Question

NO is a pollutant emitted by motor vehicles. It is formed by the reaction: (a) \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}(g)\) Once in the atmosphere, NO (through a series of reactions) adds one oxygen atom to form \(\mathrm{NO}_{2}\). \(\mathrm{NO}_{2}\) then interacts with UV light according to the reaction: (b) \(\mathrm{NO}_{2}(g) \underset{\mathrm{UV} \text { light }}{\mathrm{NO}(g)}+\mathrm{O}(g)\) These freshly formed oxygen atoms then react with \(\mathrm{O}_{2}\) in the air to form ozone \(\left(\mathrm{O}_{3}\right)\), a main component of smog: (c) \(\mathrm{O}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{O}_{3}(g)\) Classify each of the preceding reactions \((a, b, c)\) as a synthesis, decomposition, single-displacement, or doubledisplacement reaction.

Short answer

Reaction (a) is a synthesis reaction, reaction (b) is a decomposition reaction, and reaction (c) is a synthesis reaction.

Step-by-step solution

Identify Reaction a

Look at reaction (a) \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}(g)\). Since two elements, nitrogen and oxygen, are combining to form a single product, nitrogen monoxide, this reaction is a synthesis reaction.

Identify Reaction b

Examine reaction (b) \(\mathrm{NO}_{2}(g) \underset{\mathrm{UV} \text { light }}{\mathrm{NO}(g)}+\mathrm{O}(g)\). This reaction involves the breaking down of nitrogen dioxide into nitrogen monoxide and oxygen when exposed to UV light. It is a decomposition reaction because one reactant breaks down into two or more simpler products.

Identify Reaction c

Look at reaction (c) \(\mathrm{O}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{O}_{3}(g)\). This reaction involves combining an oxygen atom with an oxygen molecule to form ozone. Similar to reaction (a), this is a synthesis reaction because two substances combine to make a more complex product.

Key concepts

Synthesis Reaction

In the realm of chemical reactions, a synthesis reaction, also known as a direct combination reaction, occurs when two or more simple substances combine to form a more complex product. This type of reaction is represented by the general equation: \[\begin{equation}A + B \rightarrow AB\text{.}\end{equation}\]A classic example of a synthesis reaction is the combination of iron and sulfur to form iron sulfide. In the exercise presented, reaction (a) \[\begin{equation}N_{2}(g) + O_{2}(g) \rightarrow 2NO(g)\end{equation}\]is identified as a synthesis reaction because it involves the combination of nitrogen (N_{2}) and oxygen (O_{2}) gases to create nitrogen monoxide (NO\text{)}. This transformation showcases the hallmark of synthesis reactions: building complexity from simplicity, often releasing energy in the process.

In educational contexts, understanding synthesis reactions is crucial as they are foundational to multiple disciplines including organic chemistry, environmental science, and materials engineering. Recognizing them involves identifying the reactants that merge to create a single product.

Decomposition Reaction

Decomposition reactions represent the opposite of synthesis reactions. Here, a single compound breaks down into two or more simpler substances. The general formula for a decomposition reaction is:\[\begin{equation}AB \rightarrow A + B\text{.}\end{equation}\]These reactions often require an external source of energy, such as heat, light, or electricity, to occur. In reaction (b) from our exercise,\[\begin{equation}NO_{2}(g) \text{ under UV light } NO(g) + O(g)\text{,}\end{equation}\]nitrogen dioxide (NO_{2}) decomposes into nitrogen monoxide (NO\text{)} and oxygen (O\text{)} when exposed to ultraviolet light. It's important to point out that decomposition reactions are commonly encountered in real-world scenarios, such as the electrolysis of water to produce hydrogen and oxygen gases, or the breakdown of organic matter over time.

Students must grasp that decomposition reactions are integral in processes like recycling of materials, digestion in living organisms and the production of energy in batteries.

Chemical Reactions in Atmospheric Chemistry

Chemical reactions in atmospheric chemistry involve transformations and interactions of substances within the Earth's atmosphere, playing a pivotal role in air quality, climate change, and environmental health. In relation to our exercise, reaction (c)\[\begin{equation}O(g) + O_{2}(g) \rightarrow O_{3}(g)\text{,}\end{equation}\]is a synthesis reaction forming ozone (O_{3}), a critical component determining the composition of the atmosphere. However, it is essential to comprehend the broader context of atmospheric chemistry. It encompasses not only synthesis reactions but also decomposition, as demonstrated in reaction (b), along with other types such as photochemical reactions driven by light energy, which trigger a cascade of changes affecting the climate and air we breathe.

For example, the formation of ozone near the Earth's surface, while beneficial in blocking harmful UV radiation in the stratosphere, at ground level contributes to smog and poses health risks. Similarly, reactions involving pollutants like NO\text{ and }NO_{2}\text{, which originate from human activities like burning fossil fuels, illustrate the implications of atmospheric chemistry in environmental policy and public health. Understanding these reactions is crucial for developing strategies to reduce air pollution and mitigate climate change.