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.

Short answer

In each scenario, the system transfers energy to or receives it from the surroundings, determining the direction of energy flow.

Step-by-step solution

Understand the Concept of a System and Surroundings

In thermodynamics, a system is the part of the universe that we are focusing on, while the surroundings encompass everything else outside the system. The boundary separates these two, and energy transfer (in form of heat or work) can occur across this boundary.

Identify the System and Energy Transfer in Methane Combustion

(a) System: Methane and oxygen inside the furnace. Surroundings: Everything outside the furnace. Direction of energy transfer: Energy is transferred from the system to the surroundings as heat, because burning methane (combustion) releases energy.

Identify the System and Energy Transfer in Water Evaporation

(b) System: Water droplets on skin. Surroundings: The air and skin. Direction of energy transfer: Energy is transferred from the surroundings to the system as heat, allowing the water to absorb energy and evaporate.

Identify the System and Energy Transfer in Water Freezing

(c) System: Water at 25°C. Surroundings: Interior of the refrigerator. Direction of energy transfer: Energy is transferred from the system to the surroundings as the water releases heat to cool and eventually solidify.

Identify the System and Energy Transfer in Aluminum and Iron (III) Oxide Reaction

(d) System: Mixture of aluminum and Fe₂O₃ in the flask. Surroundings: Laboratory bench and everything else around the flask. Direction of energy transfer: Energy is transferred from the system to the surroundings as the chemical reaction is exothermic and releases heat.

Key concepts

System and Surroundings

In the world of thermodynamics, understanding what we mean by "system" and "surroundings" is crucial. When examining a process, the system is the main focus of our study; it's where all the interesting changes happen. Imagine it as your classroom. Everything within these walls would be your system.

Now, outside these walls—that's your surroundings. Surroundings include anything not part of the system but near enough to possibly interact with it. The boundary, which separates the system from the surroundings, can be real or imaginary, and through this boundary, energy can flow in or out.

Let's consider an example—if you're looking at a cup of hot chocolate, the system is the hot chocolate inside the cup. The surroundings would be the cup itself, the air around it, and everything else nearby. It's a simple way to decide what's in and what's out when observing energy changes. Keeping this distinction clear helps us effectively study various physical and chemical processes in thermodynamics.

Energy Transfer

Energy can move between the system and surroundings, and this movement is essential to understanding thermodynamics. This transfer might occur as heat or work. It's like passing the ball around in a game; energy transfers in different directions depending on the play.

For example, think about boiling water. Energy, as heat, flows from the stove to the water, transferring from the surroundings (stove) into the system (water). This boosts energy within the water, causing it to get hot enough to vaporize.

Conversely, if ice melts in a warm room, energy moves from the surroundings (warm air) to the ice (system). This energy absorption allows the ice to turn into liquid water. Observing who "gives" and who "gets" energy helps us determine the direction of energy transfer. Thus, understanding energy transfers enriches our comprehension of various thermodynamic processes.

Exothermic Reaction

Exothermic reactions are a key type of chemical reactions where energy is released, usually in the form of heat. These are like thermodynamic fireworks, with energy bursting forth from the system into the surroundings.

For example, consider a common exothermic reaction: burning wood in a campfire. The wood and oxygen are your system, and when they react, they release a great deal of energy. The surroundings, which include the air and people around the fire, receive this energy as heat.

• Common exothermic reactions include combustion.
• Heat is a sign that energy is leaving the system.
• Exothermic reactions can be detected by the increase in temperature of the surroundings.

So, whenever you feel warmth from a chemical reaction, such as heat radiating from a burning candle, you're observing the principle of exothermic reactions in real time.

Heat Exchange

Heat exchange is a fascinating aspect of thermodynamics. It's the process where thermal energy transfers between the system and its surroundings, causing temperature changes.

When we talk about heat exchange, imagine it as money changing hands. The system or surroundings "pay" by giving away heat or "earn" by taking in heat wherever there's a temperature difference.

For instance, when water evaporates from your skin after swimming, it needs energy to overcome intermolecular forces and turn into vapor. It "borrows" this energy from your body and the surrounding air. This involves absorbing energy and is a form of heat exchange essential for processes such as evaporative cooling, which helps regulate body temperature.

• Heat flows from a hotter body to a cooler one.
• Heat exchange results in thermal equilibrium eventually being achieved.

Understanding heat exchange enables us to appreciate why an iced drink feels refreshing on a hot day or why frost forms on a windshield during cold nights.