Problem 47 - General Chemistry

Question

The process of dissolving ammonium nitrate, \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), in water is an endothermic process. What is the sign of \(q ?\) If you were to add some ammonium nitrate to water in a flask, would you expect the flask to feel warm or cool?

Step-by-Step Solution

Answer

The sign of \( q \) is positive, and the flask will feel cool.

Step 1: Understanding the Process

The process described involves dissolving ammonium nitrate, \( \text{NH}_4\text{NO}_3 \), in water. This process is endothermic, meaning it absorbs heat from the surroundings to proceed. In thermodynamics, when a process absorbs heat, we need to determine the sign of the heat transfer, \( q \).

Step 2: Determining the Sign of Heat Transfer \( q \)

Since the dissolution of \( \text{NH}_4\text{NO}_3 \) is an endothermic process, it absorbs heat from the surroundings. In thermodynamic terms, when a system takes in heat, \( q \) is assigned a positive sign. Thus, \( q > 0 \).

Step 3: Predicting the Temperature Change in the Flask

As the ammonium nitrate dissolves in the water and absorbs heat (since \( q > 0 \)), it takes heat from the water and the surrounding flask. This removal of heat results in a decrease in temperature of the water and the flask, making the flask feel cooler to the touch.

Key Concepts

Ammonium Nitrate Dissolution

When ammonium nitrate (\( \text{NH}_4\text{NO}_3 \)) is dissolved in water, it is an interesting chemical process that involves the breaking down of the solid into its constituent ions within the aqueous solution.

• Ammonium nitrate is a common fertilizer with a significant role in various chemical processes.
• As it dissolves, it separates into ammonium ions (\( \text{NH}_4^+ \)) and nitrate ions (\( \text{NO}_3^- \)), dispersing these ions into the water.

This dissolution itself does not just break bonds but also interacts with water molecules, leading to changes in temperature and energy balance. The process requires energy to break the ionic bonds in the solid, which in turn affects the surrounding temperature.

Thermodynamics

Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy.

• It helps us understand how and why energy changes take place in a chemical process.
• The first law of thermodynamics, known as the law of energy conservation, is crucial here as it applies to the energy exchanged in the system.

In the dissolution of ammonium nitrate, an endothermic reaction is observed, which is a process that absorbs energy (in the form of heat) from its surroundings. This absorption is necessary for overcoming the energy required to break the ionic bonds in the solid state.

Heat Transfer Sign

In thermodynamics, the sign of the heat transfer, \( q \), indicates the direction of heat flow.

• If \( q > 0 \), the system is gaining heat from the surroundings. This is typical of endothermic processes.
• Conversely, if \( q < 0 \), the system is releasing heat to the surroundings, characteristic of exothermic processes.

For the dissolution of ammonium nitrate, since it is an endothermic process, the sign of \( q \) is positive. This indicates that heat is absorbed from the surroundings, resulting in a cooling effect on the immediate environment.

Temperature Change

The temperature change observed during the dissolution of ammonium nitrate is a direct result of heat transfer.

• As heat is absorbed by the dissolving ammonium nitrate, the water and the flask lose heat.
• This loss of heat leads to a temperature decrease, making the surrounding environment cooler.

The magnitude of this temperature change depends on the amount of ammonium nitrate dissolved and the initial temperature of the surroundings. This cooling sensation is a practical example of an endothermic reaction in action.

System and Surroundings Interaction

Understanding the interaction between the system and its surroundings is key when discussing thermodynamics.

• In our scenario, the system is the solution of ammonium nitrate and water.
• The surroundings include the flask and the external environment that interacts with the heat exchange.

When ammonium nitrate dissolves, the system pulls in heat from the surroundings. This energy transfer is crucial in maintaining the balance of energy and demonstrates the fundamental principles of thermodynamics at play. By absorbing heat, the temperature of the surroundings decreases, which is why the flask feels cool when touched.