Environmental Engineering Reference
In-Depth Information
The computer solution of these 13 equations produces consistent results for five
species, the others are negligible. The following results are given in number of moles:
n CH 4 =0
:
0664
n CO 2 =0
:
545
n CO =1
:
39
n H 2 =5
:
35
n H 2 O =1
:
52
5.2.3 Considerations Regarding Chemical Equilibrium
1. The chemical equilibrium law is valid for gaseous systems, for solutions, and
when the reactants have different phases (heterogeneous equilibrium), but it is
not generally valid because rate equations do not, in general, follow the stoi-
chiometry of the reaction as Guldberg and Waage proposed.
2. The concentrations in Equation (5.14) are those at equilibrium and not those at
the start of the reaction.
3. In Equation (5.14), the numerator contains the concentrations of the products
and the denominator the concentration of the reactants. This means that the
larger K eq , the higher the concentration of the products at equilibrium.
4. Given a reaction at a certain temperature, the numerical value of the equilib-
rium constant depends on the units of measurement chosen.
5. Given a balanced reaction, multiplying each stoichiometric coefficient with a
constant value m gives a different K eq value. In that case, K eq of the balanced
reaction with the minimum full stoichiometric coefficients becomes raised to
the power m . Conventionally, K eq of the balanced equation with the minimum
full stoichiometric coefficients is used.
6. Independently from the quantities of the reactants, the equilibrium concentra-
tions of reactants and products have to match with K eq value at that temperature.
7. The units for K eq depend upon the units used for the concentrations. Since the
concentrations of reactants and products are not dimensionless, the unit of
the equilibrium constant is represented by activity . Activity is expressed as
the dimensionless ratio [X]/c 0 where [X] signifies the molarity (concentration
in mol
m −3 ) of the reaction and c 0 is the chosen reference state. Thus, the units
are canceled and K eq becomes dimensionless.
5.2.4 Factors Affecting the Chemical Equilibrium
The factors that affect the chemical equilibrium are all external factors that can some-
how change the values of the equilibrium concentrations (partial pressures and molar
fractions) for a certain reaction. The effect of these factors is to bring the system to a
new equilibrium status with new concentration values.
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