Problem 91 - Chemistry: The Central Science

Question

An aluminum can of a soft drink is placed in a freezer. Later, you find that the can is split open and its contents frozen. Work was done on the can in splitting it open. Where did the energy for this work come from?

Step-by-Step Solution

Answer

The energy required to split the aluminum can open comes from the internal energy of the soft drink, which was reduced during the freezing process and transferred to the freezer in the form of heat. The freezer, powered by a source of electrical energy, ultimately provides the energy needed to split the can.

Step 1: Understand the freezing process

When a soft drink is placed in a freezer, the temperature of the liquid inside the can decreases, causing it to freeze. As the temperature decreases, the liquid molecules slow down and form a solid through a process called phase transition. The liquid's internal energy is reduced during this process.

Step 2: Discuss the expansion of the frozen contents

As the soft drink freezes, it expands. The volume of ice (solid) is greater than the volume of water (liquid) at the same mass and temperature, which is due to the molecular structure of the ice. So when the soft drink freezes, the contents inside the can exert a force on the can's wall, trying to expand.

Step 3: Identify the cause for splitting the can open

The force exerted on the can's wall by the frozen contents is sufficient to overcome the can's structural integrity. This force does work on the can, causing it to split open. The work done on the can is the result of the force exerted by the expanding contents, causing deformation and finally splitting the can.

Step 4: Identify where the energy to split the can comes from

The energy for the work done on the can comes from the internal energy of the soft drink, which was reduced during the freezing process. As the internal energy decreases, it is transferred from the soft drink in the form of heat to the surrounding freezer environment (a colder region). Ultimately, the freezer is powered by a source of electrical energy, which is effectively responsible for transferring the energy needed to split the can. To summarize, the energy required for splitting the aluminum can open comes from the internal energy of the soft drink, which was reduced during the freezing process and transferred to the freezer in the form of heat.

Key Concepts

Phase Transition

The concept of phase transition is crucial in understanding how substances change state. In the case of a soft drink in a freezer, a phase transition occurs when the liquid soft drink transforms into a solid-state.
When the temperature drops, the kinetic energy of the liquid's molecules decreases. This causes the molecules to slow down significantly. Ultimately, the molecular motion slows enough to allow the formation of a rigid structure, resulting in the process known as freezing.

• Phase transitions can involve changes such as melting, freezing, vaporization, or condensation.
• During a phase transition, a substance's physical properties, such as volume and density, undergo significant changes.

Freezing a soft drink not only alters its state from liquid to solid but also its volume increases, as ice occupies more space than the same mass of liquid water. Understanding this process explains why items can expand and exert pressure on their containers during freezing.

Internal Energy

Internal energy is the total energy contained within a substance, including both kinetic and potential energies of its molecules. When a soft drink freezes, the internal energy decreases. This decrease happens because the liquid turns into a structured crystalline form, losing some kinetic energy in the process.
The relationship between temperature and internal energy is fundamental:

• As the temperature decreases, the internal energy of a substance also decreases.
• In a freezing soft drink, the lost internal energy contributes to the expansion as it transitions to a solid state.

Therefore, understanding internal energy helps explain the source of work done to split open a can. The internal energy initially present in the liquid is redistributed, and some is released to the surrounding area as heat.

Heat Transfer

Heat transfer is the movement of thermal energy from a warmer object to a cooler one. In the scenario of a soft drink in a freezer, heat transfer is crucial as it allows the drink to lose enough thermal energy to reach its freezing point.
There are different modes of heat transfer:

• Conduction: Energy transfer takes place between molecules and can happen between the can and the air in the freezer.
• Convection: Involves the movement of fluid, such as the air circulating in the freezer, enhancing the cooling effect.
• Radiation: Less relevant in this context but involves energy transfer through electromagnetic waves.

Isolation provided by a freezer ensures efficient heat transfer, allowing the soft drink to cool swiftly. The decrease in internal energy due to effective heat transfer provides the necessary conditions for the can to split when the contents expand.