Chapter 13

Rates of Reaction Dr. S. M. Condren

Reaction Rates

average rate • the rate over a specific time interval instantaneous rate • the rate for an infinitely small interval Dr. S. M. Condren

Integrated Rate Laws

A ---> products rate = - ( D [A]/ D t) = k[A] m average rate rate = - (d[A]/dt) = k[A] m instantaneous rate Dr. S. M. Condren

Average vs. Instantaneous Rate

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For the reaction: The dependence of the concentration of H 2 on time is shown below. Is the reaction rate faster at point A or point B?

A, B, both rates are the same

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Rate Determining Step

• slowest step in a multistep mechanism • the step which determines the overall rate of the reaction Dr. S. M. Condren

Rate Law

• an expression which relates the rate to the concentrations and a specfic rate constant Dr. S. M. Condren

Rate Law

Reaction rate = k [A] m [B] n where m => order with regard to A n => order with regard to B overall order = m + n Dr. S. M. Condren

The empirical rate law for the reaction is Rate = k[NO 2 ][F 2 ].

Which of the following mechanisms is consistent with this rate law?

A) NO 2 (g) + F 2 (g) <=> NO 2 F(g) + F(g) fast NO 2 (g) + F(g) --->NO 2 F(g) slow B) NO 2 (g) + F 2 (g) <==> NO 2 F(g) + F(g) slow NO 2 (g) + F(g) ---> NO 2 F(g) fast C) F 2 (g) <==> F(g) + F(g) slow 2NO 2 (g) + 2F(g) ---> 2NO 2 F(g) fast A, B, C Dr. S. M. Condren

Order of Reaction

• exponent of the concentration for a reactant that implies the number of molecules of that species involved in the rate determining step • first order, exponent equals one • second order, exponent equals two Dr. S. M. Condren

Order of Reaction

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Graphical Determination of Order of Reaction This plot of ln[H 2 O 2 ] vs. time produces a straight line, suggesting that the reaction is first order.

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A Nanoscale View: Elementary Reactions

• unimolecular - rearrangement of a molecule • bimolecular - reaction involving the collision of two particles • termolecular - reaction involving the collision of three particles Dr. S. M. Condren

Unimolecular Reaction

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Bimolecular Reaction

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Collision Rate Model

Three conditions must be met at the nanoscale level if a reaction is to occur: • the molecules must

collide ;

• they must be positioned so that the reacting groups are together in a

transition state

between reactants and products; • and the collision must have enough into products.

energy

to form the transition state and convert it Dr. S. M. Condren

Transition State: Activated Complex or Reaction Intermediates

• an unstable arrangement of atoms that has the highest energy reached during the rearrangement of the reactant atoms to give products of a reaction Dr. S. M. Condren

Activation Energy

the minimum energy required to start a reaction Dr. S. M. Condren

Temperature & Reaction Rate

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Catalyst

• susbstance which speeds up the rate of a reaction while not being consumed

Homogeneous Catalysis -

a catalyst which is in the same phase as the reactants

Heterogeneous Catalysis -

a catalyst which is in the different phase as the reactants catalytic converter – solid catalyst working on gaseous materials Dr. S. M. Condren

Which curve illustrates the effect of a catalyst on the reaction diagram, given that it speeds up the rate of a reaction?

A, B, C, D

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Reaction Mechanism

• A set of elementary reactions which represent the overall reaction Dr. S. M. Condren

Catalytic Converter

H 2 O (g) 2 H 2(g) +

HCs

+ 2 NO (g) catalyst -------> CO (g) catalyst ------> N 2(g) + H 2(g) + 2 H 2 O (g) (unbalanced) CO (g) + O 2(g) catalyst --------> CO 2(g) (unbalanced) catalyst = Pt-NiO

HCs

= unburned hydrocarbons Dr. S. M. Condren

Chain Mechanisms

chain initiating step

-

the step of a mechanism which starts the chain chain propagating step(s)

-

the step or steps which keeps the chain going chain terminating step(s) - the step or steps which break the chain Dr. S. M. Condren

Chain Mechanisms

combustion of gasoline in an internal combustion engine chain initiating step - additives which generate free radicals, particles with unpaired electrons chain propagating step(s) - steps which generate new free radicals chain terminating step(s) - steps which do not generate new free radicals Dr. S. M. Condren

Enzymes

any one of many specialized organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.

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