Lecture6 - Example Arrhenius plot Physical Chemistry...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Physical Chemistry Lecture 6 Mechanisms of chemical reactions Chemical kinetics Understand the nature of reactions Measure concentration changes with time Determine effects of various parameters (e.g. pressure, temperature, relative concentration, presence of other chemical substances) Explain observed changes in chemical terms Requires integration of theory of chemical action with experimental results Development of a mechanism of reaction Distinguish between possible mechanisms by comparison of prediction to observed results Arrhenius theory Empirical equation of Arrhenius (van’t Hoff) Useful parameterization of temperature-dependent rate constants A pre-exponential factor E a activation energy Often seen in empirical analysis of kinetic data Activation energy related to the likelihood of a reactive event ) / exp( ) ( RT E A T k a Example Arrhenius plot T R E A T k a 1 ln ) ( ln Useful linear form Slope of plot gives the activation energy directly Many rate constants seem to obey this relationship Non-Arrhenius behavior Arrhenius behavior is empirical Modern chemical-reaction theory does not readily predict the Arrhenius equation Surprising how often Arrhenius behavior is seen Seen in analysis because it is simple Rate constants of some reactions do not obey Arrhenius’s simple equation Example: decomposition of diacetylene Indicates complexity of the chemical reaction Elementary reactions Chemical reactions are often more complex than presented in the balanced equation Do not always occur as a single step from initial state to final state Order is not necessarily the stoichiometric coefficient Some reactions do occur in a single step -- elementary reactions Generally involve simple mono- or bimolecular interactions May involve “unusual” species, i.e. species that are not thought of because they are not very stable Order in elementary reactions is the stoichiometry number, which is called the molecularity 1 1 2 1 2 2 2 2 ] [ ] [ ] [ ] [ 2 Br H k v HBr Br H O H k v O H O H
Background image of page 1

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

View Full DocumentRight Arrow Icon
2 Complex reactions Complex reactions are considered to be comprised of a series of elementary reactions The simultaneous occurrence of these elementary reactions leads to an overall reaction mechanism Example: reaction of hydrogen and bromine to produce HBr 2 2 2 2 2 2 2 2 2 Br Br Br Br H HBr H Br HBr Br H H HBr H Br Br Br mechanism reaction Elementary HBr Br H Sequential two-step reaction
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 02/02/2012 for the course CHEM 419 taught by Professor Staff during the Fall '10 term at University of Delaware.

Page1 / 5

Lecture6 - Example Arrhenius plot Physical Chemistry...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online