Frequently Asked Questions (FAQs)

Chapter 3

  1. What is the frequency factor and where can we get values for it? What is it dependent on?

    There are tables in the literature. For a first-order gas phase reaction, an order of magnitude value is A=1013s-1. Generally the frequency factor is independent of temperatures, however on occasion it can be a weak function of temperature. See p.944

  2. Why is the limiting reactant our basis of calculation?

    If the limiting reactant is not chosen as the basis of calculations, one could calculate a NEGATIVE concentration. See Example 3-5 (p.90).

  3. What is the relationship between the KC in chemistry (A + B <--> C)

    and the k in the rate laws?

    KC is an equilibrium constant, and k is specific rate constant and has units of time. The concentration equilibrium constant KC does not.

  4. How does the k (specific reaction rate) depend on pressure, or does it?

    ONLY in very very rare instances at very high pressures such as, 6000 atm is k a function of pressure. See p.220 and CD-ROM on "Critiquing what you read."

  5. What is the frequency factor, A, in the Arrhenius Equation; I want to know what it's physical meaning is and/or what it is a frequency factor.

    Arrhenius Equation is k = Ae-E/RT
    The frequency factor, A, is the coefficient of the exponential term. It has the same units as k. It is related to the number of collisions between molecules. See p.942 and 943.

  6. What does the overall order of the power law model indicate?

    One can classify reactions by their overall order of reaction.

  7. Who determines all the rate laws?

    These can be found in the literature, journals, books, tables, etc., see the footnote on p.75. They can also be determined in the laboratory. See Ch.5

  8. How is rA determined as a function of concentration and how do we find the rate law of a chemical reaction  if this is not elementary?

    The rate law is determined only from experiment, see chapter 5. Also see chapter 7 for a discussion of non-elementary reactions.

  9. Is the reaction rate affected by conditions other than temperature?

    It is a function of concentration of species participating in the reaction and whether or not there is a catalyst.

  10. Why are the units for k different depending on the rate law expression?

    So that -rA has always the same units (mol/dm3 s), no matter what the reaction order.

  11. Are there any factors on which the activation energy is dependent upon?

    The activation energy is a constant for each reaction (at least in a range of temperatures). It is related to the strength of the chemical bonds that must be broken and formed for the chemical reaction to take place.

  12. What type of reaction is zero-order?

    For example, if the rate law is we can see that for large values of k2CA ( k2CA >> 1) the rate law becomes , i.e. apparant zero order. If a zero-order reaction is conducted in a batch reactor, the concentration is a linear function of time (i.e. dC/dt = k and CA= CAo - kt).


Elementary Reactions

  1. How often are chemical reactions "elementary"? Is there a way to know if a given chemical reaction  is elementary?

    Many chemical reactions are elementary. The only way to find the rate law is by experiment. See chapter 5.

  2. How do we define terms for Kc if reaction is not elementary?

    The equilibrium constant and it's relationship to the concentrations of reacting species is determined by the thermodynamics Not by kinetics. See Appendix C.

Equilibrium reactions and Thermodynamics

  1. At what point can we assume that a reversible reaction is practically irreversible?

    When the equilibrium constant is very large (e.g. Ca. 1000). For irreversible reactions the rate law equation  is simpler and so are the computations involved in reactor sizing.

  2. For a reversible chemical reaction, what is the effect of temperature on the rate  law?

    Temperature affects the equilibrium constant, Kc and thus the rate law,  through the Van't Hoff equation. See chapter 8 and Appendix C. For an exothermic reaction the equilibrium is shifted back to the left and Kc decreases.

  3. For a reversible reaction, does the equilibrium constant change as a function of any thing other than temperature?

    The dimensionless equilibrium constant K is most always only function of temperature. However Kc and Kp can be functions of pressure. See Appendix C.

  4. What are activity coeffficients and under what conditions are activity coefficients necessary?

    Activity coefficients correct for deviations from ideal solutions and ideal gases, and are necessary under non-ideal solutions.

  5. When do we need to use Z, the compressibility factor in the rate law?

    Only when Z changes during the course of the reaction; that is, Z could change if we were to start the reaction at very very high pressures and end at low pressures. These conditions seldom, if ever, occur.