The conversion described by sulphur to sulphur dioxide is the oxidation of elemental sulphur by dioxygen, commonly observed when sulphur burns in air. The product sulphur dioxide (SO2) is a molecular oxide that forms directly from the elements.
Balanced chemical equation
S(s) + O2(g) → SO2(g)
The coefficients are 1 : 1 : 1, so one mole of sulphur consumes one mole of oxygen and forms one mole of sulphur dioxide.
A typical setting is excess O2 (air) with sulphur as the limiting reactant. Under strongly oxidizing conditions, further oxidation of SO2 to SO3 becomes relevant (especially with catalysts), but the direct elemental combination produces SO2.
Oxidation states and redox interpretation
Elemental sulphur has oxidation number 0. In SO2, oxygen is typically −2, so sulphur must be +4 to keep the molecule neutral.
\[ x + 2(-2) = 0 \quad \Rightarrow \quad x = +4 \]
The redox changes follow directly:
- Sulphur: \(0 \rightarrow +4\) (oxidation; loss of 4 electrons per sulphur atom)
- Oxygen: \(0 \rightarrow -2\) (reduction; gain of 4 electrons per O2 molecule)
An electron-balance view (non-aqueous, conceptual accounting) can be written as:
\[ \mathrm{S}^{0} \rightarrow \mathrm{S}^{+4} + 4e^{-} \qquad \mathrm{O_2} + 4e^{-} \rightarrow 2\,\mathrm{O}^{2-} \]
Stoichiometric relationships
Let the amount of sulphur reacted be \(n\) moles. The balanced coefficients impose the same amount \(n\) for O2 consumed and SO2 produced:
\[ n(\mathrm{S}) = n(\mathrm{O_2}) = n(\mathrm{SO_2}) \]
| Species | Coefficient | Moles (in terms of \(n\)) | Molar mass (g·mol−1) | Mass (in terms of \(n\)) |
|---|---|---|---|---|
| S | 1 | \(n\) | \(\approx 32.06\) | \(m = 32.06\,n\) |
| O2 | 1 | \(n\) | \(\approx 32.00\) | \(m = 32.00\,n\) |
| SO2 | 1 | \(n\) | \(\approx 64.06\) | \(m = 64.06\,n\) |
Visualization of sulphur to sulphur dioxide
Common pitfalls
- Misbalanced oxygen: SO2 contains two oxygen atoms, so a single O2 molecule supplies exactly the needed oxygen in the balanced equation.
- Oxidation number sign errors: oxygen in most oxides is −2, enforcing sulphur at +4 in SO2.
- Reactant state assumptions: sulphur is commonly treated as a solid reactant and oxygen as a gas; state labels do not change stoichiometric coefficients.
- Overgeneralized “valence” language: oxidation number (+4) is not the same concept as valence electrons or valency in bonding discussions.