Problem 74 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write balanced chemical equations for each of the following reactions: (a) The carbon dioxide molecule undergoes photodissociation in the upper atmosphere. (b) The carbon dioxide molecule undergoes photoionization in the upper atmosphere. (c) Carbon monoxide undergoes oxidation by ozone in the stratosphere. (d) Carbon dioxide dissolves in water to form hydrogen carbonate.

Step-by-Step Solution

Answer

(a) \(CO_2 + hv \rightarrow CO + O\) (b) \(CO_2 + hv \rightarrow CO_2^+ + e^-\) (c) \(CO + O_3 \rightarrow CO_2 + O_2\) (d) \(CO_2 + H_2O \rightarrow HCO_3^-\)

Step 1: 1. Photodissociation of carbon dioxide

When carbon dioxide undergoes photodissociation, it absorbs a photon and breaks down into carbon monoxide (CO) and an oxygen atom (O). The balanced chemical equation for this reaction is: \[CO_2 + hv \rightarrow CO + O\]

Step 2: 2. Photoionization of carbon dioxide

When carbon dioxide undergoes photoionization, it absorbs a photon and loses an electron, resulting in the formation of a positive ion (CO2+). The balanced chemical equation for this reaction is: \[CO_2 + hv \rightarrow CO_2^+ + e^-\]

Step 3: 3. Oxidation of carbon monoxide by ozone

In the stratosphere, carbon monoxide can react with ozone (O3) resulting in the formation of carbon dioxide (CO2) and molecular oxygen (O2). The balanced chemical equation for this reaction is: \[CO + O_3 \rightarrow CO_2 + O_2\]

Step 4: 4. Carbon dioxide dissolving in water to form hydrogen carbonate

When carbon dioxide dissolves in water, it forms hydrogen carbonate (HCO3-) by reacting with water molecules. The balanced chemical equation for this reaction is: \[CO_2 + H_2O \rightarrow HCO_3^-\]

Key Concepts

Photodissociation

Photodissociation is a process in which a molecule absorbs a photon of light and undergoes a chemical change. In the case of carbon dioxide, photodissociation occurs when the molecule absorbs ultraviolet light from the sun. This absorption provides enough energy for the carbon dioxide molecule (\(CO_2\)) to break apart into carbon monoxide (\(CO\)) and a single oxygen atom (\(O\)). This process is important in the upper atmosphere where high-energy photons are more prevalent.Key Points:

• Occurs when a molecule absorbs high-energy light.
• Results in the breaking of chemical bonds.
• Essential for the formation of different elements in atmospheric chemistry.

Photoionization

Photoionization is another interaction between light and matter but it differs from photodissociation. In this process, a molecule absorbs a photon and loses one or more electrons. This leads to the formation of a positively charged ion. For carbon dioxide, when it absorbs ultraviolet light, it can lose an electron, forming a CO₂⁺ ion. The electron is ejected as a free electron. Characteristics:

• Involves absorption of photons to eject electrons.
• Results in the creation of ions.
• Does not involve breaking molecules apart, just removing electrons.

This is significant in atmospheric chemistry because ions play crucial roles in many atmospheric processes.

Oxidation

Oxidation reactions are chemical processes in which a substance loses electrons. In atmospheric chemistry, one common oxidation reaction is when carbon monoxide (\(CO\)) reacts with ozone (\(O_3\)) to form carbon dioxide (\(CO_2\)) and molecular oxygen (\(O_2\)). Understanding Oxidation:

• Typically involves transfer of electrons.
• Often associated with the addition of oxygen to a substance.
• Important in environmental and atmospheric processes, as seen in smog formation and ozone depletion.

This reaction is vital in the stratosphere, helping to reduce pollutants like carbon monoxide through natural chemical cycles.

Balanced Chemical Equations

Balanced chemical equations are essential tools in chemistry. They depict the substances involved in a chemical reaction and ensure the conservation of mass. For any chemical reaction, the number of atoms of each element involved must be the same on both sides of the equation. Steps to Balance Chemical Equations:

• Write the unbalanced equation identifying reactants and products.
• Adjust coefficients to get the same number of each type of atom on both sides of the equation.
• Check to ensure all coefficients are in the simplest possible ratio.

Balancing equations like those for photodissociation, photoionization, oxidation, and reactions of gases in the atmosphere ensures that scientific observations are accurately represented and can be clearly communicated.