Problem 103 - Chemistry The Molecular Science

Question

Based on your experience, when ice melts to liquid water, is the process exothermic or endothermic? When liquid water freezes to ice at \(0{ }^{\circ} \mathrm{C}\), is this exothermic or endothermic? (Assume that the ice/water is the system in each case.) Explain your answers.

Step-by-Step Solution

Answer

Melting is endothermic; freezing is exothermic.

Step 1: Understanding Energy Changes in Melting

The process of ice melting into liquid water involves the absorption of heat energy by the system (the ice/water). Since energy is absorbed to break the hydrogen bonds in the ice to transition it into water, this process is endothermic. In an endothermic reaction, the system gains energy from its surroundings.

Step 2: Analyzing the Freezing Process

When liquid water freezes into ice, the system releases energy to its surroundings. This occurs because the water molecules lose energy as they form hydrogen bonds, transitioning from the higher energy state of liquid water to the lower energy state of solid ice. Thus, freezing is an exothermic process.

Step 3: Application of Definitions

Recall that `endothermic` refers to the absorption of energy by the system, while `exothermic` refers to the release of energy by the system. Using these definitions, confirm that melting (gaining energy) is endothermic and freezing (losing energy) is exothermic.

Key Concepts

Endothermic Process

In daily life, processes that absorb energy from their surroundings are quite common. An endothermic process is one where the system (usually a substance) absorbs energy in the form of heat. For example, when you place an ice cube on a warm surface, it absorbs heat and begins to melt. This melting process is endothermic because the ice cube requires energy to break the hydrogen bonds that hold its molecules in a solid structure. The heat energy absorbed leads to these bonds loosening, allowing the ice to become liquid water. Naturally, this means that endothermic processes often feel cold to the touch, as they take heat away from their environment.

• Melting ice is a classic example of an endothermic process.
• Other examples include photosynthesis and cooking an egg.

Understanding endothermic processes helps us see how energy transitions affect matter and contribute to everyday phenomena.

Exothermic Process

Exothermic processes occur when a system releases energy to its surroundings, typically in the form of heat. These processes often generate heat and can feel warm or hot to the touch. When liquid water freezes into ice, it demonstrates an exothermic process. As the temperature decreases, water molecules slow down and release energy. As they bond into a solid crystalline structure, energy is emitted into the surrounding environment. This release of energy is characteristic of exothermic processes.

• Freezing water into ice is an exothermic example.
• Combustion reactions, like burning wood, also release energy.

Recognizing exothermic reactions reveals how systems naturally move towards more stable, lower-energy states by emitting excess energy.

Phase Changes

Phase changes are transformations between different states of matter: solid, liquid, and gas. These changes involve energy exchange, where matter absorbs or emits energy to transition between phases. In each change, either an endothermic or exothermic process occurs, depending on the direction of the phase transition. When matter transitions from a solid to a liquid, like ice melting, the process is endothermic. Conversely, transitioning from a liquid to a solid, such as water freezing, is exothermic.

• Solid to liquid: melting (endothermic).
• Liquid to solid: freezing (exothermic).
• Liquid to gas: vaporization (endothermic).
• Gas to liquid: condensation (exothermic).

These phase changes underline important concepts in thermodynamics and are essential for understanding both natural and industrial processes.