Page 158 - Modelling in Transport Phenomena A Conceptual Approach

138 CHAPTER 5. RATE OF GENERATION

are the degradation of mechanical energy into thermal energy during viscous flow
and degradation of electrical energy into thermal energy during transmission of an
electric current.
Generation of energy can also be attributed to various other factors such as
chemical and nuclear reactions, absorption radiation, and presence of magnetic
fields. Energy generation as a result of chemical reaction will be explained in detail
in Chapter 6.
The rate of energy generation per unit volume may be considered constant in
most cases. If it is dependent on temperature, it may be expressed in various forms
R= { a+bT (5.2- 1)
such as

8, eaT
where a and b are constants.

5.3 RATE OF GENERATION IN MASS
TRANSPORT

5.3.1 Stoichiometry of a Chemical Reaction

Balancing of a chemical equation is based on the conservation of mass for a closed
thermodynamic system. If a chemical reaction takes place in a closed container, the
mass does not change even if there is an exchange of energy with the surroundings.
Consider a reaction between nitrogen and hydrogen to form ammonia, i.e.,

N2 -k 3H2 = 2NH3 (5.3-1)

If A1 = N2, A2 = H2 and A3 = NH3, Eq. (5.3-1) is expressed as

A1+3A2=2A3 (5.3-2)
It is convenient to write all the chemical species on one side of the equation and give
a positive sign to the species which are regarded as the products of the reaction.
Thus,
2A3 - Ai - 3A2 = 0 (5.3-3)

CaiAi = 0 (5.3-4)
b1
where ai is the stoichiometric coefficient of ith chemical species (positive if species
is a product, negative if species is a reactant), s is the total number of species in the
reaction, and Ai is the chemical symbol for the ith chemical species, representing
the molecular weight of species.