A fundamental concept in thermodynamics is the “system,” the specific portion of the universe under observation. Everything outside this defined area is the “surroundings,” separated from the system by a boundary. Chemical processes are analyzed based on the interaction between the system and its surroundings, which involves the exchange of matter and energy. Classifying a system depends on the nature and restrictions placed upon this exchange.
The Defining Characteristics of an Open System
An open system is defined as one that freely exchanges both matter and energy with its surroundings. Matter exchange involves the gain or loss of mass across the system’s boundary. For instance, reactants can be added to the system, or products, such as gas or vapor, can escape into the atmosphere.
The system also exchanges energy, typically in the form of heat or work. Heat exchange occurs when there is a temperature difference, allowing thermal energy to flow into or out of the system. Work exchange happens when the system expands, contracts, or interacts mechanically with its environment. The continuous, unrestricted flow of both mass and energy defines the open system’s dynamic nature.
The Role of the System Boundary
The ability of an open system to exchange matter and energy is determined by the physical nature of its boundary. For matter to move freely across this interface, the boundary must be permeable. A permeable boundary allows particles (atoms, molecules, or ions) to pass through it, facilitating the flow of mass.
Simultaneously, the boundary must be diathermal to permit the exchange of heat energy. A diathermal boundary allows thermal energy to pass through without restriction, typically because it is made of a thin or conductive material. The boundary itself can be real, such as the walls of a beaker, or imaginary, such as a defined area in the atmosphere.
Contrasting Open, Closed, and Isolated Systems
The open system is contrasted with the two other primary thermodynamic classifications: the closed system and the isolated system. The closed system permits the exchange of energy with its surroundings but prevents the exchange of matter. A sealed container that is heated or cooled represents a closed system, where heat can transfer through the walls, but the mass remains constant.
The isolated system is the theoretical extreme, preventing the exchange of both matter and energy. This system possesses an impermeable boundary that is also adiabatic, meaning it blocks all mass and heat transfer. While a perfectly isolated system is challenging to achieve in practice, a well-sealed thermos flask is often used as a close approximation.
These distinctions are fundamental to applying the laws of thermodynamics. The isolated system is the only one where the total internal energy remains constant, adhering to the first law of thermodynamics. Conversely, both open and closed systems can experience changes in internal energy because they transfer energy across their boundaries.
Real-World Examples in Chemistry and Biology
Many common real-world phenomena function as open systems, providing accessible illustrations of these thermodynamic principles. A simple chemical example is a beaker of water being heated on a hot plate without a lid. The system loses matter as water vaporizes into steam and simultaneously exchanges energy with the hot plate and the cooler air.
Another chemical example is an open-air combustion reaction, like a candle flame, which consumes oxygen (matter input) and releases carbon dioxide and water vapor (matter output) along with light and heat (energy output).
On a much larger scale, a living organism, such as a human being, is a biological open system. The body constantly exchanges matter by taking in food and oxygen and expelling waste and carbon dioxide. Energy is exchanged through metabolism, releasing heat and doing work, ensuring continuous interaction with the environment.