Question

The rate of the reaction between ${\mathrm{H}}_2$ and ${\mathrm{I}}_2$ to form $\mathrm{HI}$ increases with the intensity of visible light. (a) Explain why this fact supports a two-step mechanism. $({\mathrm{I}}_2$ vapor is purple.) (b) Explain why the visible light has no effect on the formation of $\mathrm{H}$ atoms.

Short answer

Visible light excites ${\mathrm{I}}_2$, supporting a two-step mechanism. ${\mathrm{H}}_2$ is unaffected as it doesn't absorb visible light.

Step-by-step solution

Understand the Reaction

The given reaction involves the combination of hydrogen (${\mathrm{H}}_2$) and iodine (${\mathrm{I}}_2$) to produce hydrogen iodide ($\mathrm{HI}$). The rate of this reaction is influenced by the presence of visible light.

Analyze the Impact of Light on Rate

Visible light increases the reaction rate because it provides energy that can be absorbed by the purple iodine vapor (${\mathrm{I}}_2$). This energy excites the iodine molecules, breaking them into individual iodine atoms ($\mathrm{I}$ radicals). This is because iodine is known to absorb visible light due to its color.

Consider a Two-Step Mechanism

Given that visible light facilitates the breaking of ${\mathrm{I}}_2$ into $\mathrm{I}$ radicals, a plausible two-step mechanism could be: 1) ${\mathrm{I}}_2\to 2\mathrm{I}$ (photolytic step) followed by 2) ${\mathrm{H}}_2+\mathrm{I}\to 2\mathrm{HI}$ (reaction step). This mechanism supports the idea that ${\mathrm{I}}_2$ breakdown is a separate step facilitated by light, increasing the reaction rate as more $\mathrm{I}$ radicals are available.

Explain Lack of Effect on $\mathrm{H}$ Formation

Visible light does not affect ${\mathrm{H}}_2$ because hydrogen molecules do not absorb visible light efficiently. They are primarily absorbed in the UV spectrum, so visible light does not provide the necessary energy to dissociate ${\mathrm{H}}_2$ into $\mathrm{H}$ radicals. Therefore, the formation of hydrogen atoms is not directly influenced by visible light.

Key concepts

Rate of Reaction

The rate of a reaction refers to how quickly or slowly a reaction occurs. In the context of the combination of hydrogen (${\text{H}}_2$) and iodine (${\text{I}}_2$) to form hydrogen iodide ($\text{HI}$), the rate can be significantly influenced by external factors such as light. When a chemical reaction absorbs energy from its surroundings, it can speed up. For instance, shining visible light on the reaction between ${\text{H}}_2$ and ${\text{I}}_2$ provides energy to the reactant molecules, allowing the reaction to occur more quickly. Increased reaction rates are often desirable because they reduce the time needed to produce a certain quantity of product. Evaluating the rate of reaction involves understanding both the reactants and conditions facilitating the reaction, such as temperature, pressure, and lighting.

Visible Light Effect

Visible light, part of the electromagnetic spectrum, can influence certain chemical reactions by providing energy that excite molecules. In the reaction between hydrogen and iodine, ${\text{I}}_2$ vapor absorbs visible light due to its purple color. This absorption is crucial because it provides energy to break the ${\text{I}}_2$ molecules into iodine radicals ($\text{I}$). Once these radicals are formed, they are highly reactive and can quickly combine with hydrogen to form hydrogen iodide ($\text{HI}$).

• The color of a substance is often linked to the wavelengths of light it can absorb. In this case, the purple color of iodine suggests it absorbs visible light effectively.
• When enough energy is absorbed, it can cause molecule dissociation, creating reactive radicals.

Energy from visible light, therefore, plays a pivotal role in advancing the reaction and is an integral part of its mechanism.

Photolytic Step

A photolytic step in a reaction mechanism is where light energy causes a molecule to break apart into smaller components, typically radicals. In the reaction of ${\text{H}}_2$ and ${\text{I}}_2$ to form $\text{HI}$, the photolytic step involves the dissociation of ${\text{I}}_2$ into two iodine radicals ($\text{I}$).This step is critical because these radicals are much more reactive than the original diatomic iodine molecule. They readily react with hydrogen molecules to form the product, hydrogen iodide. The energy required for this dissociation comes directly from absorbed visible light, making it a prime example of a photochemical process. Understanding photolytic processes is essential in chemistry because they elucidate how light energy can harness and propel chemical transformations. This understanding is also applied in various fields, such as environmental chemistry and phototherapy.

Two-Step Mechanism

A two-step mechanism is a sequence of reactions where the overall process occurs in two distinct stages. This type of mechanism helps explain why the rate of reaction can be affected by factors like light.For the reaction between hydrogen and iodine:

• The first step is photolytic: ${\text{I}}_2\to 2\text{I}$. This step is driven by visible light, which breaks the ${\text{I}}_2$ molecules into iodine radicals.
• The second step involves these radicals: ${\text{H}}_2+\text{I}\to 2\text{HI}$, where the highly reactive iodine radicals react with hydrogen to form the product.

This two-step pathway highlights how a complex reaction can be simplified into manageable parts. Stepwise mechanisms offer valuable insights into the roles played by each reactant and the impact of conditions like light, supporting the idea that chemical reactions often proceed through multiple intermediates before reaching the final products.