Problem 39 - Chemistry A Molecular Approach

Question

Identify each energy exchange as primarily heat or work and determine whether the sign of \(\Delta E\) is positive or negative for the system. MISSED THIS? 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.)

Step-by-Step Solution

Answer

a. Heat exchange, \(\Delta E > 0\); b. Work exchange, \(\Delta E < 0\); c. Heat exchange, \(\Delta E > 0\).

Step 1: Analyzing Evaporation of Sweat

Evaporation is a process where a liquid turns into gas. When considering the evaporating sweat, energy is required for the phase change from liquid to gas. This energy is absorbed from the skin as heat, resulting in a cooling effect. Since the evaporating sweat (system) is taking in energy, \(\Delta E > 0\), and it's primarily heat exchange.

Step 2: Evaluating Balloon Expansion

When a balloon expands, it does work against the external pressure surrounding it. The work is performed by the system (contents of the balloon) on the surroundings. As the system does work, energy is leaving the system, indicating that \(\Delta E < 0\), making the energy exchange work.

Step 3: Assessing Heating of Chemical Reaction Mixture

Heating a chemical reaction mixture involves the transfer of energy into the system through an external flame. The system absorbs heat, leading to an increase in the internal energy of the system. Hence, for the mixture (system), \(\Delta E > 0\) as this is a heat exchange.

Key Concepts

Enthalpy Change

Enthalpy change, denoted as and often expressed in units of joules or calories, represents the total heat content of a system. It's an extensive property, meaning it's dependent on the amount of substance in the system. In thermodynamics, we're interested in the change in enthalpy, , which occurs during a reaction or process. This change is an indication of the heat absorbed or released under constant pressure. For example, when sweat evaporates off your skin, the system—sweat—absorbs energy from the skin. This process is endothermic, resulting in a positive enthalpy change, revealing that the system's enthalpy has increased as heat is absorbed from the surroundings.

Heat and Work

The concepts of heat and work are central to understanding energy exchanges in thermodynamics. Heat is the transfer of thermal energy between systems due to a temperature difference, while work is the energy transfer that occurs when a force moves an object. For instance, when a balloon expands, it does work against the surrounding pressure, and energy leaves the system. This illustrates the work being performed by the system on the surroundings. Conversely, introducing heat to a chemical mixture as in our exercise, increases the system's energy, demonstrating a heat exchange. It's essential to recognize that while heat can be felt as temperature changes, work often involves a physical and noticeable movement or deformation.

Phase Change

Phase changes are physical transformations from one state of matter to another; for example, from solid to liquid (melting), liquid to gas (evaporation), gas to liquid (condensation), or liquid to solid (freezing). Each phase change involves energy exchange with the surroundings. During evaporation, energy is taken in by the system to break molecular bonds and transform the substance into a gas, which in the case of our exercise, is sweat evaporating from the skin. This process requires heat from the surroundings (the skin), thus cooling it. This endothermic process is where the concept of enthalpy change we discussed earlier is directly applied, as the phase change alters the system's enthalpy.

System and Surroundings

In thermodynamics, a 'system' refers to the part of the universe we are focused on, while everything else constitutes the 'surroundings'. When assessing energy exchange, the boundary between the system and its surroundings is where this exchange takes place. Energy can cross this boundary as heat or work, affecting the internal energy of the system, , which in turn can either increase (positive sign) or decrease (negative sign). Whether studying sweat evaporation, balloon expansion, or heating a reaction mixture, understanding these concepts helps us determine the direction of energy flow and the variability of the system's internal energy.