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 \(\mathrm{O}_{2}(g)\), (b) formation of ozone from oxygen molecules and oxygen atoms, (c) diffusion of CFCs into the stratosphere, (d) desalination of water by reverse osmosis.

Short answer

In summary, the entropy of the system experiences the following changes during each process: a) Photodissociation of \(\mathrm{O}_{2}(g)\): Entropy increases b) Formation of ozone from oxygen molecules and oxygen atoms: Entropy decreases c) Diffusion of CFCs into the stratosphere: Entropy increases d) Desalination of water by reverse osmosis: Entropy decreases

Step-by-step solution

Process A: Photodissociation of \(\mathrm{O}_{2}(g)\)

Photodissociation is a process where a molecule absorbs a photon and splits into two or more fragments. In this case, \(\mathrm{O}_{2}(g)\) absorbs a photon and breaks down into two oxygen atoms. This process leads to an increase in the system's disorder, as there are more individual particles in the products than the reactants. So, the entropy of the system increases during photodissociation.

Process B: Formation of ozone from oxygen molecules and oxygen atoms

Ozone is formed from the combination of oxygen molecules (\(\mathrm{O}_{2}\)) and oxygen atoms (\(\mathrm{O}\)). The reaction can be expressed as follows: $$\mathrm{O}_{2}(g) + \mathrm{O}(g) \rightarrow \mathrm{O}_{3}(g)$$ Here, we are converting two distinct gas particles into a single gas particle. This leads to a reduction in the disorder within the system. Hence, the entropy of the system decreases during the formation of ozone.

Process C: Diffusion of CFCs into the stratosphere

Diffusion is the process where particles spread out in a given space. In this case, CFCs diffuse from their original location and spread into the stratosphere. This spreading out leads to an increase in the disorder or randomness of the particles in the system. Therefore, the entropy of the system increases during the diffusion of CFCs into the stratosphere.

Process D: Desalination of water by reverse osmosis

Reverse osmosis is a process used to remove dissolved salts from saline water by applying pressure. This process separates salts and water molecules, resulting in a purified water output and a concentrated saline solution. In this process, the system becomes more ordered, as salts and water molecules are separated. So, the entropy of the system decreases during desalination of water by reverse osmosis.

Key concepts

Photodissociation

Photodissociation occurs when a molecule absorbs a photon and breaks into smaller fragments. A common example in environmental chemistry is the photodissociation of \(\mathrm{O}_2(g)\). When oxygen molecules absorb energy from light, they split into individual oxygen atoms.
This process increases the system's entropy, or disorder, because you start with a single molecule and end up with multiple smaller particles.
- More particles generally mean more disorder.- Photodissociation plays a significant role in atmospheric processes, especially in reactions involving ozone.
In the atmosphere, photodissociation contributes to the breakdown of pollutants and influences the balance of gases. For instance, it is part of the natural cycle that maintains the concentration of ozone in the stratosphere.

Ozone Formation

Ozone formation is a fascinating chemical reaction that involves oxygen molecules \(\mathrm{O}_2\) and oxygen atoms \(\mathrm{O}\) coming together to form ozone \(\mathrm{O}_3\). The reaction can be represented as: \[\mathrm{O}_2(g) + \mathrm{O}(g) \rightarrow \mathrm{O}_3(g)\]
This process actually decreases the system's entropy because you are combining two separate particles into one.

• Fewer particles generally mean less disorder.
• Ozone acts as a shield for the Earth, absorbing harmful ultraviolet radiation.

Though the creation of ozone results in decreased entropy locally, its presence is crucial for protecting life on Earth by filtering out UV radiation.

Diffusion in Atmosphere

Diffusion refers to the natural spreading of particles as they move from areas of higher concentration to areas of lower concentration.
When it comes to chlorofluorocarbons (CFCs) diffusing into the stratosphere, diffusion results in an increase in the system's entropy. This happens because: - CFCs spread out, leading to a more randomized distribution. - Increased randomness or spatial distribution correlates with increased entropy. Diffusion is essential for the dispersion of gases in the atmosphere, including pollutants and natural elements. It allows substances like CFCs, once released into the environment, to mix and travel upwards into the stratosphere, where they can have long-term environmental effects.

Desalination by Reverse Osmosis

Desalination by reverse osmosis is a method of removing salts from seawater to produce fresh water. This process utilizes a semi-permeable membrane to separate water from salt.
- Pressure is applied to overcome the natural osmotic pressure. - Salt ions are retained on the pressurized side while pure water passes through.
The entropy in the system decreases during desalination by reverse osmosis because the process organizes substances into ordered states: - Salt and water molecules are separated, creating more order. This decrease in entropy is counterintuitive in natural processes but vital for providing fresh water in arid regions. It requires energy to achieve, reflecting the broader principle that increasing order typically demands an input of energy.