Enthalpy and ΔH
In the context we're interested in, enthalpy is defined as the heat content of a substance. Now, you may ask, "How can you measure heat content?" This is a valid concern, since there is no one way to do this. Therefore, the usual values for entropy are calibrated with respect to the elements: since an element is already in its simplest form, it is assigned an enthalpy of zero. The enthalpy of a compound can thus be measured by finding out how much energy (in the form of heat) is given off when it is formed from its component elements. This value is called the enthalpy of formation. Usually, when a chemist refers to the "enthalpy" of a substance, he or she means the enthalpy of formation. Another common enthalpy value is the enthalpy of combustion, which is the heat given off when the substance is burned with oxygen. These values are almost exclusively measured in joules or kilojoules per mole.
Enthalpy can be used in everyday reactions to measure or predict the heat given off or absorbed. Since matter and energy are conserved (that is, energy cannot be created out of nowhere), it is logical that any heat released could be found by taking the enthalpies (of formation) of the products and subtracting the enthalpies of the reactants. In this way, one can find the amount of energy released or absorbed. Since enthalpy measures the energy change in a system (the heat the reaction gives to or gets from its surroundings), a positive value means energy flowed into the reaction from its surroundings--it is endothermic. A negative value indicates a loss of heat into the environment, so the sign is negative, indicating an exothermic reaction. Alternately, if you know the enthalpy change of a reaction, you can use it to find the enthalpy of a certain product or reactant.
The concept of enthalpy explains ΔH, which we have been using for the past several chapters. Using the Greek letter for summation, capital sigma (Σ), we can now say the equation for ΔH is:
ΔH°system = Σ H°formation(products) - Σ H°formation(reactants)
The superscript "o" signifies that the values are for standard conditions (explained on the first page of this chapter).
It is a scientific fact that substances naturally seek a state of low energy. If a reaction is exothermic, the reacting substances have achieved a lower state of energy. Therefore, most exothermic reactions are favorable (they have high K values). However, some endothermic reactions are also favorable. Explaining this requires the introduction of another form of energy, which we will discuss in a few sections. Next, however, comes a discussion of heat transfer in reactions.