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Chemistry · Ch 7 — The p-Block Elements

Sulphur Dioxide

7.15

Sulphur Dioxide

Sulphur Dioxide

Preparation

Burning sulphur in air or oxygen produces sulphur dioxide along with a small amount (6–8%) of sulphur trioxide as a by-product:

S(s)+O2(g)→SO2(g)S(s) + O_2(g) \rightarrow SO_2(g)

In the laboratory, SO2SO_2 is conveniently generated by treating a sulphite salt with dilute sulphuric acid:

SO32−(aq)+2H+(aq)→H2O(l)+SO2(g)SO_3^{2-}(aq) + 2H^+(aq) \rightarrow H_2O(l) + SO_2(g)

Industrially, sulphur dioxide arises as a by-product of roasting sulphide ores, for example iron pyrites:

4FeS2(s)+11O2(g)→2Fe2O3(s)+8SO2(g)4FeS_2(s) + 11O_2(g) \rightarrow 2Fe_2O_3(s) + 8SO_2(g)

The gas is dried, then liquefied under pressure and stored in steel cylinders for transport and use.

Physical Properties

Sulphur dioxide is a colourless gas with a sharp, pungent smell and dissolves very readily in water. Under a pressure of about two atmospheres it liquefies at room temperature, and its normal boiling point is 263 K.

Chemical Reactions

Passed into water, SO2SO_2 dissolves to give a solution of sulphurous acid:

SO2(g)+H2O(l)⇌H2SO3(aq)SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)

It reacts with sodium hydroxide solution to form sodium sulphite, which itself reacts with further SO2SO_2 to give sodium hydrogen sulphite:

2NaOH+SO2→Na2SO3+H2O2NaOH + SO_2 \rightarrow Na_2SO_3 + H_2O

Na2SO3+H2O+SO2→2NaHSO3Na_2SO_3 + H_2O + SO_2 \rightarrow 2NaHSO_3

In both its reaction with water and its reaction with alkalies, sulphur dioxide behaves very much like carbon dioxide.

In the presence of charcoal acting as a catalyst, SO2SO_2 reacts with chlorine to form sulphuryl chloride:

SO2(g)+Cl2(g)→SO2Cl2(l)SO_2(g) + Cl_2(g) \rightarrow SO_2Cl_2(l)

With oxygen, and in the presence of vanadium(V) oxide as catalyst, sulphur dioxide is oxidised further to sulphur trioxide:

2SO2(g)+O2(g)→V2O52SO3(g)2SO_2(g) + O_2(g) \xrightarrow{V_2O_5} 2SO_3(g)

When moist, sulphur dioxide acts as a reducing agent. It reduces iron(III) ions to iron(II), and it decolourises acidified potassium permanganate(VII) solution — a reaction commonly used as a simple test to identify the gas:

2Fe3++SO2+2H2O→2Fe2++SO42−+4H+2Fe^{3+} + SO_2 + 2H_2O \rightarrow 2Fe^{2+} + SO_4^{2-} + 4H^+

5SO2+2MnO4−+2H2O→5SO42−+4H++2Mn2+5SO_2 + 2MnO_4^- + 2H_2O \rightarrow 5SO_4^{2-} + 4H^+ + 2Mn^{2+}

Structure …

Figure unnumbered(no printed caption — inline diagram illustrating: "It is a resonance hybrid of the two canonical forms:")

What this figure shows. Two Lewis dot structures of sulphur dioxide (SO2), drawn in the left margin next to the paragraph, connected by a double-headed resonance arrow (<->), both bent/angular 3-atom (O-S-O) shapes. Left structure: central S atom carries one lone pair (dot-pair) on top, is double-bonded (=) to an O atom on the upper-left (shown with two lone-pair dot-sets), and single-bonded to a second O atom on the lower-right (shown with three lone-pair dot-sets, i.e. carrying extra negative-charge lone pairs). Right structure is the mirror image: the double bond is now on the lower-right O and the single bond (with three lone pairs) is on the upper-left O, S again bearing one lone pair. No figure number or caption is printed …

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