Chemical Kinetics“Rates of Reactions”

Reaction Rates

Average rate:

Change of reactant or product concentrations

over a specific time interval

Initial rate:

Rate at the beginning of a reaction.

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

Question?

Rate Determining Step

• slowest step in a multi-step mechanism

• the step which determines the overall rate of the reaction

Rate Law

• an expression which relates the rate to the concentrations and a specific rate constant

Rate Law

Reaction rate = k [A]m [B]n

where m => order with respect to A

n => order with respect to B

overall order = m + n

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, or C

Question?

Order of Reaction

• The exponent of the concentration for a reactant implies the number of molecules of that species involved in the rate determining step

• first order, exponent equals one

• second order, exponent equals two

Integrated Rate Laws

A ---> products

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

average rate

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

instantaneous rate

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

Elementary Reactions• Molecularity is the number of particles coming

together in a reaction step.• unimolecular - rearrangement of a molecule• bimolecular - reaction involving the collision of

two particles• termolecular - reaction involving the collision of

three particles (statistically unlikely)

Unimolecular Reaction

Bimolecular Reaction

Collision Rate ModelThree conditions must be met at the molecular 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 break bonds, form the transition state and convert it into products.

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

Activation Energy

the minimum energy required to start a reaction

Temperature & Reaction Rate

Catalyst• A substance 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

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

Question?

Reaction Mechanism

• A set of elementary reactions which represent the overall reaction

Car Catalytic Converter catalyst

H2O(g) + HCs -------> CO(g) + H2(g) (unbalanced)

catalyst

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

catalyst

HCs + O2(g) --------> CO2(g) + H2O(g) (unbalanced)

catalyst

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

catalyst = Pt-NiOHCs = unburned hydrocarbons

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

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

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.