Question

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

Short answer

Reactions (a) and (b) cause an increase in temperature in the stratosphere, as they involve the formation of ozone and the transfer of energy from excited ozone molecules, respectively. Therefore, the answer is (e) All of the above.

Step-by-step solution

Understanding the given reactions

We will analyze each given reaction and determine whether it contributes to an increase in temperature in the stratosphere. (a) O(g) + O2(g) -> O3*(g) In this reaction, an oxygen atom (O) reacts with an oxygen molecule (O2) to form an excited ozone molecule (O3*). (b) O3*(g) + M(g) -> O3(g) + M*(g) In this reaction, an excited ozone molecule (O3*) loses its excess energy by transferring it to another molecule (M), resulting in a non-excited ozone molecule (O3) and an excited molecule (M*). (c) O2(g) + hν -> 2O(g) In this reaction, an oxygen molecule (O2) absorbs ultraviolet radiation (hν) and splits into two oxygen atoms (O). (d) O(g) + N2(g) -> NO(g) + N(g) In this reaction, an oxygen atom (O) reacts with a nitrogen molecule (N2) to form a nitric oxide molecule (NO) and a nitrogen atom (N). Now, we will identify which reactions contribute to an increase in temperature in the stratosphere.

Identifying the reactions causing an increase in temperature

Following the analysis, we can conclude: (a) This reaction forms excited ozone (O3*) molecules, which are important for absorbing UV radiation. Thus, it contributes to an increase in temperature in the stratosphere. (b) This reaction involves the transfer of energy from an excited ozone molecule (O3*) to another molecule (M). As a result, the excess energy is dispersed to other molecules, increasing the temperature. Thus, this reaction also contributes to an increase in temperature. (c) In this reaction, an oxygen molecule (O2) absorbs ultraviolet radiation and dissociates into oxygen atoms. While this reaction is crucial for the ozone formation cycle, it does not directly contribute to an increase in temperature, since energy is used to dissociate the molecule rather than being converted to heat. (d) This reaction does not contribute directly to the formation of ozone or involve the absorption of UV radiation. Therefore, it does not contribute to an increase in temperature in the stratosphere.

Final Answer

Based on our analysis, reactions (a) and (b) are responsible for causing an increase in temperature in the stratosphere. Thus, the answer to this question is (e) All of the above, considering that there are only two reactions in this particular case, and both contribute to the heating of the stratosphere.

Key concepts

Stratospheric Reactions

Understanding how certain reactions in the stratosphere contribute to temperature changes is crucial for grasping the complex dynamics of Earth's atmosphere. The stratosphere, situated above the troposphere and below the mesosphere, is known for its temperature inversion, where temperature increases with altitude due to the absorption of ultraviolet (UV) radiation by ozone. This is counterintuitive compared to the troposphere, where temperature typically decreases with altitude.

• When oxygen atoms combine with oxygen molecules, ozone is generated, and this reaction releases energy, thereby warming the surrounding air.
• The decomposition of ozone by UV light though does not warm the stratosphere meaningfully, as most energy goes into breaking the ozone apart.

Overall, it's the formation and de-excitation of ozone that predominantly contribute to the heating of the stratosphere.

Excited Ozone Molecule

An 'excited' ozone molecule, often denoted as O3*, is an ozone molecule that has absorbed energy but has not yet released it. This excitement is due to the absorption of energy from various reactions in the stratosphere, including the collision between an oxygen atom and an oxygen molecule.

• When an excited ozone molecule returns to its non-excited state, it releases energy in the form of heat, contributing to the increased temperature in the stratosphere.
• The presence of other molecules (M), typically nitrogen or oxygen, helps in transferring this energy, stabilizing the excited ozone molecule into a non-excited state.

Recognizing how these molecules release stored energy as heat helps us understand the thermal dynamics within the stratospheric layer.

UV Radiation Absorption

In the stratosphere, UV radiation plays a pivotal role in both temperature regulation and chemical reactions. Absorption of UV radiation happens primarily through ozone molecules, which act as Earth's natural sunscreen, blocking harmful UV rays from reaching the ground level. Here's how the absorption impacts the stratosphere:

• Ozone molecules absorb UV radiation energy and break down into constituent oxygen atoms and molecules — a process that does not directly heat the stratosphere.
• However, the formation of ozone from oxygen atoms and molecules (which may have been generated from previous ozone decomposition) releases heat, contributing to the warming of the atmosphere.

This sequence of absorption and release of energy through various reactions keeps the stratosphere warmer than the layers below it.

Ozone Formation Cycle

The ozone formation cycle is a key player in the stratospheric temperature profile. It is a sequence of reactions that lead to the creation and destruction of ozone, which is central to the heat dynamics in the stratosphere.

• Oxygen molecules are split by UV light into individual oxygen atoms.
• These atoms then react with oxygen molecules to form ozone, which is often in an excited state and releases energy as it stabilizes.
• While the initial breaking apart of ozone by UV does not increase temperature, the subsequent recombination into ozone is exothermic and warms the surrounding air.

This cyclical nature of ozone transformation leads to the fluctuation of temperature and stability within the stratosphere's thermal structure.