# Test 2-Ch.16 B Kinetics Flashcard

 Arrhenius Equation
 k = A*e^(-Ea/(R*T))k = rate constantT = temperature(abs)A = constant that relates to the orientation of the colliding moleculesAlso: 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 reactionIntermediate- 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