16) chemical kinetics

7/29/2019 16) Chemical Kinetics

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CHEMICAL KINETICS


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CHEMICAL KINETICS Area of chemistry concerned with rates of

reactions How rapidly food spoils

Rate of fuel burning in automobiles

How quickly medicines work

Development of catalysts


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CHEMICAL KINETICS Reaction Rate

change in the concentration of reactants or products

per unit of time

unit: M/s

Red sphere 0.01 mol A

Blue sphere 0.01 mol BCompute the average rate of appearance of Bfrom t=0 to t=20


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CHEMICAL KINETICS Change of rate with time

C4H9Cl(aq) + H2O(l)

C4H9OH(aq) + HCl(aq)

Rates decrease as the reaction proceeds.


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CHEMICAL KINETICS Change of rate with time


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Reaction Rates and Stoichiometry


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Reaction Rates and Stoichiometry

How is the rate at which ozone disappears related

to the rate at which oxygen appears in thereaction 2 O3(g) 3 O2(g)?

-

2 .a particular instant, at what rate is O3disappearing at this time?


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Factors that Affect Reaction Rates

1. Nature of the reactants

2. Physical state of the reactants Important when reactants are of different phases (higher

SA faster reaction

3. Concentration of Reactants For most reactions higher [reactant] faster reaction

4. Temperature Increasing T will also increase the reaction rate

5. Presence of Catalyst


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THE RATE LAW An equation that relates the effect of concentration on

the reaction rate

Consider the reaction:

NH4+(aq) + NO2-(aq) N2(g) + 2H2O(l)

Rate [NH4+][NO2

-]

Rate Law: Rate = k[NH4+][NO2-]


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REACTION ORDERS

Rate = k[A]m

[B]n

. . . m and n are called reaction orders

m+n overall reaction order

The reaction orders are empirically determined.

Units of the rate constant, k

Overall order of reaction Unit of k

0 M s-1

1 s-1

2 M-1s-1

3 M-2s-1


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THE RATE LAW


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Method of Initial Rates

a. Determine the rate law of the reaction

b. Calculate the rate constant

c. Calculate the rate when [NO]= 0.050 M and [H2] = 0.150


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TEMPERATURE AND RATE

Dough rises faster in RT vs. refrigerator

Food spoils faster in RT

Temperature does not affect reactant

concentrations


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TEMPERATURE AND RATE CH3NC CH3CN (rearrangement of methyl isonitrile)


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TEMPERATURE AND RATETHE COLLISION MODEL

The reactant molecules must collide to form the products

The collision must be effective; only a small fraction of

collisions (~1 in every 1013 collisions!) results in a reaction

Correct orientation

Consider the reaction: Cl + NOCl NO + Cl


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TEMPERATURE AND RATE


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TEMPERATURE AND RATEActivation energy (Ea)

Molecules must possess a minimum amount of energy for the

reaction to take place

KE of molecules is used to bend, strain and break bonds If energy is insufficient (low T), molecules will just bounce off each

other

Hi her E slower reaction

Rate is dependent on Ea


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TEMPERATURE AND RATEActivation energy (Ea)


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Activation energy (Ea)


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REACTION MECHANISMSELEMENTARY REACTIONS

Reactions that occur in a single step

The number of molecules that act as reactants in an

elementary process gives the reactions molecularity

High molecularities are rarely encountered

Involves more than one elementary steps


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REACTION MECHANISMS

THE RATE DETERMINING STEP FOR A MULTISTEP PROCESS

the slowest step in a multistep reaction limits the overall rate

NO2 + CO NO + CO2 Rate=k[NO2]2


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CATALYSIS

CATALYST

A substance that changes the speed of the reaction

without itself undergoing a permanent chemical

change

Catalysts are very common in natural systems

HOMOGENEOUS CATALYST A catalyst that is present in the same phase as the

reactant/s

2H2O2(aq) 2H2O(l) + O2(g)


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CATALYSIS


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CATALYSIS


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CATALYSISHETEROGENEOUS CATALYST exists in a different phase

usually a metal or a metal oxide

first step involves adsorption of reactant on the surface

of the catalyst (high SAincreased rate)


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CATALYSISENZYMES biological catalysts

very specific in terms of substrate

more efficient that other nonbiological catalysts

C6H12O6(aq) + 6O2(g) 6CO2(g) + 6H2O(g) release of energy

16) chemical kinetics

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