Dr. S. M. Condren

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 = - ([A]/t) = k[A]m

average rate

rate = - (d[A]/dt) = k[A]m

instantaneous rate

Dr. S. M. Condren

Average vs. Instantaneous Rate

Dr. S. M. Condren

For the reaction:

The dependence of the concentration of H2 on time is shown below. Is the reaction rate faster at point A or point B?

A, B, both rates are the same

Dr. S. M. Condren

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[NO2][F2].

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

A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast NO2(g) + F(g) --->NO2F(g) slow

B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast

C) F2(g) <==> F(g) + F(g) slow 2NO2(g) + 2F(g) ---> 2NO2F(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

Dr. S. M. Condren

Dr. S. M. Condren

Graphical Determination of Order of Reaction

This plot of ln[H2O2] vs. time produces a straight line, suggesting that the reaction is first order.

Dr. S. M. Condren

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

Dr. S. M. Condren

Bimolecular Reaction

Dr. S. M. Condren

Collision Rate ModelThree 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 energy to form the transition state and convert it into products.

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

Dr. S. M. Condren

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

Dr. S. M. Condren

Reaction Mechanism

• A set of elementary reactions which represent the overall reaction

Dr. S. M. Condren

Catalytic Converter

catalyst2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g)

catalyst

H2O(g) + HCs -------> CO(g) + H 2(g) (unbalanced)

catalystCO(g) + O2(g) --------> CO2(g) (unbalanced)

catalyst = Pt-NiOHCs = 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 Mechanismscombustion 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.