Question

Free radicals are important in many environmentally significant reactions (see the Chemistry in the Emironment box on free radicals in this chapter). For example, photochemical smogsmog that results from the action of sunlight on air pollutants-forms in part by these two steps: \(\mathrm{NO}_{2} \stackrel{\mathrm{UV} \mathrm{Lgh}}{\longrightarrow} \mathrm{NO}+\mathrm{O}\) \(\mathrm{O}+\mathrm{O}_{2} \longrightarrow \mathrm{O}_{3}\)

Short answer

The two-step reaction involves the breaking of the \( \mathrm{NO}_2 \) molecule into \( \mathrm{NO} \) and \( \mathrm{O} \) under UV light, followed by the reaction of the free radical \( \mathrm{O} \) with \( \mathrm{O}_2 \) to form \( \mathrm{O}_3 \) (ozone).

Step-by-step solution

Understanding the Reaction Mechanism

We need to recognize that this is a two-step reaction process in which a free radical is involved. First, Nitrogen dioxide \( \mathrm{NO}_2 \) absorbs Ultraviolet (UV) light which causes it to split into Nitric oxide \( \mathrm{NO} \) and a single oxygen atom \( \mathrm{O} \) - a free radical. The energy from the UV light is sufficient to break the bond in \( \mathrm{NO}_2 \) because of the high energy of UV light.

Free Radical Formation

The first equation \( \mathrm{NO}_{2} \stackrel{\mathrm{UV} \mathrm{Lght}}{\longrightarrow} \mathrm{NO} + \mathrm{O} \) demonstrates the formation of the free radical oxygen atom \( \mathrm{O} \) when Nitrogen dioxide \( \mathrm{NO}_2 \) breaks down. This is the initiation step where the free radical is produced.

Free Radical Reaction with Oxygen

In the subsequent step, the free radical oxygen atom \( \mathrm{O} \) reacts with molecular oxygen \( \mathrm{O}_2 \) to form ozone \( \mathrm{O}_3 \) . This reaction is the propagation step of the chain reaction. It illustrates how free radicals can promote further reactions, in this case leading to the formation of ozone.

Key concepts

Photochemical Smog Formation

Understanding photochemical smog is crucial for grasping environmental chemistry. Photochemical smog is a type of air pollution characterized by the presence of harmful chemicals formed by the interaction between sunlight and airborne pollutants. The process often begins with the emission of pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicles and industrial sources.

When exposed to UV light from the sun, these substances undergo various chemical reactions. For instance, nitrogen dioxide \br\br\br(\( \mathrm{NO}_{2} \)) absorbs UV light and dissociates into nitric oxide \br\br\br(\( \mathrm{NO} \)) and a free radical oxygen atom \br\br\br(\( \mathrm{O} \)). This oxygen atom can further react with molecular oxygen to form ozone \br\br\br(\( \mathrm{O}_{3} \)), contributing to the smog. The presence of sunlight is critical in this series of reactions, hence the term 'photochemical'. These reactions not only produce ozone but also other secondary pollutants that can be harmful to human health and the environment.

In dense urban environments, photochemical smog can lead to reduced visibility and harmful air quality, affecting both respiratory health and ecosystems. Regulations aimed at controlling emissions of NOx and VOCs are crucial in managing smog formation.

Reaction Mechanisms

The term 'reaction mechanism' refers to the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Each step is a fundamental process involving changes at the molecular level, often including the formation and destruction of intermediates such as free radicals.

In the example provided from the textbook, the process of photochemical smog formation involves a clear reaction mechanism. Initially, nitrogen dioxide \br\br\br(\( \mathrm{NO}_{2} \)) absorbs the energy from UV light, which leads to the breaking of a chemical bond and the formation of nitric oxide \br\br\br(\( \mathrm{NO} \)) and a free radical oxygen atom \br\br\br(\( \mathrm{O} \)), showcasing the initiation step of the mechanism. Subsequently, the reactive oxygen atom combines with molecular oxygen to form ozone \br\br\br(\( \mathrm{O}_{3} \)), representing the propagation step. These steps constitute a chain reaction that continues as long as the necessary reactants and energy are present.

Recognizing and understanding reaction mechanisms enable chemists to predict the outcome of chemical processes and design strategies to control and optimize reactions, such as in reducing the formation of undesirable pollutants like photochemical smog.

UV Light and Chemical Reactions

UV light plays a pivotal role in driving chemical reactions, particularly in environmental chemistry. Ultraviolet (UV) radiation is a type of energy emitted by the sun, possessing higher energy than visible light, which can initiate chemical reactions through the absorption by certain molecules.

In the breakdown of nitrogen dioxide \br\br\br(\( \mathrm{NO}_{2} \)), UV light provides the necessary energy to disrupt the molecular bonds, leading to the creation of a free radical. Free radicals are highly reactive due to the presence of unpaired electrons, and they can quickly react with other substances, often leading to chain reactions. The absorption of UV light by \br\br\br(\( \mathrm{NO}_{2} \)) and subsequent free radical formation is a primary example of how UV light can influence the rate and path of chemical reactions.

Understanding the effects of UV light on chemical substances is crucial for various applications, including the development of sunscreens, materials designed to withstand sun exposure, and even the analysis of environmental impact caused by the depletion of the ozone layer, which acts as a protective shield against harmful UV radiation.