Problem 115 - Chemistry Matter and Change

Question

Which of the following physical processes are exothermic for water—freezing, boiling, condensing, subliming, evaporating? (Chapter 12)

Step-by-Step Solution

Answer

Freezing and condensing are the exothermic processes for water, as they release heat energy during the phase transition. Freezing occurs when liquid water turns into a solid, and condensing happens when water vapor turns into a liquid. In both cases, heat is released, increasing the temperature of the surroundings.

Step 1: Identify Exothermic Process

An exothermic process is any that releases heat energy.

Step 2: Assess Freezing

Freezing is exothermic. When water freezes, it releases heat, thereby lowering its internal energy and increasing the surroundings' temperature.

Step 3: Assess Boiling

Boiling is endothermic, not exothermic. This process absorbs heat energy from the surroundings to provide the energy needed for the water molecules to overcome the forces holding them together, transforming from a liquid to a gas.

Step 4: Assess Condensing

Condensation is exothermic. When water vapor condenses into liquid water, heat is released, increasing the temperature of the surroundings.

Step 5: Assess Subliming

Sublimation is generally endothermic for most substances, including water. This is because heat energy is absorbed by the water ice in order to change from a solid directly into a gas, without undergoing a liquid phase.

Step 6: Assess Evaporating

Evaporation is endothermic. When water evaporates, it absorbs energy from the surroundings, causing the overall temperature to drop. So, based on the assessments, freezing and condensation are the exothermic processes for water.

Key Concepts

Heat Energy

Heat energy is the energy that is in transit between substances due to a temperature difference. When it comes to water, different physical processes involve the absorption or release of this energy. In exothermic processes, water releases heat energy, warming its surroundings. In contrast, during endothermic processes, water absorbs heat energy, cooling the surroundings.

• Exothermic: Releases heat energy
• Endothermic: Absorbs heat energy

This transfer of heat energy plays a key role in determining whether a process is exothermic or endothermic.

Physical Processes

Physical processes involve changes in the state or appearance of a substance without altering its chemical composition. For water, some common physical processes include freezing, boiling, condensing, subliming, and evaporating.
These processes often involve changes in energy, specifically heat energy, leading to either endothermic or exothermic reactions. The distinction between these processes is made based on whether they release or absorb heat energy. Understanding these physical processes is crucial for determining if a reaction is exothermic or endothermic.

Phase Changes

Phase changes are transformations between different states of matter: solid, liquid, and gas. For example, freezing is a phase change from liquid to solid, while condensing goes from gas to liquid.
These phase changes are either exothermic or endothermic:

• Exothermic phase changes: Freezing and condensing. These release heat energy to the surroundings.
• Endothermic phase changes: Boiling, subliming, and evaporating. These absorb heat energy from the surroundings.

Understanding phase changes helps us grasp how heat energy is transferred during these transformations.

Thermodynamics

Thermodynamics is the study of energy, particularly how it is transferred and converted. It's a crucial field for understanding physical and phase changes. Exothermic and endothermic reactions are prime examples of thermodynamic processes since they involve the movement of heat energy.
Thermodynamics provides a framework for explaining how and why these processes occur, allowing us to predict whether a process will release or absorb energy. By analyzing energy changes, we gain insight into the nature of different physical and chemical transformations.