Problem 19 - Chemistry: An Atoms-Focused Approach

Question

Which of the following processes are exothermic, and which are endothermic? a. Molten aluminum solidifies. b. Rubbing alcohol evaporates from the skin. c. Fog forms over San Francisco Bay.

Step-by-Step Solution

Answer

Explain your answer. Answer: The process when molten aluminum solidifies is exothermic. This is because, during this phase change from a liquid state to a solid state, the aluminum releases heat to its surroundings as the particles move closer together and form a more ordered structure.

Step 1: Process a: Molten aluminum solidifies

When molten aluminum solidifies, it changes from a liquid state to a solid state. During this phase change, the aluminum releases heat to its surroundings as the particles move closer together and form a more ordered structure. Therefore, this process is exothermic.

Step 2: Process b: Rubbing alcohol evaporates from the skin

In this process, rubbing alcohol changes from a liquid state to a gas state as it evaporates from the skin. For this phase change to occur, rubbing alcohol needs to absorb heat from its surroundings (in this case, the skin). This is why the skin often feels cooler when rubbing alcohol is applied. Therefore, this process is endothermic.

Step 3: Process c: Fog forms over San Francisco Bay

Fog forms when water vapor in the air condenses into tiny liquid water droplets, which are suspended in the atmosphere. This process involves a phase change from gas to liquid. During this phase change, the water releases heat energy to its surroundings as the water particles come closer together to form droplets. Therefore, this process is exothermic.

Key Concepts

Phase Changes

Phase changes are transitions between different states of matter: solid, liquid, and gas. These changes occur when energy is either absorbed or released by a substance.
For example:

• Freezing (liquid to solid)
• Melting (solid to liquid)
• Condensation (gas to liquid)
• Evaporation (liquid to gas)

Each phase change involves either the absorption or release of energy, affecting whether the process is endothermic or exothermic. Understanding these changes helps explain many everyday phenomena, like ice melting or steam forming.

Heat Exothermic

In exothermic reactions, heat is released into the environment. This type of process occurs because energy is needed to break bonds in the reactants, and when products form, excess energy is released as heat.
During exothermic phase changes, like solidification and condensation, substances release heat. Consider solidification: when molten aluminum solidifies, its energy state lowers, releasing heat to its surroundings. People often feel the warmth from an exothermic reaction, as it transfers energy to the nearby air.

Condensation

Condensation is when a substance transitions from a gas to a liquid state. This is a form of exothermic process, as the gas releases heat.
A common real-world example is dew formation. Water vapor in the air loses energy and condenses into liquid dew on surfaces. This typically occurs when temperatures drop and the air can no longer hold all the moisture, leading to condensation. The release of heat in condensation can affect the temperature of the local environment, increasing it slightly.

Evaporation Endothermic

Evaporation occurs when a liquid changes into a gaseous state. This process requires energy, making it endothermic.
The liquid absorbs heat from its surroundings, increasing the movement of its particles until they can escape into the air. Evaporation not only occurs in liquids like water but also with substances like rubbing alcohol. When applied to the skin, the alcohol evaporates, absorbing heat and leaving the skin feeling cooler.

Solidification

Solidification is the change from a liquid to a solid state. During this exothermic process, liquid particles lose energy and form a structured, closely packed solid.
As particles settle into a fixed structure, they release energy in the form of heat. A classic example includes the hardening of molten aluminum, which releases a sufficient amount of heat as it cools and forms a solid.