MCAT Physics and Math Review

Chapter 3: Thermodynamics

3.2 Systems

Physicists and chemists tend to classify the world on the basis of observable phenomena and interactions between objects. Before moving on, we need to become familiar with some of the jargon that these fields have in common—specifically, thermodynamic systems and state functions. Note that the same jargon is discussed in Chapter 7 of MCAT General Chemistry Review.

SYSTEM TYPES

system is the portion of the universe that we are interested in observing or manipulating. The rest of the universe is considered the surroundings.

Isolated Systems

Isolated systems are not capable of exchanging energy or matter with their surroundings. As a result, the total change in internal energy must be zero. Isolated systems are very rare, although they can be approximated. A bomb calorimeter attempts to insulate a reaction from the surroundings to prevent energy transfer, and the entire universe can be considered an isolated system because there are no surroundings.

Closed Systems

Closed systems are capable of exchanging energy, but not matter, with the surroundings. The classic experiments involving gases in vessels with movable pistons are examples of closed systems. For thermodynamic purposes, most of what will be encountered on Test Day will be a closed system or will approximate a closed system.

Open Systems

Open systems can exchange both matter and energy with the environment. In an open system, not only does the matter carry energy, but more may be transferred in the form of heat or work. A boiling pot of water, human beings, and uncontained combustion reactions are all examples of open systems.

STATE FUNCTIONS

State functions are thermodynamic properties that are a function of only the current equilibrium state of a system. In other words, state functions are defined by the fact that they are independent of the path taken to get to a particular equilibrium state. The state functions include pressure (P), density (ρ), temperature (T), volume (V), enthalpy (H), internal energy (U), Gibbs free energy (G), and entropy (S). On the other hand, process functions, such as work and heat, describe the path taken to get to from one state to another.

MCAT Concept Check 3.2:

Before you move on, assess your understanding of the material with these questions.

1.    Which of the following thermodynamic systems transfer matter? Transfer energy?

System Type

Transfers Matter

Transfers Energy

Closed

   

Isolated

   

Open

   

2.    What is the difference between a state function and a process function?

·        State function:

·        Process function:

3.    List at least five common state functions:

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