Formation of Hydrogen Iodide from the Corresponding Elements: A Two-Step Mechanism

Introduction:
Hydrogen iodide (HI) is a binary compound composed of hydrogen (H) and iodine (I). It forms through the reaction between hydrogen gas and iodine vapor. The reaction is exothermic, releasing a significant amount of heat.

Reaction:
H2(g) + I2(g) → 2HI(g)

Two-Step Mechanism:
The formation of hydrogen iodide from the corresponding elements can be explained by a two-step mechanism:

Step 1: Homolytic Cleavage of I2
In the first step, iodine gas (I2) undergoes homolytic cleavage, meaning that the I-I bond is broken, and two iodine radicals (I●) are formed. This process is initiated by the absorption of energy, such as heat or light.

I2(g) → 2I●(g)

Step 2: Combination of Hydrogen and Iodine Radicals
In the second step, the iodine radicals (I●) react with hydrogen gas (H2) to form hydrogen iodide (HI). This is a bimolecular step, where two species collide and react with each other.

I●(g) + H2(g) → HI(g)

The overall reaction can be obtained by combining these two steps:

H2(g) + I2(g) → 2HI(g)

Explanation:
The reaction proceeds via a two-step mechanism because it involves the formation of highly reactive iodine radicals (I●) in the first step. These radicals are highly reactive and can quickly react with hydrogen gas to form hydrogen iodide.

The two-step mechanism is supported by experimental evidence, such as the observation of iodine radicals during the reaction and the dependence of the reaction rate on the concentrations of both hydrogen and iodine.

Conclusion:
The formation of hydrogen iodide from the corresponding elements follows a two-step mechanism. In the first step, iodine gas undergoes homolytic cleavage to form iodine radicals, and in the second step, these radicals react with hydrogen gas to produce hydrogen iodide. Understanding the reaction mechanism is crucial for studying the kinetics and thermodynamics of this chemical process.