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If Im not wrong, the reaction between H2 and I2 to form HI is exothermic and reversible.
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 + 578 nm radiation → H2 + 2 I → I - - - H - - - H - - - I → 2 HI
It depends...I2(s), ΔHof = 0 kJI2(g), ΔHof = (+)62.44 kJ