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, (c) diffusion of CFCs into the stratosphere, (d) desalination of water by reverse osmosis.

Short answer

(a) The photodissociation of O2(g) increases the number of particles in the system, leading to an increase in entropy. (b) The formation of ozone from oxygen molecules and oxygen atoms decreases the number of particles in the system, leading to a decrease in entropy. (c) The diffusion of CFCs into the stratosphere increases entropy due to the increased dispersal of energy. (d) The desalination of water by reverse osmosis results in a more ordered state of the system, leading to a decrease in entropy.

Step-by-step solution

Process (a) Photodissociation of O2(g)

Photodissociation is the process of breaking a molecule into its constituent atoms or fragments under the influence of light. In the case of O2(g), the photodissociation process can be represented as follows: \[O_2(g) \xrightarrow{h \nu} 2O(g)\] As we can see, the number of particles in the system increases from 1 O2 molecule to 2 O atoms. An increase in the number of particles generally leads to an increase in entropy due to increased dispersal of energy. Therefore, the entropy of the system increases during this process.

Process (b) Formation of ozone from oxygen molecules and oxygen atoms

Ozone is formed from oxygen molecules and oxygen atoms via the following reaction: \[O_2(g) + O(g) \rightarrow O_3(g)\] In this case, two particles (one O2 molecule and one O atom) are combining to form one new particle (O3 molecule). A decrease in the number of particles generally leads to a decrease in entropy. Therefore, the entropy of the system decreases during this process.

Process (c) Diffusion of CFCs into the stratosphere

Diffusion is the process of particles spreading out from a region of high concentration to a region of lower concentration to reach a state of equilibrium. When CFCs (chlorofluorocarbons) diffuse into the stratosphere, their concentration decreases as they spread out among the other particles in the stratosphere, leading to an increase in entropy due to increased dispersal of energy. Therefore, the entropy of the system increases during this process.

Process (d) Desalination of water by reverse osmosis

Reverse osmosis is a process in which water is forced through a semi-permeable membrane to separate it from dissolved salts and impurities. In this process, water molecules that pass through the membrane are separated from the salt molecules and impurities, leading to a more ordered state of the system. This results in a decrease in entropy. Therefore, the entropy of the system decreases during this process.

Key concepts

Photodissociation

When molecules break apart under the influence of light, this is called photodissociation. It is a fascinating process because it involves the energy from light causing a bond in the molecule to break, resulting in smaller fragments. Taking the example of oxygen gas (\(O_2\)), photodissociation occurs as follows: \[ O_2(g) \xrightarrow{h u} 2O(g) \].
This reaction shows how one oxygen molecule transforms into two separate oxygen atoms.
Because the system ends up with more particles than it started with, energy is dispersed more widely, leading to an increase in entropy. Some key points about photodissociation:

• It requires energy input, often from ultraviolet light.
• It leads to an increase in particle number, and therefore, entropy.
• It plays a crucial role in atmospheric and environmental chemistry.

Ozone Formation

The formation of ozone involves the reaction between oxygen molecules and individual oxygen atoms. This can be represented by the equation: \[ O_2(g) + O(g) \rightarrow O_3(g) \].
In this process, two separate particles combine to form a single ozone molecule. While ozone is crucial for blocking harmful ultraviolet radiation from reaching the Earth's surface, this reaction involves a decrease in entropy.
This is because the number of particles decreases, leading to less dispersal of energy. Important details about ozone formation include:

• The reaction is vital for creating the ozone layer, which protects life on Earth.
• Due to fewer molecules being present in the final state, entropy decreases.
• Ozone formation is a natural atmospheric process involving complex chemical pathways.

Diffusion

Diffusion is a process driven by the natural tendency of particles to spread out from areas of higher concentration to areas of lower concentration. When substances like chlorofluorocarbons (CFCs) diffuse into the stratosphere, they spread out among the particles already present there. This spreading out of particles leads to an increase in entropy, as the system moves towards a more disordered state.
Key aspects of diffusion include:

• Particles move from high to low concentration, eventually reaching equilibrium.
• It increases entropy due to the wider dispersal of particles.
• Diffusion is an essential concept in natural processes and industrial applications.

Reverse Osmosis

Reverse osmosis is a purification process where water molecules are pushed through a semi-permeable membrane, separating them from salts and impurities. This technique is largely employed in desalinating seawater.
Through reverse osmosis, water molecules are placed on one side, creating a more ordered state and consequently decreasing entropy in the system.
Some critical points about reverse osmosis include:

• It requires an external pressure to push water through the membrane.
• The separation process means fewer free-moving particles in the water.
• Widely used for obtaining clean water in both industrial and household settings.