Problem 67 - Chemistry and Chemical Reactivity

Question

For each of the following, define a system and its surroundings, and give the direction of energy transfer between system and surroundings. (a) Methane burns in a gas furnace in your home. (b) Water drops, sitting on your skin after a swim, evaporate. (c) Water, at \(25^{\circ} \mathrm{C},\) is placed in the freezing compartment of a refrigerator, where it cools and eventually solidifies. (d) Aluminum and \(\mathrm{Fe}_{2} \mathrm{O}_{3}(\mathrm{s})\) are mixed in a flask sitting on a laboratory bench. A reaction occurs, and a large quantity of energy is evolved as heat.

Step-by-Step Solution

Answer

(a) From system to surroundings; (b) From surroundings to system; (c) From system to surroundings; (d) From system to surroundings.

Step 1: Define System and Surroundings - Methane Combustion

**System**: The methane gas undergoing combustion in the gas furnace. **Surroundings**: Everything outside the furnace, including the home environment. **Energy Transfer Direction**: Energy is transferred from the system to the surroundings as heat, warming the home.

Step 2: Define System and Surroundings - Evaporation

**System**: The water droplets on your skin. **Surroundings**: The air and skin around the droplets. **Energy Transfer Direction**: Energy is transferred from the surroundings (skin and air) to the system as heat, which allows the water to evaporate.

Step 3: Define System and Surroundings - Freezing Water

**System**: The water placed in the freezing compartment. **Surroundings**: The air inside the refrigerator. **Energy Transfer Direction**: Energy is transferred from the system to the surroundings as heat, cooling and eventually solidifying the water.

Step 4: Define System and Surroundings - Aluminum and Fe2O3 Reaction

**System**: The mixture of aluminum and \( \mathrm{Fe}_{2} \mathrm{O}_{3}(\mathrm{s}) \) in the flask. **Surroundings**: The flask and the laboratory environment. **Energy Transfer Direction**: Energy is transferred from the system to the surroundings as heat, indicating an exothermic reaction.

Key Concepts

System and Surroundings

In thermochemistry, the concept of a system and its surroundings is fundamental. A **system** refers to the part of the universe being studied, usually where the reaction or process occurs. On the other hand, **surroundings** encompass everything outside the system. By clearly defining system and surroundings, one can effectively analyze energy changes in various processes. For example, when methane combusts in a furnace, the system includes the methane and the combustion process itself, while the surroundings are the home environment that receives the heat generated. Recognizing these boundaries is key to understanding how energy flows, indicating whether it enters or exits a system.

Energy Transfer

Energy transfer is a critical aspect of thermochemistry. It involves the movement of energy from one place to another, usually in the form of heat. In the context of methane combustion, energy is transferred as heat from the system (methane gas) to the surroundings (home), warming the space.

• During evaporation, such as when water droplets evaporate from your skin, energy is absorbed from the surroundings to the system, causing the phase change in water.
• Conversely, in freezing water, energy is released from water as it turns from liquid to solid, transferring to the surroundings and cooling the air in the refrigerator.

Energy changes are directional, always moving from areas of high energy to low, which allows processes like combustion or freezing to occur.

Methane Combustion

Methane combustion is a chemical reaction where methane (CH₄) reacts with oxygen (O₂) to produce carbon dioxide (CO₂), water ( H₂O), and energy. This process is exothermic, meaning it releases energy. Inside a gas furnace, methane's combustion releases heat, which transfers to the surroundings, in this case, heating the home.

• This reaction is an example of how chemical energy stored in bonds is converted to thermal energy, effectively making use of fuel for warmth.
• The energy release exemplifies energy transfer from a high-energy state (chemical bonds in methane) to a lower-energy state (thermal energy in the air).

Understanding this process illustrates the importance of energy-efficient systems in everyday applications.

Evaporation

Evaporation is the process by which liquid water transitions into a gas. It occurs when water molecules gain enough energy to overcome attraction forces and enter the air. When you step out of a pool, water droplets on your skin evaporate, drawing heat from your body's surface and surrounding air—this results in a cooling sensation.

• In evaporation, energy is transferred to the system (water droplets) from the surroundings (your skin and the air).
• This process is endothermic, as it requires energy absorption to proceed.

Learning about evaporation helps comprehend how cooling happens naturally and is vital in various natural cycles and technological applications.

Freezing Water

Freezing is the transformation of liquid water to ice when its temperature decreases to 0°C or below. When water is placed in a refrigerator's freezing compartment, energy is removed from the water, causing its molecules to lose kinetic energy and form solid ice. This process demonstrates energy transfer from the water to the surrounding colder air in the refrigerator.

• The release of energy to the surroundings is an exothermic process, evident in the freezing of water.
• This phenomenon is crucial for preserving foods and is a basic principle in thermodynamics.

Understanding freezing highlights how energy conservation principles apply in everyday life, showing how thermal energy management is essential for efficient cooling systems.