false

Unlock Document

Chemistry

CHEM 001B

Hill

Summer

Description

Chapter 12: Chemical Kinetics
Focus on the question – How fast does it happen?
o More closely we’ll look at the speeds, or rates at which chemical
reactions occur
The area of chemistry concerned with reaction rates and sequence of steps
by which reactions occur is called chemical kinetics
o Subject of crucial environmental, biological, and economic
importance
12.1 Reaction Rates
Chemical reactions differ greatly
o Examples:
Combination of Na and Br, occur instantly
Rusting of Fe, imperceptibly slow
To describe a reaction rate quantitatively
o Need to specify
How fast the concentration of a reactant or product changes per
unit time
o Rate =
The reaction rate can be defined either as the increase in the concentration
of a product per unit time or as the decrease in the concentration of a
reactant per unit time
o Example:
2 N O254NO + O 2 2
If we want to find the rate formation of O 2n the
decomposition of N O2 5
o 1. Use this equation:
Rate of formation of O =2
=
o 2. Plug in the values necessary
o 3. Solve
o The most common units :
M/s mol/(L*s)
o Defined in terms of concentration because we want the rate to be
independent of the scale of the reaction
o Don’t forget a minus sign when calculating the rate of disappearance
of reactant because reaction rate is defined as a positive quantity
Example:
Rate of decomposition of N O 2 5
=
-5
= 1.9 x 10 M/s
o Depend on the concentration of at some of the reactants and therefore
decrease as the reaction mixture runs out of reactants, as indicated by
the decreasing slopes of the curves
The slope of the tangent to a concentration-versus-time curve at a time t is
called the instantaneous rate at that particular time
The instantaneous rate at the beginning of a reaction (t = 0) is called the
initial rate
12.2 Rate Laws and Reaction Order
Ddexxxxdxxedx9drrx9wzxz
The dependence of the reaction rate on the concentration of each reactant is
given by an equation called the rate law
o Rate = - = k[A] [B] n
k is the proportionality constant called the rate constant
exponents m and n in the rate law indicate how sensitive the
rate is to changes in [A] and [B]
Small positive numbers
If m is negative, the rate decreases as [A] increases
If m is zero, the rate is independent of the concentration
of A
If m = 1 and [A] is doubled, the rate doubles
If m = 2 and [A] is doubled, [A] quadruples and the rate
increases by a factor of 4 The values of the exponents m and determine the reaction order with respect
to A and B
The sum of the exponents (m + n) defines the overall reaction order
o Example:
2
Rate = k[A] [B] m = 2, n = 1 so m + n = 2 + 1 = 3
12.3 Experimental Determination of a Rate Law
Methods to determine the values of the exponents in a rate law
o The method of initial rates
Carry out a series of experiments in which the initial rate of a
reaction is measured as a function of different sets of initial
concentrations
Ways to determine rate law:
o Establish the reaction order
o Evaluate the numerical value of the rate constant k
Rate = k zeroth order
Rate = k[A] first order
Rate = k[A][B] second order
2
Rate = k[A][B] third order
The rate depends on the concentrations, whereas the rate constant does not
The rate is usually expressed in units of M/s, units of the rate constant
depend on the overall reaction order
Example:
o Initial rate date for the decomposition of gaseous N O a2 55 degrees
are as follows:
Experiment Initial N2O 5 Initial rate of Decomposition
-5
1 .020 3.4 x 10
2 .050 8.5 x 10 -5
a) What is the rate?
Compare experiments 1 and 2 and when we do find that both the
initial N O and initial rate of decomposition come out to be 2.5
2 5
and with this we can say the rate is proportional to the
concentration of N O2a5d therefore the rate law is of first order
Rate = - = k [N 2 ]5m
b) What is the value of the rate constant? -3 -1
Solve by substituting k = = = 1.7 x 10 s
c) What is the initial rate of decomposition of N O2at555 degrees
when its initial concentration is .030M?
Substitute initial concentration of .030 M and rate constant from
part b
-5
Rate = - = k [N 2 ]5= (.030 M) = 5.1 x 10 M/s
12.4 Integrated Rate Law
A first order reaction is one whose rate depends on the concentration of a
single reactant raised to the first power
o Rate = - = k[A]
With calculus it’s possible to convert the rate law to another form,
integrated rate law
o ln = - kt
ln = natural log
[A] = concentration of A at some initial time
0
[A] =tconcentration of A at any time t thereafter
= fraction of A that remains at time t
o Concentration-time equation that lets us calculate the concentration of
A or the fraction of A that remains at any time t
o Used to calculate the time required for the initial concentration of A to
drop to any particular value or to any

More
Less
Related notes for CHEM 001B

Join OneClass

Access over 10 million pages of study

documents for 1.3 million courses.

Sign up

Join to view

Continue

Continue
OR

By registering, I agree to the
Terms
and
Privacy Policies

Already have an account?
Log in

Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.