Ch. 15: Chemical Kinetics

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To be useful, reactions must occur at a

reasonable rate

Chemical kinetics concerns

reaction rates

Reaction mechanism

series of steps by which a reaction takes place

To track concentration vs. time, the _________ _________ _________ is determined by a spectrometer when the two reactants mix in a chamber

volumetric flow rate

Reaction Rate

change in concentration of a reactant or product per unit time

Δ[A]/Δt =

average reaction rate

It is customary to work with __________ reaction rates

positive

The instantaneous reaction rate at any time can be found by computing the ________ of the ________ ________.

slope of the tangent line

Stoichiometry determines

relative rates

The reaction rate changes with

time

To avoid complications, we study the rate of reaction under conditions where the ________ __________ makes only a negligible contribution.

reverse reaction

The reaction rate depends only on the concentrations of the

reactants

Rate=k[A]^n is called a _______ ________ in which k is the __________ constant and n is the ______ of the reactant, both which are determined by __________.

rate law; rate; order; experiment

The rate constant can be affected by changes in

temperature

Differential rate law (or just rate law) expresses how the ______ depends on __________.

rate; concentration

Integrated rate law expresses how _______ depend on _______.

concentrations; time

We can work backwards from the ______ _______ to find the steps by which the reaction occurs.

rate law

If rate/concentration=1, then the reaction is

first order

The overall reaction order is

the sum of the orders of the reactants

For a first order reaction, plotting _____ vs. time will give a straight line.

ln[A]

In a first order reaction, the slope of the line=

-k

The time required for a reactant to reach half of its original concentration

half-life

Does half-life for a first order reaction depend on concentration?

no

t 1/2=

0.693/k

Does half-life for the second order reaction depend on concentration?

yes

t 1/2 (second order) =

1/ k[A]o

For a second order reaction each successive half-life is _______ the preceding one.

double

For a zero-order reaction, the rate is

constant

Zero-order reactions are most often encountered when a substance such as a __________ ___________ or an ____________ is required for the reaction to occur.

metal surface; enzyme

In a pseudo-first order rate law, the concentration of one reactant is _______ compared with the concentrations of all the others.

small

intermediate

a species that is neither a reactant nor a product but that is formed during the reaction sequence

A reaction whose rate can be written from its molecularity

Elementary step

Molecularity

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

The rate-determining step is the _______ step.

slowest

What is the steady-state approximation?

The assumption that the concentration of any intermediate remains constant as the reaction proceeds

d[I]/dt=

0

Rate laws obtained by assuming steady-state conditions are often___________.

complicated

To use steady-state approximation, first write the proposed _____________, construct a steady-state expression for each ______________ in which the sum of the rate laws that produce I are then set equal to the sum of the rate laws that consume I, solve for [I1], [I2],....construct an overall _______ ________ for the reaction, and substitute the concentrations of the intermediates to obtain an overall rate law that contains only reactant and/or __________ concentrations.

mechanism; intermediate; rate law; product

Chemical reactions ________ ______ when the temperature is increased.

speed up

The collision model explains that molecules must collide to _______.

react

Only a small fraction of __________ produces a reaction because not all reach the threshold energy.

collisions

Activation energy

The energy that must be overcome to produce a chemical reaction

Activated complex/transition state

The arrangement of atoms found at the top of the potential energy "hill"

ΔE has no effect on the

rate of the reaction

The rate depends on the size of the

activation energy, Ea

Only collisions with energy greater than the ________ ___________ are able to react.

activation energy

The fraction of effective collisions increases ____________ with temperature.

exponentially

Number of collisions with at least activation energy=

(total number of collisions)e^(-Ea/RT)

In order for a collision to lead to a reaction, the collision must involve enough _________ to produce the reaction and the relative ___________ of the reactants must allow formation of any new bonds necessary to produce products.

energy; orientations

zpe^(-Ea/RT) =

k

steric factor

the fraction of collisions with effective orientations (p)

In the Arrhenius equation, A is the

pre-exponential factor or frequency factor

most rate constants obey the ___________ ___________ to a good approximation, indicating that the collision model is physically reasonable.

Arrhenius equation

You can speed up a reaction by raising its

temperature

What do enzymes do?

They increase the rates of reactions even though the body temperature remains constant.

A substance that speeds up a reaction without being consumed itself is called a

catalyst

Enzymes are __________ catalysts.

biological

A catalyst allows the reaction to occur along a pathway with a lower _________ ___________, making a much larger fraction of collisions effective at a given temperature.

activation energy

A homogenous catalyst is one that is present in the ____ ________ as the reacting molecules.

same phase

A heterogeneous catalyst is one that exists in a _______ ________ than that of the reacting molecule, usually as a solid.

different phase

__________ refers to the collection of one substance on the surface of another substance.

Adsorption

___________ refers to the penetration of one substance into another.

Absorption

Heterogeneous catalysis involves four steps:

1) Adsorption and activation of the reactants 2) Migration of the adsorbed reactants on the surface 3) Reaction among the adsorbed substances 4) Escape, or desorption, of the products.

The most effective catalytic materials are transition _______ ________ and ________ metals such as palladium and platinum.

metal oxides; noble

_________ quickly destroys much of a converter's catalytic efficiency.

Lead

NO forms ozone in the ________ atmosphere, which is harmful, and destroys ozone in the ________ atmosphere.

lower; upper

A _________ is a species with an unpaired electron.

radical

As a rule of thumb, when there is an increase of 10 degrees Celsius, the reaction rate

doubles

Photochemistry

the interaction of radiation with molecules which produces a chemical change

If A+B--> products, and [A]>>B, then [A] remains about _______. [B] will then exhibit "_______-_____ ______" kinetics and ln[B] vs t will be linear.

constant; pseudo-first order

OH+CH4 --> CH3 + H2O, -d[OH]/dt=

k[OH][CH4], must make [CH4]>>[OH] in order to make [OH] "pseudo-first order

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