduced to $Br^-$. …

The "dual behaviour" of thiosulphate refers to its ability to be oxidised to different products ($S_4O_6^{2-}$ and $SO_4^{2-}$) when reacting with different oxidising agents (iodine and bromine). This difference arises because sulphur in thiosulphate is in an intermediate oxidation state, allowing it to be oxidised to varying extents. The extent of oxidation is determined by the strength of the oxidising agent.

Let's break down the reactions by calculating the oxidation states of sulphur and the halogens.

1. Analyse the reaction with Iodine:

   The reaction is: $2S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2I^-$

   • Oxidation state of sulphur in $S_2O_3^{2-}$ (thiosulphate ion):

     Let the oxidation state of sulphur be $x$. Oxygen typically has an oxidation state of $-2$.

     $2x + 3(-2) = -2$

     $2x - 6 = -2$

     $2x = 4$

     $x = +2$

     So, the average oxidation state of sulphur in $S_2O_3^{2-}$ is $+2$.

   • Oxidation state of sulphur in $S_4O_6^{2-}$ (tetrathionate ion):

     Let the oxidation state of sulphur be $y$. Oxygen has an oxidation state of $-2$.

     $4y + 6(-2) = -2$

     $4y - 12 = -2$

     $4y = 10$

     $y = +2.5$

     So, the average oxidation state of sulphur in $S_4O_6^{2-}$ is $+2.5$.

   • Oxidation state of iodine:

     In $I_2$, the oxidation state of iodine is $0$ (elemental form).

     In $I^-$, the oxidation state of iodine is $-1$.

   • Conclusion for the iodine reaction:

     Sulphur's oxidation state increases from $+2$ to $+2.5$, meaning thiosulphate is oxidised.

     Iodine's oxidation state decreases from $0$ to $-1$, meaning iodine is reduced.

     Iodine acts as an oxidising agent.

2. Analyse the reaction with Bromine:

   The reaction is: $S_2O_3^{2-} + 2Br_2 + 5H_2O \rightarrow 2SO_4^{2-} + 2Br^- + 10 H^+$

   • Oxidation state of sulphur in $S_2O_3^{2-}$:

     As calculated above, the average oxidation state of sulphur is $+2$.

   • Oxidation state of sulphur in $SO_4^{2-}$ (sulphate ion):

     Let the oxidation state of sulphur be $z$. Oxygen has an oxidation state of $-2$.

     $z + 4(-2) = -2$

     $z - 8 = -2$

     $z = +6$

     So, the oxidation state of sulphur in $SO_4^{2-}$ is $+6$.

   • Oxidation state of bromine:

     In $Br_2$, the oxidation state of bromine is $0$ (elemental form).

     In $Br^-$, the oxidation state of bromine is $-1$.

   • Conclusion for the bromine reaction:

     Sulphur's oxidation state increases from $+2$ to $+6$, meaning thiosulphate is oxidised.

     Bromine's oxidation state decreases from $0$ to $-1$, meaning bromine is reduced.

     Bromine acts as an oxidising agent.

3. Compare the extent of oxidation and oxidant strength:

   In both reactions, thiosulphate is oxidised. However, the extent of oxidation is different:

   • With iodine, sulphur is oxidised from $+2$ to $+2.5$.
   • With bromine, sulphur is oxidised from $+2$ to $+6$.

   Since bromine oxidises sulphur to a much higher oxidation state ($+6$) compared to iodine ($+2.5$), it implies that bromine is a stronger oxidising agent than iodine. A stronger oxidant can force a reactant to lose more electrons, resulting in a higher oxidation state for the oxidised product. …