Example 4.5: Assigning Oxidation Numbers
Follow the guidelines in this section of the text to assign oxidation numbers to all the elements in the following species:
(a)
\(\ce{H2S}\)
(b)
\(\ce{SO3^2-}\)
(c)
\(\ce{Na2SO4}\)
Solution
(a) According to guideline
1, the oxidation number for H is
+1.
Using this oxidation number and the compound’s formula, guideline
4 may then be used to calculate the oxidation number for sulfur:
\(\mathrm{charge}_{\mathrm{\ce{H2S}}} = 0 = 2 \cdot + 1 + 1 \cdot \mathrm{ON}_{\mathrm{\ce{S}}}\)
\(\mathrm{ON}_{\mathrm{\ce{S}}} = 0 - 2 \cdot + 1 = - 2\)
(b) Guideline
3 suggests the oxidation number for oxygen is
-2.
Using this oxidation number and the ion’s formula, guideline
4 may then be used to calculate the oxidation number for sulfur:
\(\mathrm{charge}_{\mathrm{\ce{SO3^2-}}} = - 2 = 3 \cdot (- 2) + 1 \cdot \mathrm{ON}_{\mathrm{\ce{S}}}\)
\(\mathrm{ON}_{\mathrm{\ce{S}}} = - 2 - 3 \cdot × - 2 = + 4\)
(c) For ionic compounds, it’s convenient to assign oxidation numbers for the cation and anion separately.
According to guideline
2, the oxidation number for sodium is
+1.
Assuming the usual oxidation number for oxygen (
-2 per guideline
3), the oxidation number for sulfur is calculated as directed by guideline 4:
\(\mathrm{charge}_{\mathrm{\ce{SO4^2-}}} = - 2 = 4 \cdot (- 2) + 1 \cdot \mathrm{ON}_{\mathrm{\ce{S}}}\)
\(\mathrm{ON}_{\mathrm{\ce{S}}} = - 2 - 4 \cdot (- 2) = + 6\)