Tài liệu Chemical Kinetics pdf

Types of Rate LawsDifferential Rate Law: expresses how rate Integrated Rate Law: expresses how concentration depends on time... Method of Initial RatesInitial Rate: the “instantaneous ra

Chemical Kinetics

The area of chemistry that concerns reaction rates

Reaction Rate

Change in concentration (conc) of a reactant

Rate = conc of A at time 2 conc of A at time 1

0.0003 70s

O20.0025

0.005 0.0075 0.0100

0.0006 70s

12_1575

Rate Laws

Rate = k [NO2]n

k = rate constant

n = rate order

Types of Rate Laws

Differential Rate Law: expresses how rate

Integrated Rate Law: expresses how

concentration depends on time

C4H9Cl + H2O → C4H9OH + HCl

Method of Initial Rates

Initial Rate: the “instantaneous rate” just after the reaction begins

The initial rate is determined in several experiments using different initial

concentrations

NH4+(aq) + NO2-(aq) → N2(g) 2H2O(l)

Overall Reaction Order

Sum of the order of each component in the rate law

rate = k[H2SeO3][H+]2[I−]3

The overall reaction order is 1 + 2 + 3 = 6

2ClO2 + 2OH- → ClO3- + ClO2- + H2O

1 Determine the rate law for the reaction

2 Calculate the rate constant

First-Order Rate Law

Integrated first-order rate law is

CH3NC → CH3CN

Conversion of methyl isonitrile to acetonitrile

Half-Life of a First Order Rx

12_294

[ N2O5]0 0.1000

0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900

Types of Radioactive Decay

Types of Radioactive Decay

High energy photon

Th series of decays   → Pb

spontaneous beta production)

202

80 Hg (1.53:1 ratio)

110

48 Cd (1.29:1 ratio)

6

3 Li (1:1 ratio)

Unstable region (too many protons;

spontaneous positron production)

Number of protons (Z)

20 40 60 80 100 120

0 0

Rate of Decay

rate = kN

The rate of decay is proportional to the

number of nuclides This represents a order process

first-Decay of Strontium-90

21_477

1 life

2 lives

3 lives 4 half-

Radioactive Decay

Second-Order Rate Law

Integrated rate law is

Zero Order Rate Laws

Integrated Form

A Summary

1 Simplification: Conditions are set such

that only forward reaction is important

2 Two types: differential rate law

integrated rate law

3 Which type? Depends on the type of

data collected - differential and

integrated forms can be interconverted

A Summary

(continued)

4 Most common: method of initial rates

5 Concentration v time: used to determine

integrated rate law, often graphically

6 For several reactants: choose conditions

under which only one reactant varies

significantly (pseudo n-order conditions)

Table 12.6 Summary of the Kinetics for Reactions of the Type aA → Products That Are Zero, First, or Second Order in [A]

Order

Integrated rate law [A] = -kt + [A]0 ln[A] = -kt + ln[A]0

[A]

1

= kt +

[A]01

Plot needed to give a straight line [A] versus t ln[A] versus t

[A]

1 versus t

Relationship of rate constant

Slope = -k Slope = -k Slope = k

to the slope of straight line

Reaction Mechanism

reaction occurs

reactants become products - it is a

summary of the overall process

Often Used Terms

Intermediate: formed in one step and used up

in a subsequent step and so is never seen as a product.

Molecularity: the number of species that must collide to produce the reaction indicated by

Collision Model

Key Idea: Molecules must collide to react

However, only a small fraction of

collisions produces a reaction Why?

Arrhenius: An activation energy must be overcome

Arrhenius Equation

produce the reaction (must equal or exceed the activation energy)

formation of new bonds

Catalyst: A substance that speeds up a

reaction without being consumed

Enzyme: A large molecule (usually a protein) that catalyzes biological reactions.

Homogeneous catalyst: Present in the same phase as the reacting molecules.

Heterogeneous catalyst: Present in a different phase than the reacting molecules.

Lower Ea for Catalyzed Rx

Uncatalyzed pathway

3 Reaction of the adsorbed substances.

4 Escape, or desorption, of the products

Steps:

END