Identify each of the following energy exchanges as primarily heat or work and determine whether the sign of AU is positive or negative for the system. a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.) b. A balloon expands against an external pressure. (The contents of the balloon is the system.) c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.)
The Correct Answer and Explanation is :
Let’s analyze each scenario to determine whether the energy exchange is primarily heat or work and assess the sign of the change in internal energy (ΔU) for the system:
a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.)
- Energy Exchange: This is primarily a heat exchange. As sweat evaporates, it absorbs heat from the skin, leading to a cooling effect.
- Sign of ΔU: For the system (evaporating sweat), the internal energy increases because it absorbs heat from the surroundings (the skin). Therefore, ΔU is positive for the system.
b. A balloon expands against an external pressure. (The contents of the balloon are the system.)
- Energy Exchange: This is primarily a work exchange. As the balloon expands, it does work against the external pressure.
- Sign of ΔU: For the system (the balloon’s contents), the internal energy decreases because the system does work on the surroundings. Therefore, ΔU is negative for the system.
c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.)
- Energy Exchange: This is primarily a heat exchange. The external flame transfers heat to the reaction mixture, increasing its temperature.
- Sign of ΔU: For the system (the reaction mixture), the internal energy increases due to the heat added from the flame. Therefore, ΔU is positive for the system.
Explanation:
In thermodynamics, energy can be transferred to or from a system as heat or work. Heat transfer occurs due to a temperature difference between the system and its surroundings, while work involves energy transfer resulting from a force acting over a distance.
- Scenario a: Sweat evaporation is an endothermic process where the liquid phase absorbs heat to become vapor, leading to a cooling effect on the skin. Since the system (evaporating sweat) absorbs heat, its internal energy increases, resulting in a positive ΔU.
- Scenario b: When a balloon expands against external pressure, it performs work on the surroundings by pushing against the pressure. This work requires energy, which the system supplies by decreasing its internal energy, leading to a negative ΔU.
- Scenario c: Heating the reaction mixture with an external flame adds thermal energy to the system, increasing its internal energy. This heat transfer results in a positive ΔU for the system.
Understanding these energy exchanges is crucial in thermodynamics, as they dictate how systems interact with their surroundings and how energy transformations occur in various processes.