Test 2-Ch.16 B Kinetics Flashcard

Arrhenius Equation

k = A*e^(-Ea/(R*T))

k = rate constant

T = temperature(abs)

A = constant that relates to the orientation of the colliding molecules

Also: ln(k2/k1) = -Ea/R((1/T2)-(1/T1))

Effect of Temperature on Reaction Rate
Temperature affects the rate by affecting the rate constant
Activation Energy

-Does NOT change with temperature

-it is the minimum energy the molecules must have to react

Graphical Determination of the Activation Energy
A plot of lnk vs. 1/T gives a straight line with slope = -Ea/R
Collision Theory

-the reaction rate as the result of particles colliding w/ each other to react

-thus, concentration are multiplied in the rate law

-only those collision w/ enough energy to exceed Ea can lead to reaction

Effect of Temperature on Collisions

-a temperature rise enlarges the fraction of collisions w/ enough energy to exceed the Ea

 

Fraction of Molecular Collisions (f)

f = e^(-Ea/(R*T))

– the magnitudes of both Ea and T affect the fraction of sufficiently energetic collisions

Reversible Reactions
A reversible reaction has two activation energies
Comparing Ea of fwd and rev reactions

-exothermic process-f of reactant collisions w/ energy exceeding Ea(fwd) is larger than the f of product collision w/energy exceeding Ea(rev) and fwd reaction is faster

-endothermic- opposite

Relation of Ea,T,k, and the rate

-smaller Ea(or higher T) = larger k = increased rate

-larger Ea(or lower T) = smaller k = decreased rate

Effective Collisions

-molecules must collide so that the reacting atoms make contact

-collison must have enough energy and a particular nolecular orientation

Frequency Factor(A)

-the product of the collision frequency Z and an orientation probability factor, p

*A = pZ

The Transition State

-At the moment of a head-on collision, the molecules stop and their Ek is converted to Ep of the collision

-if the Ep is less than the Ea, the molecules recoil

Transition State or Activated Complex

-neither a reactant or a product but a transitional species with partial bonds

-it’s extremely unstable

-the Ea is the quantity needed to stretch and deformm the bonds in order to reach the transition state

– a transition state can go in either direction

Reaction Mechanism/Intermediate

Mechanism- a sequence of single reaction steps that sum to the overall reaction

Intermediate- a substance that is formed and used up during the overall reaction

Reaction Energy Diagram

-the potential energy of the system during the reaction as a smooth curve

-deltaH of rxn = Ea(fwd) – Ea(rev)

Elementary Reactions

-each describes a single molecular event, not made up simpler steps

-can be unimolecular or bimolecular

Termolecular Elementary Steps
-rarely occur bc the probability of three particles colliding simultaneously w/ enough energy and w/ an effective orientation is small
Rate Law for an Elementary Step

-can be deduced from the reaction stoich.

-we use the equation coefficients as the reaction orders in the rate law for an elementary step: that is, the reation order equals molecularity

The Rate-Determining Step/

Rate-Limiting Step

;

-slower than the others, so it limits how fas the overall reaction proceeds.

-its rate law represents the rate law for the overall reaction

-a reactant can have a reaction order of zero because it takes part in the reaction only after the rate-determining step

Correlation Mechanism with the Rate Law

-the elementary steps must add up to the overall balanced equation

-the elementary steps must be physicall reasonable (why termolecular step is rare)

-the mechanism must correlate with the rate law (must correlate with experimental facts)

-cannot include intermediates

Mechanisms and Transition States
Each step in the mechanism has its own transition state
Catalyst

-a substance that increases the rate w/o being consumed in the rxn

-causes a lower Ea which in turn makes the rate constant larger and the rate higher

How A Catalyst Functions

-Does not yield more product than one w/o a catalyst but it yields the product more quickly

-causes a lower Ea by providing a different mechanism for the reaction

Homogeneous Catalyst
-exists in solution w/ the reaction mixture
Heterogeneous Catalyst
-speeds up a rxn that occurs in a separate phase

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