It is useful to discuss classification of enthalpies. We begin by considering enthalpies of physical change, namely phase changes. These are also enthalpies that correspond to the latent heat required for a conversion of a substance from one phase to another. Then we consider various kinds of enthalpy of reaction, hydrogenation, combustion etc. There is also an enthalpy assicated with individual chemical bonds and thus we can add these enthalpies to make molecules. These are not highly accurate, but they serve as an example of the versatility of the enthalpy state function.
Definition of the standard state
The standard state is extremely important in thermodynamics. Yet, it is one of the most confusing aspects in initially. The standard state is defined as 1 bar of pressure for a gas (we often use 1 atm since it is almost the same as a bar and more common) or 1 molar of concentration for a solute in solution. You might wonder why the number 1 is used in both definitions. For now we will point out that if our correction to the standard state uses a logarithm then the number 1 is very handy for a defintion since the ln(1) = 0. Therefore, our correction will be zero if the substance is in the standard state. We will show how these corrections enter into the description once we have discussed free energy. At that point the true meaning of the standard state will become clear. In addition to this definition we will also define the reference state for enthalpy as zero for all elemental compounds in their most stable forms at 298 K. This definition is necessary since enthalpy is a relative function, not an absolute one. Therefore we need to specify a zero of enthalpy for reference. Using this zero we can define standard enthalpies of formation as reactions to form any compound from the elements that make it up.
Thermodynamic cycles
The additivity of enthalpy is one of its most useful properties. We can use known enthalpies to deduce unknown values. This is cone using thermodynamic cycles. Sometimes we simply add up reactions to find the unknown, but the principle is the same as a cycle. We are looking for ways to find out information on unknown enthalpies by using a collection of known enthalpies. As long as we known all but one, the fact that enthalpy is a state property allows us to find the missing one by using the property that the enthalpy change around a closed cycle is equal to zero.
