Problem 110 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

The entropy changes for each process are as follows: (a) The entropy increases during photodissociation of O₂(g) due to increased randomness. (b) The entropy decreases during the formation of ozone from oxygen molecules and atoms, as the number of particles is reduced. (c) The entropy increases during the diffusion of CFCs into the stratosphere, as it results in a more uniform distribution. (d) The entropy decreases during desalination of water by reverse osmosis, as the separation decreases the randomness in the system.

Step 1: Process (a): Photodissociation of O₂(g)

Photodissociation of O₂(g) is a process where a molecule of oxygen gas (O₂) absorbs energy from light and breaks down into two oxygen atoms (O). The reaction can be represented as: \( \mathrm{O}_{2}(g) \rightarrow 2\,\mathrm{O}(g) \) This process increases the randomness in the system as the number of particles increases from 1 molecule to 2 atoms, with more possible energy states. Thus, the entropy of the system increases during photodissociation of O₂(g).

Step 2: Process (b): Formation of ozone from oxygen molecules and oxygen atoms

The formation of ozone involves the combination of an oxygen molecule (O₂) and an oxygen atom (O) to form an ozone molecule (O₃). The reaction can be represented as: \( \mathrm{O}_{2}(g) + \mathrm{O}(g) \rightarrow \mathrm{O}_{3}(g) \) In this case, the number of particles in the system is reduced from 2 (1 molecule and 1 atom) to 1 (1 molecule). The randomness is reduced, and the entropy of the system decreases during the formation of ozone from oxygen molecules and oxygen atoms.

Step 3: Process (c): Diffusion of CFCs into the stratosphere

Diffusion is the process where molecules move from regions of higher concentration to lower concentration in order to reach a uniform distribution. In this case, CFCs (chlorofluorocarbons) are diffusing into the stratosphere. When comparing the initial and final states of the system, the uniform distribution of CFCs throughout the stratosphere increases the randomness. Therefore, the entropy of the system increases during the diffusion of CFCs into the stratosphere.

Step 4: Process (d): Desalination of water by reverse osmosis

In the process of reverse osmosis, water is separated from dissolved salt particles by applying pressure to force the water through a semipermeable membrane. The membrane allows only water to pass through, leaving the salts behind. Hence, the system is divided into pure water and concentrated salt solution. The separation of mixed particles (water and salt) decreases the randomness in the system. As such, the entropy of the system decreases during the desalination of water by reverse osmosis.

Key Concepts

Photodissociation

Photodissociation is a process where light energy breaks chemical bonds within a molecule. For instance, when oxygen gas (O_2) absorbs ultraviolet light, it splits into two separate oxygen atoms (O). This process is represented as:
\[\mathrm{O}_{2}(g) \rightarrow 2\,\mathrm{O}(g)\]
In this transformation, a single oxygen molecule turns into two oxygen atoms, leading to an increase in the number of particles. More particles mean more ways these particles can be arranged and more possible energy states, which is a measure of entropy. Hence, the entropy of a system increases during photodissociation because randomness or disorder increases. This is important in many natural processes such as the breakdown of chemical substances in the atmosphere, affecting how molecules like oxygen behave under sunlight.

Ozone Formation

Ozone formation involves the combination of an oxygen molecule (O_2) and an individual oxygen atom (O) to create an ozone molecule (O_3). This chemical reaction can be represented as:
\[\mathrm{O}_{2}(g) + \mathrm{O}(g) \rightarrow \mathrm{O}_{3}(g)\]
During this reaction, two particles (one oxygen molecule and one oxygen atom) combine to form one ozone molecule. This decrease in the number of particles leads to a decrease in randomness or disorder within the system. As a result, the entropy decreases when ozone is formed. Ozone plays a crucial role in protecting life on Earth by absorbing harmful ultraviolet radiation in the stratosphere. The balance between the formation and destruction of ozone is vital for maintaining the protective ozone layer.

Diffusion

Diffusion is the natural movement of particles from areas of higher concentration to areas of lower concentration until they are evenly distributed. For example, when chlorofluorocarbons (CFCs) are released, they gradually spread upwards into the stratosphere through the process of diffusion.

• This movement toward uniformity increases the overall entropy of the system.
• The particles start in a concentrated area and end in a spread-out, evenly distributed state.

The change from a state of lower to higher randomness signifies an increase in entropy. This process is fundamental in many natural phenomena such as gas exchange in the atmosphere and also impacts how pollutants like CFCs affect atmospheric chemistry.

Reverse Osmosis

Reverse osmosis is a process used to remove impurities, like salt, from water. It involves forcing water through a semipermeable membrane.

• The water passes through, but the impurities are left behind.
• This process separates water into two parts: pure water and a concentrated salt solution.

During reverse osmosis, the mixed state of water and salt is separated into two distinct parts. This separation reduces the randomness in the system because the molecules are more organized after the process than before. Therefore, the entropy of the system decreases during reverse osmosis. This technique is widely used in water purification, making seawater or brackish water safe for drinking by removing dissolved salts and other unwanted particles.