FundChemReaxEngCh2

FundChemReaxEngCh2 - _~.2,f~ Rate Constants of Elementary...

Info icon This preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
_________ ~~___ .... 2...,f~ Rate Constants of Elementary Reactions 2.1 I Elementary Reactions Recall from the discussion of reaction networks in Chapter 1 that an elementary re- action must be written as it proceeds at the molecular level and represents an irre- ducible molecular event. An elementary reaction normally involves the breaking or making of a single chemical bond, although more rarely, two bonds are broken and two bonds are formed in what is denoted a four-center reaction. For example, the reaction: is a good candidate for possibly being an elementary reaction, while the reaction: is not. Whether or not a reaction is elementary must be determined by experimentation. As stated in Chapter 1, an elementary reaction cannot be written arbitrarily and must be written the way it takes place. For example (see Table 1.4.3), the reaction: (2.1.1) cannot be written as: (2.1.2) since clearly there is no such entity as half a molecule of dioxygen. It is important to note the distinction between stoichiometric equations and elementary reactions (see Chapter 1) is that for the stoichiometric relation: (2.1.3)
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
S4 CHAPTER 2 Rate Constants of Elementary Reactions one can write (although not preferred): NO + ~02 = N0 2 (2.1.4) The remainder of this chapter describes methods to determine the rate and tem- perature dependence of the rate of elementary reactions. This information is used to describe how reaction rates in general are appraised. 2.2 I Arrhenius Temperature Dependence of the Rate Constant The rate constant normally depends on the absolute temperature, and the functional form of this relationship was first proposed by Arrhenius in 1889 (see Rule III in Chapter 1) to be: k = Ii exp[ -E/(RgT)J (2.2.1) EXAMPLE 2.2.1 I where the activation energy, E, and the pre-exponential factor, A, both do not de- pend on the absolute temperature. The Arrhenius form of the reaction rate constant is an empirical relationship. However, transition-state theory provides a justification for the Arrhenius formulation, as will be shown below. Note that the Arrhenius law (Equation 2.2.1) gives a linear relationship between In k and T~ 1• The decomposition reaction: can proceed at temperatures below 100°C and the temperature dependence of the first-order rate constant has been measured. The data are: 288 298 313 323 338 1.04 X 1O~5 3.38 X 1O~5 2.47 X 1O~4 7.59 X 1O~4 4.87 X 10 3 Suggest an experimental approach to obtain these rate constant data and calculate the acti- vation energy and pre-exponential factor. (Adapted from C. G. Hill, An Introduction to Chem- ical Engineering Kinetics & Reactor Design. Wiley, New York, 1977.) Answer Note that the rate constants are for a first-order reaction. The material balance for a closed system at constant temperature is: dt
Image of page 2
CHAPTER 2 Bate Constants of Elementary Reactions 55 where nN,o, is the number of moles of NzOs.
Image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern