Sulfur and Iodine

"...the union of hydrogen and sulphur to form gaseous H2S evolves +3.6 units of heat, while 0.8 unit is absorbed in the formation of hydriodic acid gas. The author has made new experiments upon this point. Dry hydrogen sulfide was inclosed with a little iodine in a sealed tube and heated to 500°, but no reaction took place. On the other hand, dry hydrogen iodide and sulphur in a similar arrangement reacted immediately, even in the cold, and on heating to 100° the reaction was complete."

The same article goes on to say that the reverse of this reaction takes place in the presence of water. That when the tube containing the hydrogen sulfide and iodine was opened under water, it reacted back into sulfur and hydroiodic acid. Yet the reaction could be reversed yet again, by making the water contain more than 52% hydroiodic acid.

If iodine is dissolved in 52% HI solution, and hydrogen sulfide is passed into the solution, there is no observable reaction.

Reciprocal Displacements between Oxygen, Sulfur, and the Halogens, when combined with Hydrogen.
by Berthelot
Scientific American: Supplement, Volume 7, No. 175, p2791

Apparently hydrogen iodide is a much stronger reducing agent in its anhydrous form. The reactions must be:

(2)HI + S --> I2 + H2S
H2S + I2 + (2)H2O --> S + (2)I- + (2)H3O+
 
Sulfur dioxide also oxidizes hydrogen sulfide at room temperature. When SO2 is simultaneosuly bubbled with H2S into water, the water turns a milky white color, with a slight yellowish tinge.

(2)H2S + SO2 --> (2)H2O + (3)S

or if you prefer
(16)H2S + (8)SO2 --> (16)H2O + (3)S8


Iodine oxidizes aqueous solutions of sulfur dioxide to sulfuric acid. This only works to make dilute sulfuric acid.

H2O + SO2 + I2 --> H2SO4 + (2)HI

Highly concentrated sulfuric acid reacts with hydrogen iodide in essentially the reverse reaction. This is not the entire reaction, because sulfur, and even some H2S gas, is also produced.

H2SO4 + (2)HI --> H2O + SO2 + I2

Concentrated sulfuric acid will also similarly react with sodium iodide, and will oxidize hydrogen bromide.
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