Preparation
The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.
- 2 I2 + N2H4 → 4 HI + N2
When performed in water, the HI must be distilled.
HI can also be distilled from a solution of NaI or other alkali iodide in concentrated hypophosphorous acid (note that sulfuric acid will not work for acidifying iodides as it will oxidize the iodide to elemental iodine).
Another way HI may be prepared is by bubbling hydrogen sulfide steam through an aqueous solution of iodine, forming hydroiodic acid (which is distilled) and elemental sulfur (this is filtered).
- H2S +I2 → 2 HI + S
Additionally HI can be prepared by simply combining H2 and I2. This method is usually employed to generate high purity samples.
- H2 + I2 → 2 HI
For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H2 and I2. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I2, about 578 nm, the rate increases significantly. This supports a mechanism whereby I2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H2 molecule and break the H—H bond:
- H2 + I2 → H2 + 2 I → I – - – H – - – H – - – I → 2 HI (hν, 578 nm)
In the laboratory, another method involves hydrolysis of PI3, the iodine equivalent of PBr3. In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid in two steps. The first step involves the reaction of iodine with phosphorus:
- 3 I2 + 2 P → 2 PI3
The second step involves water reacting with phosphorus triiodide
- 2 PI3 + 6 H2O → 6 HI + 2 H3PO3
Read more about this topic: Hydrogen Iodide
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