Application of the First Law to Chemical and Physical Changes

In comparing the quantity of work done in the two different expansions (Figs. 7-8 and 7-11), we found them to be different, thereby proving that work is not a state function. Additionally, the quantity of work performed is greater in the two-step expansion (Fig. 7-11) than in the single-step expansion (Fig. 7-8). We leave it to the interested student to demonstrate, through Feature Problem 125, that the maximum possible work is that done in a reversible expansion (Fig. 7-12). 

Three different paths leading to the same internal energy change in a system 

A sample can be heated very slowly or very rapidly. The darker shading in the illustration indicates a higher temperature. Which of the two sets of diagrams do you think corresponds to reversible heating and which to spontaneous, or irreversible, heating  

In this section, we apply the first law of thermodynamics to a system that undergoes a chemical change. Let s represent the chemical change as follows  

We arrive at the following conclusion For a constant-volume process, such as a reaction in a bomb calorimeter, the heat transferred is equal to the internal energy change. All, of the system. 

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The First Law of Thermodynamics 7-6 Application of the First Law to Chemical and Physical Changes... 

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

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