All of the processes involved in science have corresponding energy changes, however there are some chemical and physical changes that are of particular interest, either historically, or because they can be used to obtain other information about energy changes that are difficult to find directly.
The actual value of a specific change can be affected by the temperature at which it is measured, so a standard set of conditions is defined to ensure that the data produced in an experiment is relevant. Standard state conditions are 1 atmosphere pressure and molar quantities.
Note: This is not to be confused with S.T.P. which, in terms of the gas laws, is defined as 273K and 1 atmosphere pressure. Energy measurements are usually quoted under standard conditions and represented by the subway sign (shown right, more correctly called a superscript plimsoll line). The temperature, while not formally included in standard state conditions is quoted and is usually 298K.
In Chapter 4.2