Problem 106 - Chemistry The Central Science In Si Units

Question

The following processes were all discussed in Chapter \(18,\) "Chemistry of the Environment." Estimate whether the entropy of the system increases or decreases during each process: (a) photodissociation of \(\mathrm{O}_{2}(g),(\mathbf{b})\) formation of ozone from oxygen molecules and oxygen atoms, \((\mathbf{c})\) diffusion of CFCs into the stratosphere, (d) desalination of water by reverse osmosis.

Step-by-Step Solution

Answer

Entropy increases in (a) and (c), and decreases in (b) and (d).

Step 1: Understanding Entropy

Entropy, a measure of randomness or disorder in a system, generally increases when a system becomes more disordered. Processes that increase the number of particles or spread them more widely tend to increase entropy, while those that result in more ordered arrangements tend to decrease entropy.

Step 2: Analyzing Photodissociation of \(\mathrm{O}_2(g)\)

Photodissociation of \(\mathrm{O}_{2}(g)\), referring to the breaking down of oxygen molecules into individual oxygen atoms under the influence of light, leads to an increase in the number of particles. The increase in particle number increases the disorder and thus, entropy of the system.

Step 3: Formation of Ozone from Oxygen Molecules and Atoms

The formation of ozone \((\mathrm{O}_3)\) from oxygen molecules and atoms involves the combination of atoms to form larger, more complex molecules. This combining process tends to decrease the number of independent particles and makes the system more ordered, leading to a decrease in entropy.

Step 4: Diffusion of CFCs into the Stratosphere

Diffusion is the process in which particles spread out to occupy more available space. As CFCs move from a concentrated area to the more expansive stratosphere, the particles become more randomly distributed, resulting in increased disorder and thus, an increase in entropy.

Step 5: Desalination of Water by Reverse Osmosis

Reverse osmosis in desalination involves removing dissolved salts from water molecules, which decreases the randomness by creating a separation between pure water molecules and ions. This separation increases the order of the system, resulting in a decrease in entropy.

Key Concepts

Photodissociation

Photodissociation is an intriguing chemical process where molecules break apart into atoms or smaller molecules when exposed to light. This process primarily involves the absorption of photon energy, leading to the breaking of chemical bonds. For example, in the case of

• Oxygen photodissociation, light energy separates \( \mathrm{O}_2 \) (Oxygen) into two separate oxygen atoms.
• The increase in the number of particles, from one molecule to two separate atoms, leads to a higher level of disorder or randomness.
• As a result, there is an increase in entropy, because the system has become more chaotic with more particles moving independently.

Photodissociation is a crucial process in atmospheric chemistry, as it plays a significant role in various reactions, including the formation of ozone.

Ozone Formation

Ozone formation is a critical process for protecting life on Earth, as ozone absorbs harmful ultraviolet radiation. The formation of ozone

• Begins with the splitting of an oxygen molecule \( \mathrm{O}_2 \) into two free oxygen atoms under ultraviolet light.
• These individual oxygen atoms then collide with other \( \mathrm{O}_2 \) molecules to form ozone (\( \mathrm{O}_3 \)).
• This process reduces the number of independent particles, as individual atoms form more complex molecules (ozone), thereby decreasing system randomness.
• The decrease in independent, random particles means a decrease in the system’s entropy.

Ozone creation is important for stabilizing the atmosphere, but it results in a more ordered system, highlighting the dynamic balance in atmospheric chemistry.

Diffusion

Diffusion is a naturally occurring phenomenon where particles move from regions of higher concentration to areas of lower concentration. This movement is driven by the tendency to reach equilibrium or uniform distribution. Consider the diffusion of chlorofluorocarbons (CFCs):

• When CFCs are released, they spread from their concentrated release point into the vastness of the stratosphere.
• The particles move randomly and spread out, increasing the overall disorder of the system.
• This spreading out results in an increase in entropy, as the distribution of particles becomes more even and less concentrated.

Diffusion is a fundamental concept in chemistry and physics, showcasing how energy and material transportation work to create balance and disorder at the same time.

Desalination

Desalination is an essential process for converting seawater into freshwater, crucial for potable water and agriculture. The method commonly used is reverse osmosis, where

• Seawater is pushed through a semi-permeable membrane.
• This membrane allows only water molecules to pass through, effectively separating dissolved salts from the water.
• This process takes a disordered mixture of salt and water molecules and creates distinct, more ordered systems: fresh water and concentrated salt brine.
• Because the system moves towards order—separating molecules that were once evenly mixed—the entropy decreases.

Understanding desalination highlights the practical applications of entropy in engineering and technology, as we utilize these scientific principles to solve global water scarcity issues.