chapter 9 phenomenological chemical kinetics (empirical/classical) -- experimental aspects

Chapter 9 Phenomenological chemical kinetics (empirical/classical)

-- experimental aspects

§ 9.1 Introduction of chemical kinetics

Levine p. 528-529

17.1 reaction kinetics

rate of reaction, rate laws, reaction mechanism

17.2 measurement of reaction rates

17.3 integration of rate law

first-order reactions; second-order reactions; third-order reactions

zero-order reaction;

n-th-order reaction

1.1 details about a chemical reaction

1) Stoichiometric: balance of the equation

2) Thermodynamic: Spontaneity and equilibrium

3) Kinetic: Rate and mechanism

The factor concerned by thermodynamics is the inherent tendency or the possibility and the advancement of the reaction but not the reality and the rate.

is spontaneous. Fortunately, this reaction is too slow to be detectable.

Thermodynamics and kinetics concern different aspects of reaction, for example:

. ,

2 2 3N 3H 2NHCat T p

Thermodynamics:

reaction direction (spontaneity); maximum yield (equilibrium).

Kinetics:

rate (catalyst, temperature), mechanism

Chemical kinetics:

a branch of chemistry that concerns reaction rate and mechanism.

Main concerns of chemical kinetics:

1) Reaction rate: factors that determines the rate of reaction, rate law.

2) Reaction mechanism: the series of steps by which a reaction takes place.

3) structure-dependence of reactivity: the correlation between molecular structure and reaction potential.

1.2 reaction mechanism and elementary reaction

H2 + I2 = 2 HI

1) I2 = 2I

2) 2I + H2 = 2HI

H2 + Br2 = 2 HBr1) Br2 = 2 Br

2) Br + H2 = HBr + H

3) H + Br2 = HBr + Br

4) H + HBr = H2 + Br

5) 2 Br = Br2

the detailed way by which the reactants are converted into products,

or the series of steps by which a reaction takes place.

the detailed way by which the reactants are converted into products,

or the series of steps by which a reaction takes place.

Reaction mechanism / reaction pathway

multistep reaction

The reaction that completes in one act is called elementary reaction. The sum of the elementary reactions and their series is called the mechanism of the overall reaction.

1) Br2 = 2Br

2) Br + H2 = HBr + H

3) H + Br2 = HBr + Br

4) H + HBr = H2 + Br

5) 2 Br = Br2

The species such as H and Br formed in one step and consumed

in a subsequent step and never seen as a product is called an

intermediate.

1.3 Classification of reactions

1) elementary reactions

According to the number of molecules involved in the reaction, elementary reactions can be divided into three kinds:

unimolecular reaction: (decomposition, isomerization) I2 = 2I

bimolecular reaction: (combination) Br + H2 = HBr + H

termolecular reaction: 2I + H2 = 2HI

The probability of three-molecule colliding simultaneously is very

small, termolecular reaction are quite rare. The elementary reaction

with four reactant molecules in gaseous phase has not been found.

According to the number of elementary reactions

involved, the overall reaction can be classified into:

2) overall reactions

+

Ethylene butadiene cyclohexene

Find differences between mechanisms described in physical

chemistry and those shown in organic chemistry.

Find differences between mechanisms described in physical

chemistry and those shown in organic chemistry.

simple reaction

Diels-Alder addition:

complex reaction

Overall reaction

Hydrolysis of nitrile