Problem 93 - Chemistry The Science in Context

Question

Which of these processes result in an entropy decrease of the system? a. Diluting hydrochloric acid with water b. Boiling water c. \(2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)\) d. Making ice cubes in the freezer

Step-by-Step Solution

Answer

Answer: Option d: Making ice cubes in the freezer

Step 1: Option a: Diluting hydrochloric acid with water

When hydrochloric acid (HCl) is diluted with water, the HCl molecules are spread out in the solvent, increasing the randomness or disorder of the system. As a result, the process increases the entropy of the system, and therefore this option is not the one we are looking for.

Step 2: Option b: Boiling water

Boiling water is a process in which liquid water turns into water vapor (gas). In the gaseous state, the water molecules are farther apart and more randomly distributed, leading to an increase in disorder and consequently, an increase in entropy. Therefore, this option is not the one we are looking for.

Step 3: Option c: \(2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)\)

In this chemical reaction, 2 particles of NO and 1 particle of O2 combine to form 2 particles of NO2. The total number of gas particles on both sides of the equation is the same (3 particles). However, the distribution of particles has changed, making it difficult to assess the change in entropy directly, and this option might not be the one we are looking for.

Step 4: Option d: Making ice cubes in the freezer

When making ice cubes in the freezer, liquid water is cooled and turns into a solid state - ice. In the solid state, the water molecules are in a more ordered and less random arrangement than in the liquid state. Thus, the process of making ice cubes results in a decrease in the entropy of the system. This option is the correct answer. The process that results in an entropy decrease of the system is:

Step 5: Answer

Option d: Making ice cubes in the freezer

Key Concepts

Decrease in Entropy

Entropy is a measure of the randomness or disorder within a system. When entropy decreases, it means the system becomes more ordered and less random. This often occurs during processes where particles transition from a dispersed state to a more structured arrangement.
For example, when you cool water to form ice, the water molecules arrange themselves in a fixed, orderly pattern that characterizes the solid phase of matter. This process results in a decrease in entropy because the molecules are no longer free to move around as they were when the water was in liquid form.

• In a solid, such as ice, molecules are tightly packed in a lattice structure.
• This leads to reduced molecular motion compared to the liquid state.
• The result is a decrease in randomness, hence a decrease in entropy.

Recognizing processes that decrease entropy can be crucial for understanding thermodynamic reactions and conditions.

Phase Change

Phase changes involve the transition of a substance from one state of matter to another, such as from solid to liquid, liquid to gas, or in reverse processes. Each phase has different levels of entropy due to varying molecular arrangements and energy states. When moving from a gas to a liquid or a liquid to a solid, entropy decreases.
Consider the process of making ice cubes: water changes from liquid to solid. This transformation involves a phase change where the water molecules lose energy and form a more ordered, fixed structure as they become ice. Such processes are typically exothermic, meaning they release heat to the surroundings. The reduced energy state and ordered arrangement result in a decrease in entropy.

• Phase changes include melting, freezing, condensation, and evaporation.
• The direction of a phase change determines whether entropy increases or decreases.
• A key factor in phase change is the transfer of energy as heat.

Understanding phase changes is essential for grasping how materials behave under different thermal conditions.

Thermodynamics

Thermodynamics is the study of energy transformations in physical and chemical processes. It's crucial for understanding how changes in energy and matter affect systems and their surroundings. One fundamental concept in thermodynamics is entropy, which plays a pivotal role in predicting the spontaneity of processes.
The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, implying that systems naturally progress towards disorder. The decline in entropy is often an indication of energy being released or utilized in a system to obtain a more ordered state.

• Entropy helps determine the spontaneity of chemical reactions and processes.
• A process with increased entropy likely occurs naturally.
• Understanding thermodynamics is vital for chemical engineering and environmental science.

Thermodynamics provides the theoretical framework to comprehend energy flow in everything from engines to biological systems.

Disorder in Chemistry

In chemistry, disorder refers to the degree of randomness in a system. It is often directly related to the concept of entropy. Processes that increase disorder, such as dissolving substances or vaporization, increase entropy. Conversely, processes that decrease disorder reduce entropy.
For instance, when making ice, water molecules move from a less ordered liquid state to a highly ordered solid state, reducing disorder. This reduction in disorder is synonymous with a decrease in entropy. Understanding how reactions and physical changes influence disorder helps predict the behavior of substances in chemical processes.

• Molecules in a gas state are highly disordered compared to solids.
• Reactions can lead to order and lower disorder through the formation of structured compounds.
• The concept of disorder explains why some reactions require energy input to proceed.

Recognizing the role of disorder in chemistry is key to mastering how substances interact and change.