STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

352 General Engineering and Science
the thermodynamic properties U and H are functions of the state variables, viz.
temperature (T) and pressure (P), and its state of aggregation [e.g., liquid (e), gas or
vapor (g), solid (s), etc.]. Here, we will briefly review certain basic definitions and
terminology employed in the area of thermochemistry and consider some applications
pertaining to combustion of fuels.
Heat of Reaction (AH,). The heat of a chemical reaction carried out at constant
pressure (P) is given by the difference between the total enthalpies of the reactants
and products.
where the subscripts i andj refer to reactant i and productj, n represents the number
of moles, and symbols XR and &. imply summations over all reactants (i = 1, 2, . . .)
and all products (i = 1, 2, . . .), respectively. AHr has units of calories or BTUs; its
value depends on the amounts and physical states of the reactants and products as
well as on the reaction conditions. Note that AHr is negative for an exothermic reaction
in which heat is spontaneously liberated and is positive for an endothermic reaction in
which heat is absorbed from the surroundings.
Standard-State Enthalpy Changes (AH'). To expedite calculations, thermochemical
data are ordinarily presented in the form of standard-state enthalpy changes of the
system AH"(T,P), with the requirement that materials start and end at the same
temperature (T) and pressure (P) and in their standard states of aggregation, Le.,
It has been traditional to choose the reference state as P = 1 standard atmosphere
and T = 25°C (77'F) in expressing AHo values. Examples include standard heats of
reaction AH:, heats of formation AH:, heats of combustion AH:, heats of
vaporization AH: or a, heats of solution AH:, etc. To avoid confusion, the standard
state of aggregation of each substance taking part in the thermochemical process
must be specified by an appropriate letter symbol adjoining its chemical formula.
The standard state for a gas is the ideal gas at 1 atm and specified T. The standard
state for a solid is its stable crystalline form (e.g., rhombic sulfur) or amorphous form
existing at the specified P and T. In the absence of such information, the normal
state of aggregation of the material at given P and T is assumed. Tabulated values of
standard-state enthalpy changes (AHo) are readily available from a number of sources
including handbooks and textbooks [59-63,65,66].
Standard Heat of Reaction. This is the standard enthalpy change accompanying a
chemical reaction under the assumptions that the reactants and products exist in
their standard states of aggregation at the same T and P, and stoichiometric amounts
of reactants take part in the reaction to completion at constant P. With P = 1 atm and
T = 25°C as the standard state, AH:(T,P) can be written as
AH;(25"C,l am) = ~:jH~(250C,1 atm)-~viH~(25"C,1 am)
P
Because v represents the stoichiometric coefficient of a given species, the value of
AH: clearly depends on the way the stoichiometric equation is written for the reaction.
It is conventional to express the above equation in a simplified form as
R