Problem 20 - Chemistry: An Atoms-Focused Approach

Question

Which of the following processes are exothermic, and which are endothermic? a. Ice cubes solidify in the freezer. b. Ice cubes in a frost-free freezer slowly lose mass. c. Dew forms on a lawn overnight.

Step-by-Step Solution

Answer

Question: Categorize the following processes as exothermic or endothermic: a) Ice cubes solidify in the freezer b) Ice cubes in a frost-free freezer slowly lose mass c) Dew forms on a lawn overnight Answer: a) Exothermic b) Endothermic c) Exothermic

Step 1: Process a: Ice cubes solidify in the freezer

When ice cubes solidify in the freezer, they change from a liquid state to a solid state. This phase change requires the molecules in the liquid to lose energy. As the energy is lost in the form of heat to the surroundings, this process is exothermic.

Step 2: Process b: Ice cubes in a frost-free freezer slowly lose mass

In a frost-free freezer, the air is circulated to prevent the buildup of ice, which can cause the ice cubes to sublime. Sublimation is the process of a solid directly changing to a gas state, bypassing the liquid state. In this case, the ice cubes absorb energy from the surroundings to sublime, making this process endothermic.

Step 3: Process c: Dew forms on a lawn overnight

Dew forms on a lawn overnight when water vapor in the air condenses to form liquid water droplets. This phase change from gas to liquid involves the release of energy as the water vapor molecules slow down and form bonds with each other in the liquid state. As energy is released in the form of heat, this process is exothermic.

Key Concepts

Phase Changes

Phase changes are important concepts in thermodynamics where a substance transitions from one state of matter to another. The primary states of matter involved in phase changes are solid, liquid, and gas. Each phase change requires or releases energy, resulting in different processes becoming either exothermic or endothermic. Phase changes include:

• Melting: Solid to liquid
• Freezing: Liquid to solid
• Vaporization: Liquid to gas
• Condensation: Gas to liquid
• Sublimation: Solid to gas
• Deposition: Gas to solid

Each of these processes involves a transfer of energy. Understanding whether energy is absorbed or released can help us determine if the process is endothermic or exothermic, which is key in solving thermodynamic problems.

Exothermic Processes

Exothermic processes are those where energy is released, usually in the form of heat, into the surroundings. This release comes as substances undergo phase changes that lead to increased stability. For example, when ice cubes solidify in a freezer (freezing), the molecules lose energy and become more ordered in their solid form. This transition from a liquid to a solid releases heat to the surroundings, characterizing it as an exothermic process. Similarly, the formation of dew on grass at night involves the condensation of water vapor into liquid. This phase change releases heat energy as water molecules bond together, indicating dews forming is also an exothermic process. In essence, exothermic processes tend to cool down the environment as they transfer energy away.

Endothermic Processes

Endothermic processes absorb energy, making them essential for understanding how energy drives changes in matter. In these processes, energy is typically absorbed in the form of heat from the surroundings, resulting in cooler surroundings. An excellent example of endothermic reaction is sublimation, as seen when ice cubes lose mass in a frost-free freezer. Here, ice transforms directly from solid to gas, requiring energy input to break intermolecular bonds. This absorption of energy from the environment characterizes it as an endothermic process. During endothermic reactions, the surroundings often feel cooler because heat is absorbed to facilitate the phase change. Understanding endothermic processes is crucial, especially in calculating energy balances in chemical reactions and thermodynamic systems.