Enthalpy in chemical change

Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes. 

We have seen that the heat supplied to a system at constant pressure is equal to the enthalpy increase. For many chemical reactions it is possible to make a direct determination of the heat change at constant pressure for other reactions indirect methods (to be discussed later) can be used. In these ways it has proved possible to build a considerable body of data on the enthalpy changes of chemical processes of all kinds and these data have been of great value in an understanding of molecular structure and in various other ways, The study of enthalpy changes in chemical systems is known as thermochemistry. 

The total number of known chemical reactions is enormous, and it would be very inconvenient if one had to tabulate enthalpies of reaction for all of them. We can avoid having to do this by tabulating enthalvies of formation of chemical compounds, which are the enthalpy changes associated with the formation of 1 mol of the substance from the elements in their standard states. From these enthalpies of formation it is possible to calculate enthalpy changes in chemical reactions. This will be discussed on page 165. - ... 

In Chapter 4 we dealt with enthalpy changes in chemical reactions and found that it was very useful to determine enthalpies of formation of compounds the standard enthalpy of formation, AH, is the enthalpy increase for the process in which a compound is formed, at one atmosphere pressure, from the elements in their standard states. If we know the standard enthalpies of formation of all compounds in a chemical reaction it is a simple matter see equation 4.31) to calculate the enthalpy change in the reaction. 

The way in which equilibrium constants vary with temperature is a matter of considerable importance in thermodynamics. It leads us to a very convenient - v GG-experimental procedure for measuring enthalpy changes in chemical reactions. -... 

Then we discuss how to establish standard values for enthalpy changes in chemical reactions and how to use them to calculate enthalpy changes for reactions. 

So far we have considered enthalpy changes for simple physical processes such as temperature changes and phase transitions. But the importance of chemistry to the energy economy arises from the fact that there are enthalpy changes in chemical reactions as well. This enthalpy change is commonly referred to as the heat of reaction. Because many reactions are carried out under constant pressure conditions, this term is sensible, even if slightly imprecise. 

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