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

(a) The entropy of the system increases during photodissociation of O2(g) as the number of particles increases, resulting in greater disorder. (b) The entropy of the system decreases during the formation of ozone from oxygen molecules and oxygen atoms, as the number of particles decreases, indicating a decrease in disorder. (c) The entropy of the system increases during the diffusion of CFCs into the stratosphere as the molecules disperse and the overall randomness increases. (d) The entropy of the system decreases during desalination of water by reverse osmosis, as the purified water has a more ordered structure compared to the starting mixture.

Step-by-step solution

(a) Photodissociation of O2(g)

Photodissociation is the process in which a molecule absorbs a photon and breaks apart into smaller fragments. When O2(g) absorbs a photon and undergoes photodissociation, it breaks down into free oxygen atoms: $${\mathrm{O}}_2(\mathrm{g})\stackrel{\text{photon}}{\to }2\mathrm{O}(\mathrm{g})$$ The number of particles increases in this process, and as a result, there is a greater degree of randomness or disorder in the system. Hence, the entropy of the system increases during photodissociation of O2(g).

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

Ozone is formed from the combination of an oxygen molecule and an oxygen atom: $${\mathrm{O}}_2(\mathrm{g})+\mathrm{O}(\mathrm{g})\to {\mathrm{O}}_3(\mathrm{g})$$ In this process, the number of particles decreases, indicating a decrease in disorder or randomness. Therefore, the entropy of the system decreases during the formation of ozone from oxygen molecules and oxygen atoms.

(c) Diffusion of CFCs into the stratosphere

When CFCs (chlorofluorocarbons) spread or diffuse into the stratosphere, the molecules disperse, and the overall randomness of the system increases. The dispersion of particles from an area of higher concentration to a region of lower concentration reflects an increase in entropy. In conclusion, the entropy of the system increases during the diffusion of CFCs into the stratosphere.

(d) Desalination of water by reverse osmosis

Reverse osmosis is a process in which water is forced through a semi-permeable membrane by applying pressure, separating the water molecules from dissolved salts and other impurities. This process results in two separate systems: one with purified water and another with concentrated salts and impurities. The purified water has a more ordered structure compared to the starting mixture, indicating a decrease in entropy in that part of the system. Hence, the entropy of the system decreases during desalination of water by reverse osmosis.