# Chap. 13

Chemical Kinetics

## • Rate or speed a chemical reaction occurs.

## • Kinetic refers to the reaction rate. Does the rxn proceed rapidly enough? Is the
rxn product favored and will we get the desired products?

# Average rate of reaction

## • Measured by the decrease in concentration of a
reactant or an increase in concentration of a product over a time interval.

##

# Reactants ---> Products

A à B

## • Average Rate = - [A] / time

## • Or

## • Average Rate = [B] / time

##

## • Change
in concentration([ ] means Molarity) / time interval

## • Rate
is always a + quantity

##

# plot of concentration vs time

from experimentation

## • 1.
Instantaneous rate: at a
specific time this is the negative of the slope of a tangent line.

## • 2. Initial
rate: at time = 0 this is the negative
slope of the tangent line.
Instantaneous rate at time zero.

#

# Factors affecting reaction rates

## • 1. The nature
of the reacting species

## • a. Aqueous

##

## • b.
Type of reaction requiring a specific orientation

#

#

## • 2. Have
different phases or layers where the rxn occurs at the boundary of the
subdivision. Directly related to
surface area of the contact site.

#

## • 3. Temperature
of the reactants: Reaction rate is
temperature dependent, so a Temperature is given.

## • increase in
temperature, increase in rate!!

#

#

## • 4. Collison
theory: The rate is proportional to the
number of molecular collisions per unit time.
Related to the concentration of
the reactants at a fixed temperature.

#

#

## • 5.
Calalyst: a substance that
increases the rate of a chemical reaction without itself being consumed.

## • rate_{catalysed}> rate_{uncatalyzed}

#

## • 6. Energy of
activation: E_{a} minimum amount of energy required to
initiate a chemical reaction forming an activated complex (transition
state).

## • Arrhenius equation:
p. 532

##

#

# Rate Law or rate equation

## • A mathmatical
equation determined experimental for the effect of concentrations of reactants
on reaction rates at constant temperature.

##

##

# aA + bB --> cC + dD

## •
Rate = k [A]^{a}[B]^{b}

##

## •
k = rate constant for a particular rxn at a
specific temperature. Need to be able to calculate the units!!! Greater the k,
faster the reaction

## •
[ ] = Molarity of each reactant

## •
a, b, etc = order of reaction with respect
to each are exp. determined

# Order of reaction

## •
0 is called zero order

## •
1 is called first order

## •
2 is called second order

## •
3 is called third order

##

## •
Overall order = sum of
the order of rxn with respect to each reactant NOT RELATED TO COEFFICIENTS

# NO + O_{3} à NO_{2} + O_{2} at 25^{o}C

## • 1. What is the general rate equation?

# Given

## •
exp [NO]
[O_{3}] Rate = M/s

##

## •
1. 1.00 3.00 .660

## •
2. 1.00 6.00 1.32

## •
3. 1.00 9.00 1.98

## •
4. 2.00 9.00 7.92

## •
Give the rate law with
the orders of rxn. What is the value of
the rate constant?

# CH_{3}CHO à CH_{4} + CO at 100^{o}C

## • Exp. [CH_{3}CHO] rate = M/s

## • 1. 1.75 x 10^{-3} 2.06 x 10^{-11}

## • 2. 3.50 x 10^{-3} 8.24 x 10^{-11}

## • 3. 7.00 x 10^{-3} 3.30 x 10^{-10}

## • Give
the general rate law with the orders of reaction.

## • Calculate
the rate constant.

# ICL + H_{2} à HCl + I_{2} at 225 K

## • Exp. [ICl] [H_{2}]
Rate = M/s

## • 1 1.5x10^{-3 }1.5x10^{-3 }3.7x10^{-7}

^{• }2. 3.0x10^{-3 }1.5x10^{-3 }7.4x10^{-7}

## • 3. 3.0x10^{-3 }4.5x10^{-3 }2.2x10^{-6}

## • General
rate law,Rate constant

^{• }Rate
if you have 4.5x10^{-3} ICl^{
}7.5x10^{-3 }H_{2}?^{}

# others

# Integrated rate equation

## • Related
to the overall order of the rxn.

## • Four
equations

## • Half-life:
t_{1/2} time required for the
concentration of a reactant to decrease to half of the original.

# 1^{st} order overall

## • Integrated
rate equation, p. 520

##

## • ln ([A] / [A_{o}]) = **-**kt

## • half-life, p. 525

## • t_{1/2 }= 0.693 / k

##

# 2^{nd} order overall

## • Have
either one reactant that is second order or two reactants, each first order,
with the same concentration. P. 527

##

## • 1/ [A]
– 1/ [A_{o}] = kt

## • t_{1/2 }= 1/ k[A_{o}]

# problems