kinetics reaction rates. collision theory factors affecting reaction rate potential energy diagrams...
TRANSCRIPT
Kinetics
Reaction Rates
Reaction Rates
Collision theory
Factors affecting reaction ratePotential energy diagrams
temperature
concentration
Surface area
catalystsActivated complex
Activation energy
Reaction Rates
A measure of how quickly a reaction occurs An experimental, measurable quantity Rate = change in property/change in time
Example: Speed = miles/hour Speed = distance/time
Chemical kinetics: the study of reaction rates and the factors that affect them
What could we measure for a reaction?
Easily measured properties include: Change in mass of a solid Change in concentration Temperature changes pH changes Gas volume changes Color changes
We must also measure changes in TIME!
Writing Rate Expressions For a general reaction
aA + bB cC + dD General form:
We need to modify the rate expression to compensate for stoichiometry The reaction has only one rate for a given set of
conditions Convention: all reaction rates are positive
time
Dmoles
dtime
Cmoles
ctime
Bmoles
btime
Amoles
aRate
1111
Measuring Rates
Average Rate
Initial Rate Calculate average rate
for early part of data when plot is nearly linear
time
molesXrateaverage
What happens to the rate over time?
Compare average rate at beginning vs. average rate at end
Reaction rates typically slow down over time Why?
There are fewer moles of reactants left, and therefore fewer collisions.
Collision Theory
Molecules must collide in order to react. They must collide with
the correct orientation. “Effective collision”
Has appropriate orientation; molecules may react.
“Ineffective collision” Doesn’t have needed
orientation; particles will separate.
Collision Theory, cont.
Molecules must collide in order to react. They must have enough
energy to react. Activation Energy, Ea
The minimum energy that reactants must have for the reaction to occur
Potential Energy Diagrams
Activation Energy: from reactants to top of “hill”
Transition State Aka Activated Complex High energy state, where
bonds are broken and new bonds are formed
Hrxn = energy of products – energy of reactants
Potential Energy Diagrams
Which reaction would you expect to be fastest? Slowest? Why?
Reactions with a smaller activation energy will occur
more quickly than reactions with a larger Ea.
Collision Theory
Basic premise: More collisions = faster reaction rate More collisions = greater likelihood for effective
collisions
How can we speed up the rate of a reaction?
Increase temperature Particles move more quickly, so more possible
collisions More particles are likely to have enough energy to
overcome activation energy barrier
Increase concentration More particles, so more possible collisions
Increase surface area More particles are exposed, so more collisions are
possible
Catalysts Speed up reaction rates,
without being consumed Homogeneous vs.
heterogeneous catalysts Enzymes Catalytic RNA Catalytic antibodies Catalytic converter in car
engine Effectively lower the
activation energy of the reaction May even change the
mechanism of the reaction