The Richard Stockton College of New Jersey
Chemistry Program, School of Natural Sciences and Mathematics
PO Box 195, Pomoma, NJ
CHEM 3410: Physical Chemistry I — Fall 2008
September 5, 2006
Lecture 2: Thermodynamic Variables, Systems, Processes & the First Law
References
1. Levine, Physical Chemistry, Sections 1.2, 2.1–2.4
Key Concepts
•Thermodynamic variables can be categorized based on how they change when the system size is
changed. Intensive quantities, such as pressure and temperature, are independent of system size.
Extensive variables, such as volume, mass, surface area, depend on the size of the system.
•Thermodynamic variables exist in conjugate pairs. The product of the units of these pairs is always
energy. They always consist of one intensive variable (a force), for example pressure (P), and one
extensive variable (displacement), like volume (V). They appear paired in work terms like −PdV , γdA,
etc.
•Thermodynamic systems can be described by determining what can pass through the boundary between
system and surroundings (isolated, closed, adiabatic, open). Processes can be described in a similar
way by looking at parameters or variables that are constant (adiabatic, isochoric, to name a few).
•Processes can also be described based on their reversibility.
–Areversible process is an idealized processes that is always in equilibrium and has no dissipative
processes. A reversible processes can occur forward and backward.
–An irreversible process occurs in one direction spontaneously. Most real processes are irreversible.
•The First Law of Thermodynamics is a way of expressing the principle of conservation of energy.
In the framework of classical thermodynamics, the energy of interest is the internal energy (U). The
internal energy accounts for energy stored within a material in bonds, vibrations, etc.
•The mathematical form of the first law is:
dU =δq +δw
where δq is the heat flow into (δq > 0) or out of (δq < 0) the system and δw is the work done on
(δw > 0) or by (δw < 0)the system. The dindicates an exact differential, namely that the change in
internal energy is path independent. The δ’s indicate inexact differentials that are path dependent.
•Work: from physics is defined as the force (F) used to move an object through some displacement (dl).
We will be dealing with other kinds of work, but they will always have the same general form, a force
or intensive variable, multiplied by a displacement in the conjugate extensive variable: −P dV ,γdA,
or in general Y dX, where Yis an intensive variable and Xis it’s conjugate extensive variable.
•Heat: the “workless” transfer of energy. The quantity of energy that flows across a boundary because
of a temperature difference.
•It is important to note that (1) heat an work only refer to energy transfer and (2) heat and work
are not conserved quantities, they are path dependent.
Related Exercises in Levine
Exercises 1.2, 2.2, 2.4
