Problem 20 - Chemistry The Science in Context

Question

Which of the following processes are exothermic, and which are endothermic? (a) frost forms on a car window in the winter; (b) water condenses on a glass of ice water on a humid summer afternoon; (c) adding ammonium nitrate to water causes the temperature of the solution to decrease.

Step-by-Step Solution

Answer

Question: Determine whether each of the given processes is exothermic or endothermic: (a) Frost formation on a car window, (b) Water condensation on a glass of ice water, (c) Adding ammonium nitrate to water. Answer: (a) Exothermic, (b) Exothermic, (c) Endothermic

Step 1: (a) Frost formation on a car window

Frost forms on a car window in the winter when water vapor in the air comes into contact with the cold surface of the window and turns into ice. This process requires the water vapor to lose heat, which is given to the surroundings. Since heat is released to the surroundings, the process is exothermic.

Step 2: (b) Water condensation on a glass of ice water

Water condenses on a glass of ice water on a humid summer afternoon because the cold surface of the glass causes the water vapor in the air to turn into liquid water droplets. In this process, the water vapor loses heat to the surroundings, which is the cold surface of the glass. Since heat is released to the surroundings, the process is exothermic.

Step 3: (c) Adding ammonium nitrate to water

When ammonium nitrate is added to water, it dissolves and the temperature of the solution decreases. This implies that the dissolving process absorbs heat from the surroundings, which is the water and causes the temperature of the water to decrease. Since heat is absorbed from the surroundings, the process is endothermic.

Key Concepts

Exothermic Processes

Exothermic processes are fascinating because they involve the release of heat to the surrounding environment. This means these processes give off energy, often making the surroundings feel warmer. In simpler terms, think of exothermic processes as a way of losing heat to the environment. A classic example is lighting a campfire, where heat is released as the wood burns and emits warmth and light.
In the context of phase changes, frost forming on a car window during winter is an exothermic process. As water vapor in the air meets the cold glass surface, it begins to change into ice. During this transformation, heat is released from the water vapor into the surrounding air, allowing it to crystallize into frost. Similarly, when water vapor condenses on a glass of ice water on a humid evening, it also releases heat to the cooler glass surface as it turns from a gas into a liquid.
In both examples, the key point is that energy is exiting the system; in other words, heat is leaving, resulting in these processes being exothermic in nature.

• Heat exits the system.
• Surroundings often become warmer.
• Examples include condensation and freezing.

Endothermic Processes

Endothermic processes are quite the opposite of exothermic ones. They absorb heat from their surroundings, often causing the environment to feel cooler. These processes require an input of energy to proceed. For example, in the kitchen, you might be familiar with the process of baking, where heat is absorbed by food to facilitate cooking.
Let's look at a chemical example like dissolving ammonium nitrate in water. When ammonium nitrate is added to water, it begins to dissolve, and if you were to track the temperature, you'd notice a drop. This temperature decrease indicates that heat is being absorbed from the water (the surroundings) to dissolve the ammonium nitrate. The energy it absorbs is used to break the bonds in the ammonium nitrate and facilitate the dissolving process.
To put it simply, endothermic processes require energy from outside to continue, resulting in a temperature drop in the surroundings.

• Heat is absorbed by the system.
• Surroundings become cooler.
• Typical examples include melting and evaporation.

Phase Changes

Phase changes are an integral part of thermochemistry because they involve changing the state of a substance from solid to liquid, liquid to gas, or vice versa. These changes are often accompanied by the absorption or release of heat, making them central to understanding exothermic and endothermic processes.
When a substance changes from one phase to another, like from ice to water or water to steam, it undergoes a specific phase change process. For instance:

• Melting: Ice turning to water is an endothermic process, as it requires heat absorption.
• Freezing: Water turning to ice is exothermic, due to heat release.
• Evaporation: Water turning to steam is endothermic, absorbing heat.
• Condensation: Steam turning back to water is exothermic, releasing heat.

These examples illustrate how each phase change is linked to the flow of energy, either absorbing or releasing heat, which in turn impacts the surrounding environment. Recognizing these concepts helps in distinguishing between endothermic and exothermic processes and better understanding the energy changes involved with phase transitions.