Question

For each of the following, define a system and its surroundings and give the direction of heat transfer between system and surroundings. (a) Methane is burning in a gas furnace in your home. (b) Water drops, sitting on your skin after a dip in a swimming pool, 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 heat is evolved.

Short answer

Heat transfers from the system to the surroundings in parts (a), (c), and (d). In part (b), heat transfers from the surroundings to the system.

Step-by-step solution

Define the system and surroundings for part (a)

For methane burning in a gas furnace, the system is the methane and oxygen within the furnace. The surroundings are the interior of the furnace and the room/house containing the furnace.

Determine heat transfer direction for part (a)

When methane burns, it releases heat as part of the combustion process. Therefore, heat transfers from the system (burning methane and oxygen) to the surroundings (furnace and house).

Define the system and surroundings for part (b)

For water drops on the skin evaporating, the system is the water present on the skin. The surroundings are the rest of the body and the air around the person.

Determine heat transfer direction for part (b)

Evaporation is an endothermic process requiring heat, so heat transfers from the surroundings (skin and ambient air) into the system (water drops).

Define the system and surroundings for part (c)

The system is the water placed inside the refrigerator's freezing compartment. The surroundings are the interior of the freezing compartment (everything excluding the water).

Determine heat transfer direction for part (c)

As water cools and solidifies, heat flows from the system (water) to the surroundings (freezing compartment), causing the water to lose heat and eventually freeze.

Define the system and surroundings for part (d)

The system consists of the aluminum and \(\text{Fe}_2\text{O}_3(s)\) in the flask. The surroundings are the flask itself and the laboratory environment outside the flask.

Determine heat transfer direction for part (d)

The reaction between aluminum and \(\text{Fe}_2\text{O}_3(s)\) evolves heat, making it an exothermic reaction. Therefore, heat transfers from the system (reactants and products) to the surroundings (flask and ambient air).

Key concepts

Heat Transfer

Heat transfer is a fundamental concept in thermodynamics, involving the movement of thermal energy from one place to another. It moves from areas of higher temperature to areas of lower temperature. This can occur in three primary ways: conduction, convection, and radiation. Each method impacts different systems and surroundings in unique ways.

Conduction refers to heat transfer through direct contact. For example, when you touch a hot stove, heat is conducted from the stove to your hand.

Convection involves the movement of heated particles within fluids (liquids or gases), distributing heat through currents. An example is the warm air rising in a heated room.

Radiation involves heat transfer through electromagnetic waves, such as feeling the warmth of the sun on your skin. Understanding how heat transfer occurs is key to analyzing thermodynamic processes in systems and surroundings.

System and Surroundings

In thermodynamics, identifying the system and its surroundings is crucial for analyzing heat transfer. The system is the part of the universe being studied, while everything outside it comprises the surroundings. This defines the border where interactions, such as energy exchange, happen.

This concept helps us understand examples like methane burning in a furnace. Here, the system includes the methane and oxygen, while everything else forms the surroundings. Knowing what constitutes the system enables a clearer understanding of how energy flows, which is essential for predicting outcomes of various reactions.

Endothermic and Exothermic Processes

Chemical reactions can either absorb or release heat. Endothermic processes require energy intake from the surroundings, whereas exothermic processes release energy into the surroundings.

An example of an endothermic reaction is water evaporating from your skin. During this process, heat is absorbed from the skin and ambient air, cooling the body. On the other hand, methane combustion in a furnace is exothermic, as it releases heat into the surroundings, warming the room.

Identifying a process as endothermic or exothermic helps in understanding how energy changes during chemical reactions and impacts the environment around them.

Combustion

Combustion is a specific type of chemical reaction where a substance combines with oxygen and releases energy. This typically happens in the form of heat and light. Commonly occurring in fuels like methane, combustion is crucial for applications like heating and engine functioning.

When methane burns in a furnace, it reacts with oxygen to form carbon dioxide and water, releasing significant heat energy. This process efficiently transfers energy from the system (methane and oxygen) to the surroundings, making it an essential mechanism for domestic and industrial heat production.

Understanding combustion reactions helps us harness these processes safely and efficiently, ensuring that energy requirements in homes and industries are met responsibly.