The oxidation number of S in the thiosulfate ion S2O3^2- is 11.
-2
-4
+2
+4
+6
The correct answer and explanation is :
The correct oxidation number of sulfur (S) in the thiosulfate ion, S2O32−\mathrm{S_2O_3^{2-}}, is +2.
Explanation:
The thiosulfate ion S2O32−\mathrm{S_2O_3^{2-}} consists of two sulfur atoms and three oxygen atoms, carrying an overall charge of −2-2.
To find the oxidation state of sulfur, we apply the following rules and approach:
- Oxidation state of oxygen: Oxygen almost always has an oxidation state of −2-2, except in peroxides or when bonded to fluorine. In thiosulfate, the three oxygens contribute a total of 3×(−2)=−63 \times (-2) = -6.
- Overall charge of the ion: The thiosulfate ion carries a charge of −2-2.
- Unknown oxidation states: The sulfur atoms can have different oxidation states because thiosulfate is unique—one sulfur is in a different environment than the other.
Step-by-step:
Let’s assign oxidation numbers:
- Let the oxidation state of the central sulfur (the one bonded to oxygens) be xx.
- Let the oxidation state of the other sulfur (usually the “terminal” or “bridging” sulfur) be yy.
We know: x+y+3(−2)=−2x + y + 3(-2) = -2 x+y−6=−2x + y – 6 = -2 x+y=4x + y = 4
Known structural facts:
- In thiosulfate, one sulfur atom is bonded to three oxygens (similar to sulfate) and usually has a higher oxidation state.
- The other sulfur atom is bonded only to sulfur and contributes differently.
In sulfate SO42−\mathrm{SO_4^{2-}}, sulfur is +6. Thiosulfate replaces one oxygen with sulfur.
Common oxidation states for sulfur in thiosulfate are:
- One sulfur at +5
- The other sulfur at −1-1
Sum: +5+(−1)=4+5 + (-1) = 4
This satisfies x+y=4x + y = 4.
Thus, the average oxidation state per sulfur atom is: 42=+2\frac{4}{2} = +2
Conclusion:
- The average oxidation number of sulfur in S2O32−\mathrm{S_2O_3^{2-}} is +2.
- This corresponds to the options given.
Additional notes:
The oxidation state is an average or formalism in such ions because the two sulfur atoms have different environments. The sulfur connected to oxygens is more oxidized (+5), and the other sulfur is reduced (-1). But when asked simply, the overall average oxidation number is +2.