Problem 24 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following reactions in the stratosphere cause an increase in temperature there? (a) \(\mathrm{O}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{O}_{3}^{*}(g)\) (b) \(\mathrm{O}_{3}^{*}(g)+\mathrm{M}(g) \longrightarrow \mathrm{O}_{3}(g)+\mathrm{M}^{*}(g)\) (c) \(\mathrm{O}_{2}(g)+h \nu \longrightarrow 2 \mathrm{O}(g)\) (d) \(\mathrm{O}(g)+\mathrm{N}_{2}(g) \longrightarrow \mathrm{NO}(g)+\mathrm{N}(g)\) (e) All of the above

Step-by-Step Solution

Answer

The reactions (a) and (d) are exothermic and cause an increase in temperature in the stratosphere.

Step 1: Analyze Reaction (a)

Analyze the reaction: \(\mathrm{O}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{O}_{3}^{*}(g)\). It consists of oxygen atoms combining to form ozone (\(\mathrm{O}_3\)). The ozone is in an excited state denoted by the asterisk. As it is a combination reaction, it is usually exothermic, meaning it releases energy.

Step 2: Analyze Reaction (b)

Analyze the reaction: \(\mathrm{O}_{3}^{*}(g)+\mathrm{M}(g) \longrightarrow \mathrm{O}_{3}(g)+\mathrm{M}^{*}(g)\). In this reaction, the excited ozone transfers its energy to another molecule M (termed the collision partner). The result is the ground state ozone and an excited-state molecule M. This reaction is energy-transfer, and does not affect the overall temperature.

Step 3: Analyze Reaction (c)

Analyze the reaction: \(\mathrm{O}_{2}(g)+h \nu \longrightarrow 2 \mathrm{O}(g)\). In this photodissociation reaction, a photon (hν) supplies the energy required to dissociate diatomic oxygen (\(\mathrm{O}_2\)) into two single oxygen atoms. It is an endothermic reaction as it absorbs energy.

Step 4: Analyze Reaction (d)

Analyze the reaction: \(\mathrm{O}(g)+\mathrm{N}_{2}(g) \longrightarrow \mathrm{NO}(g)+\mathrm{N}(g)\). This reaction involves the formation of a nitrogen monoxide molecule from the combination of a single oxygen atom and a nitrogen molecule, followed by a dissociation into nitrogen monoxide and an atomic nitrogen. The change in energy cannot be deduced directly without the help of thermodynamic data, but it is known that it is exothermic reaction.

Step 5: Answer the question

From our analysis, reactions (a) and (d) are exothermic and cause an increase in temperature in the stratosphere. Therefore, the correct answer is (a) and (d).

Key Concepts

Ozone Formation

In the stratosphere, ozone formation is a critical process that plays a vital role in protecting life on Earth. Ozone, a molecule comprising three oxygen atoms, is primarily formed when a free oxygen atom (O) bumps into an oxygen molecule (O₂) to create ozone (O₃). This is represented by the reaction \( \text{O}(g) + \text{O}_2(g) \rightarrow \text{O}_3(g) \).
At times, this ozone is in an excited state, indicated by an asterisk as \( \text{O}_3^* \). The excited ozone will eventually transfer its energy and settle into a more stable state. This process is important as it contributes to the balance and concentration of ozone in the stratosphere, where it absorbs harmful ultraviolet radiation.

Exothermic Reactions

Exothermic reactions in the stratosphere are processes that release energy into the surrounding environment, typically in the form of heat. When molecules combine to form ozone, as seen in the reaction \( \text{O}(g) + \text{O}_2(g) \rightarrow \text{O}_3^*(g) \), energy is released. This release of energy can cause a temperature increase, contributing to the warming of the stratosphere.
This is a combination reaction because it involves the joining of atoms or molecules to form a more complex product and release energy. Understanding exothermic reactions is essential to predicting temperature changes and studying atmospheric chemistry.

Photodissociation

Photodissociation is a process where a molecule absorbs a photon (a particle of light) and splits into two or more smaller fragments. In the stratosphere, photodissociation is critically important for the production of singular oxygen atoms. For example, when an oxygen molecule (O₂) absorbs a photon (denoted as \( hu \)), it splits into two separate oxygen atoms \( \text{O}_2(g) + hu \rightarrow 2 \text{O}(g) \).
This process requires energy input from the sun, making it an endothermic reaction, as it absorbs energy. Photodissociation is vital for the continual regeneration of ozone and other atmospheric processes that regulate the earth's temperature and climate.

Temperature Change

Temperature changes in the stratosphere can result from chemical reactions that either release or absorb energy. Exothermic reactions such as \( \text{O}(g) + \text{O}_2(g) \rightarrow \text{O}_3^*(g) \), release energy and raise the surrounding temperature. Conversely, endothermic reactions like \( \text{O}_2(g) + hu \rightarrow 2 \text{O}(g) \) absorb energy, which can lead to cooling.
The balance between these reactions is essential for maintaining the temperature and stability of the stratosphere. The interactions between different chemical species and the incoming solar radiation drive complex processes impacting climate regulation.

Stratosphere

The stratosphere is the second layer of Earth's atmosphere, located above the troposphere and below the mesosphere. One of its defining features is the ozone layer, which plays a critical role in absorbing the majority of the sun’s harmful ultraviolet radiation.
The stratosphere's composition and temperature are influenced by various chemical reactions, particularly those involving ozone. Changes in the concentration of ozone can affect the temperature, as different reactions either release or absorb heat. As a result, understanding stratospheric chemistry, including processes like ozone formation and photodissociation, is crucial for monitoring environmental changes and protecting life on Earth.