Problem 23 - Chemistry The Central Science In Si Units

Question

(a) According to the first law of thermodynamics, what quantity is conserved? (b) What is meant by the intemal energy of a system? (c) By what means can the internal energy of a closed system increase?

Step-by-Step Solution

Answer

(a) Energy is conserved. (b) Internal energy is the total microscopic energy within a system. (c) Internal energy increases by heat addition or work done on the system.

Step 1: Understanding the First Law of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed from one form to another. This implies that total energy within a closed system remains constant. So, the quantity that is conserved according to this law is energy.

Step 2: Defining Internal Energy

Internal energy is the total energy contained within a system. This includes all forms of energy at the molecular level, such as kinetic energy from molecular motion and potential energy from molecular positions and interactions. It represents the microscopic energy of a system that is not visible externally.

Step 3: Means of Increasing Internal Energy in a Closed System

In a closed system, internal energy can increase through two primary means: heat addition and work done on the system. When heat is added, it increases the total internal energy by increasing molecular motion or interactions. Similarly, work done on the system (such as compression) also increases the internal energy by changing molecular arrangements or velocities.

Key Concepts

Energy conservation

The concept of energy conservation is rooted in the First Law of Thermodynamics, which is a fundamental principle of physics. This law states that energy cannot be created or destroyed. Instead, it can only change form or flow from one system to another. For instance, chemical energy in a battery can convert into electrical energy. Despite these transformations, the total energy within a closed system remains constant, like a pie that can be sliced differently but always has the same total size.
Understanding energy conservation is crucial as it helps explain how energy operates within our universe. By keeping in mind that energy is always conserved, we can better predict the outcomes of physical processes, whether it's a car engine converting fuel into motion or the sun providing heat to the Earth.

Internal energy

Internal energy is an essential concept in understanding thermodynamics. It refers to the total amount of microscopic energy contained within a system. This isn't something you can see directly, like the visible motion of a car, but rather the energy at the molecular or atomic level.
Internal energy includes various forms:

• Kinetic energy: from the movement of molecules.
• Potential energy: from the forces between molecules.
• Vibrational and rotational energy: specific to molecular structures.

Knowing the internal energy of a system is key to calculating how that system behaves under different conditions. Imagine a pot of water being heated; its internal energy increases as the water molecules move faster, even if the amount of water (or its mass) does not change.

Closed system

A closed system is a type of thermodynamic system where matter does not enter or leave the system, though energy can be transferred in or out. This concept is important because it helps us focus on energy changes without the complexity of material exchanges.
In a closed system, the internal energy can increase or decrease due to:

• Heat addition: transferring heat into the system raises its energy, similar to adding heat to a closed pot causing more vigorous molecular motion.
• Work done on the system: mechanical work or compression leads to changes in the arrangement and motion of molecules, thus increasing internal energy.

Thinking of a closed system helps simplify the study of energy conservation and energy transformation by keeping the amount of matter constant while observing how energy moves and changes form within the system.