rates of reactions.ppt

8/14/2019 Rates of Reactions.ppt

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Chemical Kinetics

Rates of Reaction


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Kinetic = movement or change

Chemical kinetics is the area of

chemistry concerned with the speed, or

rates, at which a chemical reaction

occurs

Reaction rate is the change in the

concentration of a reactant or a product

with time (M/s)

Why reactions have such vastly different

rates?


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Reactants Products

A B

tRate

][

tRate

][

Where:

[A]and [B]are the changes in concentrations

(molarities) over a time period t.

[A]is a negative quantity

The rate of reaction is a positive quantity, so a

minus sign is needed to in the rate expression to

make the rate positive


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http://bouman.chem.georgetown.edu/S02/lect2/fig1.gif


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Reactants

Products


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ExampleIn an aqueous solutions, molecular bromine reacts with

formic acid as follows:

Br2(aq)+ HCOOH(aq) 2Br2-(aq)+ 2H+(aq)+ CO2 (g)

Measuring the change (decrease ) in [Br2] at some initial

time and then at some final time enable s us to

determine the average rate of the reaction during that

interval. {Data in the Table provided}

Rates of the reaction between Br2and Formic acid

Time (s) [Br2] (M) Rate (M/s) k = (rate/[Br2] )(s-1)

00.0 0.0120

50.0 0.0101 3.52 x 10-5

100.0 0.00846

150.0 0.00710

200.0 0.00595

250.0 0.00500 1.75 x 10-5


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t

rrateAverage

][ 2

initialfinal

initialfinal

tt

rBrrateAverage

][][

22

sMs

MrateAverage /1080.3

0.50

)0120.00101.0( 5

Using the data provided in the Table let us calculate theover the 1st50s time interval.

If we choose the 1st100 s as time interval the averagerate would then be given by:

sMs

MrateAverage /1054.3

0.100

)0120.000846.0( 5


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Reaction Rates

average rate the rate over a specific time

interval

instantaneous rate ( ) the rate for an infinitely small

interval

(instantaneous = Physics existing ormeasured at a particular instant)


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How to distinguish between average

rate and instantaneous rate?

The distance between Makkah and Elmadinahis 512 miles.

It takes 11.5 hours to go from one city toanother at the average speed of 512

miles/11.4 hours or 44.9 mph.

If the car is traveling at 55.3 mph after 3 hoursdeparture, then the instantaneous speed ofthe car is 55.3 mph

Speed of the car may increase or decrease,but the instantaneous rate of the reactionmust always decrease with time.


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Going back to the previous Table conc. of

Br2 is doubled, the rate of reaction alsodoubled. Thus the rate is directly

proportional to the Br2 conc.


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Rate Determining Step

slowest step in a multistepmechanism

the step which determinesthe overall rate of thereaction


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Rate Law

an expression which relates therate to the concentrations and a

specific rate constant


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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

Concept Test


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Order of Reaction

exponent of the concentration for a

reactant that implies the number of

molecules of that species involved inthe rate determining step

first order, exponent equals one

second order, exponent equals two


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Order of Reaction


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Graphical Determination of Order of Reaction

This plot of ln[H2O2] vs.

time produces a straight

line, suggesting that the

reaction is first order.


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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


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Unimolecular Reaction


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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 reactinggroups are together in a transition statebetween reactants and products;

and the collision must have enough energyto form the transition state and convert itinto products.


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Transition State:

Activated Complex or

Reaction Intermediates

an unstable arrangement of atoms that has

the highest energy reached during therearrangement of the reactant atoms to give

products of a reaction


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Activation Energy

the minimum energy required to start a reaction


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Temperature & Reaction Rate


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Catalyst

susbstance which speeds up the rate of areaction while not being consumed

Homogeneous Catalysis - a catalyst which is

in the same phase as the reactantsHeterogeneous Catalysis - a catalyst which

is in the different phase as the reactants

catalytic converter solid catalyst working on gaseous materials


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Which curve

illustrates the

effect of a catalyston the reaction

diagram, given

that it speeds up

the rate of areaction?

A, B, C, D

Concept Test


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Reaction Mechanism

A set of elementary reactions which

represent the overall reaction


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Catalytic Convertercatalyst

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

catalyst

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

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

catalyst

CO(g) + O2(g)--------> CO2(g) (unbalanced)catalyst = Pt-NiO

HCs= unburned hydrocarbons


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Chain Mechanisms

combustion of gasoline in an internalcombustion engine

chain initiating step - additives which

generate free radicals, particles withunpaired electrons

chain propagating step(s) - steps which

generate new free radicals chain terminating step(s) - steps which do

not generate new free radicals


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ENZYMES

any one of many specialized organic

substances, composed of polymers

of amino acids, that act as catalyststo regulate the speed of the many

chemical reactions involved in the

metabolism of living organisms.


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rates of reactions.ppt

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