Question

Which of the following two assumptions is (are) invoked when using a bomb calorimeter to measure energy of combustion? 1\. The system is adiabatic with respect to the environment. 2\. The system maintains an approximately constant pressure throughout the process. 1\. I only 2\. II only 3\. I and II 4\. neither I nor II

Short answer

Both assumptions 1 and 2 are used when working with a bomb calorimeter to measure the energy of combustion. The bomb calorimeter is considered an adiabatic system with respect to the environment and maintains an approximately constant pressure throughout the process. Therefore, the correct answer is 3. I and II.

Step-by-step solution

Understanding Bomb Calorimetry

A bomb calorimeter is designed to measure the heat released during a chemical reaction, such as combustion. It consists of a strong metal container in which the reaction occurs, known as the "bomb." The bomb is sealed and placed inside a larger container filled with water, and the temperature change of the water is recorded as heat flows from the bomb to the water. Knowing the specific heat capacity of water and the mass of water, the energy released during the combustion can be calculated.

Assumption 1: Adiabatic System

An adiabatic system is one in which no heat is exchanged with the surroundings. In a bomb calorimeter, the bomb's design and construction aim to minimize heat exchange with the environment. The calorimeter is surrounded by insulation to reduce heat loss to the environment. This insulation ensures that almost all the heat released during combustion is absorbed by the water inside the calorimeter. Therefore, the bomb calorimeter is assumed to be adiabatic with respect to the environment.

Assumption 2: Constant Pressure System

The second assumption involves constant pressure throughout the process. In a bomb calorimeter, the bomb is sealed, and the reaction occurs at a fixed volume. Although the pressure inside the bomb may change due to the combustion reaction, the system's overall pressure remains approximately constant. This is because the pressure change inside the bomb is negligible compared to the atmospheric pressure acting on the system. Thus, the bomb calorimeter can be assumed to maintain a constant pressure throughout the process.

Conclusion

Both assumptions 1 and 2 are used when working with a bomb calorimeter to measure the energy of combustion. The bomb calorimeter is considered an adiabatic system with respect to the environment and maintains an approximately constant pressure throughout the process. This means that the correct answer is 3. I and II.

Key concepts

Energy Measurement

Energy measurement in bomb calorimetry is a fascinating feat of science that allows us to accurately determine the amount of heat released during a chemical reaction, specifically combustion. Bomb calorimeters play a critical role in this process.

To measure energy, the calorimeter uses its "bomb," a robust metal container where the reaction takes place. It is submerged in water within another container. This setup is crucial because it lets us record the temperature change in the water, which helps us determine the heat released. Thanks to the known specific heat capacity of water and its mass, we can calculate the energy released with precision.

This precise method of energy measurement helps us understand the energy content of various substances, guiding researchers and industry professionals to make informed decisions based on accurate energy data.

Adiabatic System

An adiabatic system is essential in the operation of a bomb calorimeter. In physics, an adiabatic process is one where no heat is exchanged with the surroundings. This principle is crucial in calorimetry.

The bomb calorimeter is designed to mimic an adiabatic system. The strong metal "bomb" and insulating materials ensure minimal heat escape during the combustion process. This insulation is vital because it guarantees that nearly all the heat generated remains within the system, leading to more accurate energy measurements.

The adiabatic design ensures that any measured temperature change in the water is solely due to the heat of the reaction. This characteristic improves the validity and reliability of calorimetric measurements, making the bomb calorimeter an invaluable tool in scientific research.

Constant Pressure

Understanding constant pressure in the context of bomb calorimetry is a bit different. The system is sealed and operates at constant volume, not pressure, yet the calorimeter is often treated as if it maintains constant pressure.

The combustion reaction inside the bomb results in changes in pressure. However, this pressure change remains negligible when compared with the external atmospheric pressure.

For practical purposes, the small fluctuations in pressure do not significantly affect the measurements, so we often consider the pressure to be essentially constant for the purpose of calculations. This assumption simplifies the math involved and aligns with the principles of thermodynamics, which favor simplification.

Combustion Reaction

A combustion reaction in a bomb calorimeter is the heart of the measurement process. This reaction involves the rapid combination of a substance with oxygen to release energy in the form of heat.

In the controlled environment of the calorimeter's "bomb," substances combust while the system captures and measures the generated heat. The precise control and closed system environment means that the energy released can be accurately assessed.

By analyzing this heat exchange, a bomb calorimeter provides valuable insights into the energetic properties of different materials, such as fuels. It helps to determine not only the energy output but also the efficiency and potential environmental impact of a given substance. This makes understanding combustion reactions vital for both scientific and practical applications.