Identify each energy exchange as primarily heat or work and determine whether the sign of ΔE 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.) Identify energy exchanges as primarily heat or work. Determine whether the sign of E is positive or negative for the system.

Verified step by step guidance

1. Step 1: Understand the concept of energy exchange in thermodynamics, where energy can be transferred as heat (q) or work (w). The change in internal energy (ΔE) of a system is given by the equation ΔE = q + w.
2. Step 2: For part (a), identify that the process of sweat evaporating involves heat. The system (evaporating sweat) absorbs heat from the skin, which means heat is entering the system. Therefore, q is positive, and since no work is done, ΔE is positive.
3. Step 3: For part (b), recognize that when a balloon expands, it does work on the surroundings by pushing against external pressure. This is work done by the system, so w is negative. If no heat is exchanged, ΔE is negative.
4. Step 4: For part (c), determine that the reaction mixture being warmed by an external flame involves heat. The system (reaction mixture) absorbs heat from the flame, so q is positive. If no work is done, ΔE is positive.
5. Step 5: Summarize the findings: (a) Heat exchange with positive ΔE, (b) Work exchange with negative ΔE, (c) Heat exchange with positive ΔE.

Key Concepts

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle is crucial for analyzing energy exchanges in a system, as it helps determine how energy flows in and out, affecting the internal energy (ΔE) of the system.

Heat vs. Work

In thermodynamics, heat is the energy transferred due to a temperature difference, while work is the energy transferred when a force is applied over a distance. Understanding the distinction between these two forms of energy transfer is essential for categorizing energy exchanges in the given scenarios and determining their impact on the system's energy.

Sign Convention for Energy Changes

The sign convention for energy changes indicates that a positive ΔE signifies an increase in the system's internal energy, while a negative ΔE indicates a decrease. This convention is vital for interpreting the results of energy exchanges, as it helps assess whether the system is gaining or losing energy during processes like evaporation, expansion, or heating.