Lecture 1

Introduction to Thermodynamics

As the name implies, Thermodynamics is the study of the flow (dynamics) of heat (thermo), or more generally, of energy. We will be looking specifically at the flow of energy associated with chemical reactions, Thermochemistry. Implicit in this definition is the idea that energy is flowing from somewhere to somewhere else. It is up to us to define these locations. The energy flows between a System and its Surroundings.

An example of an open system is a fertilized hen's egg. The system is the chick embryo and shell, the surroundings are the nest and the hen. Carbon dioxide passes out of the system, through the shell, and oxygen is taken up through the shell, to sustain the embryo's metabolism. So, there is exchange of mass between the system and the surroundings. Heat flows from the hen's body, through the shell to the embryo, so there is also an exchange of energy, making this an open system.

An example of a closed system would be a stoppered beaker on a hot plate. Because the beaker (system) is stoppered, there is no exchange of mass between the system and the surroundings. Heat does flow from the hot plate, through the flask, to the system, so there is an exchange of energy. This is a closed system.

There is only one example of an isolated system, one in which there is no exchange of either energy or mass, and that is our Universe. Experimentally, it is impossible to set up an isolated system, but it is a theoretically interesting concept.

We can set up a system in the lecture room, a sealed plastic bag containing a chunk of dry ice. The surroundings are the lab bench on which it is sitting, and a book placed on top of the bag. We will leave this system to evolve during the rest of the lecture.

When we talk about the energy in a system, we are referring to the Internal Energy of that system, which is given the symbol, E. The Internal Energy of a system is the sum of all of the kinetic and potential energies of the particles in that system:

E = K.E. + P.E.

Kinetic energy is the energy a body possess due to its motion. Kinetic energy is described the the equation:

K.E. = 1/2 mv2

where m is the mass of the object and v is its velocity. Remember that Temperature is a measure of the average kinetic energy of the molecules in a system. This motion is called translational motion and involves random movement through space.

Potential energy is the energy that a body possesses due to its position relative to other bodies.

In the cartoon version of potential and kinetic energy, the anvil held high above Wiley Coyote's head possesses potential energy, through its gravitational attraction to the Earth. When the anvil is released, the potential energy is converted to kinetic energy, energy of motion. When it strikes the hapless coyote, it is reconverted to potential energy.

Let's consider a more scientific description of kinetic and potential energy. Kinetic energy is energy of motion. The kinetic energy in molecules can be broken down into three different components:

K.E.total = K.E.translation + K.E.rotation + K.E.vibration

Translational K.E. describes the random movement through space of the molecules in the system. Absolute Temperature, K, is a measure of the average translational kinetic energy of a system. Rotational and vibrational kinetic energy arise from the more ordered motion of atoms in molecules. K.E.rotation arises from the rotation of a molecule and K.E.vibration involves the stretching and bending of the bonds between the atoms of the molecule. These are illustrated in the figure below:

Potential Energy is energy of position. The energy arises through the interaction of charged particles over distance.

P.E. = Q1 Q2 / r

where Q1 and Q2 are charges and r is the distance between them. Like kinetic energy, potential energy can also be broken down into components:

P.E.total = P.E.n-n + P.E.e-e + P.E.e-n + P.E.IMF

P.E.n-n represents the nuclear-nuclear interaction of atoms in a molecule. Since nuclei carry positive charges, the sign of this component of the potential energy carries a positive sign. It is difficult to hold two positive charges next to each other. The potential energy is repulsive in this case. P.E.e-e represents the electron-electron interaction of atoms in a molecule. Again, this will be a repulsive force, since both q1 and q2 are negatively charged. P.E.e-n represents the attraction of the nucleus of one atom (+) for the electron cloud of an adjacent atom (-). This is an attractive force, and is the force which leads to bond formation. The final component, P.E.IMF represents the attractive intermolecular forces. These range from very strong, ion-ion interactions, through ion-dipole, hydrogen bonds, and dipole-dipole interactions to the weakest intermolecular forces, the London Dispersion Forces. All of these forces involve the attraction of opposite charges over distances.

Energy can be exchanged between the system and surroundings in two forms, as heat, q, or as work, w.

Heat, given the symbol, q, is defined as the exchange of energy along a Temperature gradient. Energy flows from a warmer body to a colder body until both are at the same Temperature, thermal equilibrium. By convention, if heat leaves the system, it is given a negative value. All properties lost by the system are given this sign. When heat leaves the system, the process is said to be exothermic. When heat enters the system from the surroundings, it is given a positive sign, as are all properties gained by the system. A process in which the system gains heat is called endothermic.

Looking at the dry ice system set up in the Lecture room, we find that the lab bench is now quite cold. This indicates that heat has flowed from the surroundings (bench top) into the system, making this an endothermic process, with q > 0.

Work is given the symbol w. There are many forms of work. Later in the course we will examine electrical work. For now, we will only consider mechanical work. It is defined as:

w = force x distance

force = Pressure x Area = Pressure x m2
distance = m
work = Pressure x m3 = Pressure x Volume

We can see that precisely this sort of Pressure/Volume work has occurred in our dry ice system. The bag containing the dry ice has inflated (changed in Volume), exerting enough pressure on the book to raise it off the bench top by about 0.1 m. The system has performed mechanical work on the surroundings (the book). For this system, w <0. Work "leaves" the system. We will define mechanical work involving volume changes as:

w = -Pext DV

where Pext is the external Pressure, against which the system is working, and DV is the change in volume of the system,

Vfinal - Vinitial. In our system, Pext is 1 atmosphere, and the change in volume is positive, since Vfinal > Vinitial This means that for this system, w<0, as it should be for a system which has performed work on the surroundings.