Lecture 6 Flashcards
(27 cards)
Each elementary reaction has ___ significance. It describes ___________.
physical
it describes a single step between the conversion of reactants and products.
it also describes an actual physical event on the molecular scale
Global reactions are just___________.
the result of a curve-fitting exercise over a limited range of conditions.
True/False: There is not necessarily any physical meaning to reaction rate laws obtained for global reactions.
True
What do reaction rates express?
A change in moles that occurs within a certain volume or within a certain weight of catalyst per unit of time.
What is the units of reaction rate by volume (3 options)?
moles/(volumetime)
moles/(ft^3hr)
moles/(liter*sec)
What is the units of reaction rate by weight of catalyst?
moles/(lbmin)
moles/(masstime)
moles/(kg*sec)
True/False: Units are the same for elementary and global reactions?
True
What are the two different ways to express reaction rate?
volume or weight of catalyst
What is reaction order?
refers to the exponent to which the concentrations are raised
What order is r=kfCa
First order
What is the order of r=kfCa^2
second order
What is the order of r=kfCaCb^2
third order
What kind of coefficients and reaction orders do elementary reactions have?
integer: 1,2,3 etc
What kind of coefficients and reaction orders do Global reactions have?
fractional, zero, or negative
What does zero order mean?
There is no molecules that participate
What does fourth order imply?
4 body collisions which are so rare as to be unimportant in our class
True/False: Global reaction orders are contrained?
False they are not constrained.
What do global reactions describe?
the net behavior of the reacting system, but not the individual steps required to get from the reactant to the products
What do global reactions relate?
These are balanced reactions that correctly relate moles of reactant to moles of product
What are two ways in which you can observe an elementary reaction in the labratory?
REMPI-MS (Resonance-enhanced multiphoton ionization mass spectrometry)
LI-TOF-MS (Laser-ionization time-of-flight mass spectrometry)
Why can you only observe elementary reactions in complex lab procedures?
They involve unstable reactive intermediates like OH radicals that drive flame chemistry but cannot be isolated as stable species
What are the ways to observe Global reactions?
gas chromatography (gc), liquid chromatography (lc) or spectroscopy
What do you plot to analyze a Global reaction?
concentration as a function of time, then fit the data to a rate law
What does the graph of zero order look like?
negative linear slope
