Unit I: First and Second Laws


Aristotle, 384–322 BC

The ancient Greeks thought that there were only four elements: earth, air, fire, and water. As a matter of fact, you can explain a large number of natural phenomena with little more. The first and second laws of thermodynamics can be developed and illustrated quite completely with just solid blocks (earth), ideal gases (air), steam property tables (water), and heat (fire). Without significantly more effort, we can include a number of other “elements”: methane, carbon dioxide, and several refrigerants. These additional species are quite common, and charts that are functionally equivalent to the steam property tables are readily available.

The first and second laws provide the foundation for all of thermodynamics, and their importance should not be underestimated. Many engineering disciplines typically devote an entire semester to the “earth, air, fire, and water” concepts. This knowledge is so fundamental and so universal that it is essential to any applied scientist. Nevertheless, chemical engineers must quickly lay this foundation and move on to other issues covered in Units II, III, and IV. The important thing for chemical engineers to anticipate as they move through Unit I is that the principles are at the core of the entire text and it will be necessary to integrate information from Unit I in the later units. The key is to follow the methods of applying systematically the first law (energy balance) and the second law (entropy balance). Watch carefully how the general equations are quickly reduced to the specific problem at hand. Especially watch how the systems of equations are developed to match the unknown variables in the problem. Learn to perform similar reductions quickly and accurately for yourself. It takes practice, but thorough knowledge of that much will help immensely when it comes to Unit II.

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