Thermochemistry pressure-volume work

Sections 5.3 and 5.4 When a gas is produced or consumed in a chemical reaction cKcurring at constant pressure, the system may perform pressure-volume work against the prevailing pressure. For this reason, we define a new state function called enthalpy, H, vdiich is important in thermochemistry. In systems where only pressure-volume work due to gases is involved, the change in the enthalpy of a system, AH, equals the heat gained or lost by the system at constant pressure. For an endothermic process, AH > 0 for an exothermic process, AH < 0. 

Energetics. Regardless of the gas phase combustion kinetics and thermochemistry, burning will only be possible if the energy balance is favorable. The first law of thermodynamics for a constant pressure gas phase process in which all of the work is pressure-volume (P-V) work states that the internal energy change dLT is related to the change in heat content dQ ... 

Now that we have analyzed some hypothetical applications of the first law of thermodynamics we should ask how these principles apply to chemistry and chemical reactions. The key concept is that elements react to form compounds, presumably to form lower energy situations, but that is not always the case as we will see in the next chapter. Even so, most reactions do result in a lower energy. Since energy is involved, we may think that Af7 is the key to thermochemistry, but we have already mentioned that often pressure and/or volume changes occur during a reaction. Even though these may be small effects, we know that we should work with A//. Thus, a question for an experimental science like chemistry is How can we measure A// Calorimetry involves mostly simple mathematics but is really the main part of thermochemistry. Careful measurements of Affcomb are the backbone of thermochemistry. 

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