Physical Chemistry

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INTRODUCTION 1.1 What is thermodynamics?
Thermodynamics is the science which has evolved from the original investigations in the 19th century into the nature of “heat.” At the time, the leading theory of heat was that it was a type of fluid, which could flow from a hot body to a colder one when they were brought into contact. We now know that what was then called “heat” is not a fluid, but is actually a form of energy – it is the energy associated with the continual, random motion of the atoms which compose macroscopic matter, which we can’t see directly. This type of energy, which we will call thermal energy, can be converted (at least in part) to other forms which we can perceive directly (for example, kinetic, gravitational, or electrical energy), and which can be used to do useful things such as propel an automobile or a 747. The principles of thermodynamics govern the conversion of thermal energy to other, more useful forms. For example, an automobile engine can be though of as a device which first converts chemical energy stored in fuel and oxygen molecules into thermal energy by combustion, and then extracts part of that thermal energy to perform the work necessary to propel the car forward, overcoming friction. Thermodynamics is critical to all steps in this process (including determining the level of pollutants emitted), and a careful thermodynamic analysis is required for the design of fuel-efficient, low-polluting automobile engines. In general, thermodynamics plays a vital role in the design of any engine or power-generating plant, and therefore a good grounding in thermodynamics is required for much work in engineering. If thermodynamics only governed the behavior of engines, it would probably be the most economically important of all sciences, but it is much more than that. Since the chemical and physical state of matter depends strongly on how much thermal energy it contains, thermodynamic principles play a central role in any description of the properties of matter. For example, thermodynamics allows us to understand why matter appears in different phases (solid, liquid, or gaseous), and under what conditions one phase will transform to another. 1



The composition of a chemically-reacting mixture which is given enough time to come to “equilibrium” is also fully determined by thermodynamic principles (even though thermodynamics alone can’t tell us how fast it will get there). For these reasons, thermodynamics lies at the heart of materials science, chemistry, and biology. Thermodynamics in its original form (now known as classical thermodynamics) is a theory which is based on a set of postulates about how macroscopic matter behaves. This theory was developed in the 19th century, before the atomic nature of matter was accepted, and it makes no reference to atoms. The postulates (the most important of which are energy conservation and the impossibility of complete conversion of heat to useful work) can’t be derived within the context of classical, macroscopic physics, but if one accepts them, a very powerful theory results, with predictions fully in agreement with experiment. When at the end of the 19th century it finally became clear that matter was composed of atoms, the physicist Ludwig Boltzmann showed that the postulates of classical thermodynamics emerged naturally from consideration of the microscopic atomic motion. The key was to give up trying to track the atoms individually and instead take a statistical, probabilistic approach, averaging over the behavior of a large number of atoms. Thus, the very successful postulates of classical thermodynamics were given a firm physical foundation. The science of statistical mechanics begun by Boltzmann encompasses everything in classical thermodynamics, but can do more also. When combined with quantum mechanics in the 20th century, it became possible to explain essentially all observed properties of macroscopic matter in...
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