Question

(a) Under what condition will the enthalpy change of a process equal the amount of heat transferred into or out of the system? (b) During a constant- pressure process the system absorbs heat from the surroundings. Does the enthalpy of the system increase or decrease during the process?

Short answer

(a) The enthalpy change of a process will equal the amount of heat transferred into or out of the system under the condition that the process occurs at constant pressure (ΔP = 0) and there is no work done by or on the system (ΔV = 0). (b) During a constant-pressure process when the system absorbs heat from the surroundings, the enthalpy of the system increases (Δ\(H = Q_P > 0\)).

Step-by-step solution

Part (a) - Condition for Enthalpy Change Equals Heat Transfer

Enthalpy (H) is a thermodynamic property that includes both the internal energy of the system (U) and the product of pressure (P) and volume (V). The relationship can be denoted as: \(H = U + PV\) The enthalpy change, hence the difference between the final and initial enthalpy ΔH, is given by: Δ\(H = H_{final} - H_{initial}\) We are given that the enthalpy change equals the amount of heat transferred into or out of the system (Q). Thus, Δ\(H = Q\) Now, let's consider the heat transfer happening at constant pressure (ΔP = 0). Therefore, Δ\(H = Q_P\) where ΔP = 0 and Q_P is the heat transferred at constant pressure. At constant pressure, the condition for enthalpy change equaling the amount of heat transferred is that there is no work done by or on the system. In other words, there is no change in the system's volume (ΔV = 0). Hence, under this condition, the enthalpy change of a process will equal the amount of heat transferred into or out of the system.

Part (b) - Enthalpy Change at Constant Pressure and Absorption of Heat

In this part, we are given that during a constant-pressure process, the system absorbs heat from the surroundings. As previously mentioned, at constant pressure, the enthalpy change (ΔH) equals the amount of heat transferred into or out of the system (Q_P): Δ\(H = Q_P\) Since the system is absorbing heat from the surroundings, Q_P is positive: \(Q_P > 0\) Therefore, the enthalpy of the system increases during the process: Δ\(H = Q_P > 0\) In conclusion, during a constant-pressure process, when the system absorbs heat from the surroundings, the enthalpy of the system increases.

Key concepts

Thermodynamics

Thermodynamics is a branch of physics that deals with the relationships between heat and other forms of energy. In the context of the exercise, it focuses on understanding how the enthalpy change of a system, a central concept in thermodynamics, corresponds to the heat transfer occurring within that system.

The exercise illustrates a fundamental law of thermodynamics, which is the conservation of energy. This law states that the total energy of an isolated system remains constant – it can neither be created nor destroyed, only changed from one form to another. When a process takes place at constant pressure and no work is performed by or on the system, any heat added or removed changes the system's enthalpy. The change in enthalpy under such conditions is a direct measure of the heat exchange with the surroundings. Understanding this relationship is crucial for students, as it lays the groundwork for analyzing many real-world processes, from industrial chemical reactions to atmospheric physics.

To help students better understand thermodynamics, it's essential to emphasize the practical applications of these concepts, such as in engines, refrigerators, and even biological systems, where energy transformation is a key process.

Constant Pressure Process

In a constant pressure process, the pressure of the system remains unchanged throughout the entire process. This scenario is common in many applications, especially in chemistry and engineering, such as reactions taking place in open beakers where the pressure is atmospheric pressure.

Understanding a constant pressure process is critical because many thermodynamic processes occur at atmospheric pressure and thus can be analyzed under these conditions. When a system undergoes a process without a change in pressure, it simplifies the calculation and interpretation of heat transfer, as highlighted in the given exercise. Students can be reminded that the constancy of pressure allows us to directly relate heat transfer to the enthalpy change, facilitating problem-solving and analyses.

Educational resources often use visual aids to help elucidate this concept. Diagrams that emphasize volume changes, such as expansion and compression at constant pressure, can be particularly valuable in enhancing students' grasping of related phenomena such as work done by the system and the heat involved.

Heat Transfer

Heat transfer is a mechanism by which thermal energy is exchanged between physical systems, depending on the temperature and phase of the matter. The exercise asks us to focus on heat transfer in relation to enthalpy change at constant pressure, which is a pivotal part of understanding how energy moves in thermodynamic processes.

There are three primary modes of heat transfer: conduction, convection, and radiation. In the context of the exercise, we are considering heat transfer through conduction or convection at constant pressure, which leads to a change in the system's enthalpy. When a system absorbs heat, the thermal energy increases, leading to an increase in the enthalpy of the system. Conversely, if the system loses heat, its enthality decreases.

To assist students in internalizing these concepts, it can be useful to present real-life examples, such as heating a pan of water on a stove (where the pan and the water represent the system) and observing how the water's temperature (and thus its enthalpy) increases as it absorbs heat from the stove.