Textbook Question
During a deep breath, our lungs expand about 2.0 L againstan external pressure of 101.3 kPa. How much work is involvedin this process (in J)?
Ch.5 - Thermochemistry
Chapter 5, Problem 34
(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 releases heat to the surroundings. Does the enthalpy of the system increase or decrease during the process? (c) In a constant-pressure process, _x001F_H = 0. What can you conclude about _x001F_E, q, and w?
Verified step by step guidance1
Step 1: Understand the relationship between enthalpy change (ΔH) and heat transfer (q) in thermodynamics. Enthalpy change equals the heat transferred into or out of the system at constant pressure. Therefore, ΔH = q when the process occurs at constant pressure.
Step 2: For part (b), consider the scenario where the system releases heat to the surroundings at constant pressure. According to the first law of thermodynamics, if the system releases heat, q is negative. Since ΔH = q at constant pressure, the enthalpy of the system decreases.
Step 3: For part (c), analyze the condition where ΔH = 0 in a constant-pressure process. This implies that there is no net heat transfer into or out of the system, meaning q = 0.
Step 4: Use the first law of thermodynamics, ΔE = q + w, where ΔE is the change in internal energy and w is the work done on or by the system. Since q = 0, ΔE = w.
Step 5: Conclude that if ΔH = 0 and q = 0, then the work done (w) is the only contributor to the change in internal energy (ΔE). Therefore, ΔE = w, indicating that any change in internal energy is due to work done on or by the system.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy (H)
Enthalpy is a thermodynamic property that represents the total heat content of a system. It is defined as the sum of the internal energy of the system plus the product of its pressure and volume (H = U + PV). In constant-pressure processes, the change in enthalpy (ΔH) is equal to the heat exchanged (q) with the surroundings, making it a crucial concept for understanding heat transfer in chemical reactions.
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First Law of Thermodynamics
The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. In the context of a chemical process, this means that the change in internal energy (ΔE) of a system is equal to the heat added to the system (q) minus the work done by the system (w), expressed as ΔE = q - w. This principle is essential for analyzing energy changes during chemical reactions.
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Constant-Pressure Process
A constant-pressure process is a thermodynamic process that occurs at a fixed pressure, allowing for the direct relationship between heat transfer and enthalpy change. In such processes, if the system releases heat (q < 0), the enthalpy of the system decreases (ΔH < 0). Understanding this relationship is vital for predicting how energy changes affect the state of a system during chemical reactions.
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Related Practice
Textbook Question
How much work (in J) is involved in a chemical reaction ifthe volume decreases from 33.6 L to 11.2 L against a constantpressure of 90.5 kPa?
Textbook Question
A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in Figure 5.4. When 0.49 kJ of heat is added to the gas, it expands and does 214 J of work on the surroundings. What are the values of H and E for this process?
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