Page 160 - Modelling in Transport Phenomena A Conceptual Approach

140 CHAPTER 5. RATE OF GENERATION

or,
f a2(0) f Q3(1) = 0 (4)

crl(0) + 42) + cr3(3) = 0 (5)
Solutions of Eqs. (4) and (5) give

If we take a3 = 2, then a1 = - 1 and a2 = - 3. Hence, the reaction becomes

N2 + 3H2 = 2NH3
Comment: Stoichiometric coeficients have units. For example, in the above
equation the stoichiometric coeficient of H2 indicates that there are 3 moles of H2
per mole of N2.

5.3.2 The Law of Combining Proportions

Stoichiometric coefficients have the units of moles of i per mole of basis species,
where basis species is arbitrarily chosen. The law of combining proportions states
that

moles of i reacted
(moles of ilmole of basis species) = moles of basis species (5.3-8)

(5.3-9)

where E is called the molar extent of the reaction4. Rearrangement of Eq. (5.3-9)
gives
- 1 (5.3-10)
Note that once E has been determined, the number of moles of any chemical species
participating in the reaction can be determined by using Eq. (5.3-10).
The molar extent of the reaction should not be confused with the fractional
conversion variable, X, which can only take values between 0 and 1. The molar
extent of the reaction is an extensive property measured in moles and its value can
be greater than unity.
It is also important to note that the fractional conversion may be different for
each of the reacting species, i.e.,

(5.3-11)

4The term E has been given various names in the literature, such as, degree of advancement,
reaction of coordinate, degree of reaction and progress variable.