Chapter 14
Chemical Kinetics
Chemical Kinetics
• Study of rxn rates (how fast rxn
progresses)
– Measured in [X]/t
• Also deals with reaction mechanisms
– Steps rxn goes through to move from
reactants to products
Factors That Influence Reaction Rate
Under a specific set of conditions, every reaction has its own
characteristic rate, which depends upon the chemical nature of
the reactants.
Four factors can be controlled during the reaction:
1.
2.
3.
4.
Concentration - molecules must collide to react;
Physical state - molecules must mix to collide;
Temperature - molecules must collide with enough energy to react;
The use of a catalyst.
Expressing Rxn Rates
• Rates are expressed in some unit quantity
per time
– SI units for rates of distance (speed) are in m/s
– Chemical rxn rate units are M/s
• Consider the rxn A→B
–
• Rxn rates given can be initial,
instantaneous, or average
Expressing Rate
aA
1
rate =
a
+
[A]
t
bB
cC
1
= -
b
+
[B]
t
dD
1
= +
c
[C]
t
1
[D]
d
t
= +
The numerical value of the rate depends upon the substance that
serves as the reference. The rest is relative to the balanced
chemical equation.
Practice
PROBLEM:
Determine the average rxn rate over the course of the entire
experiment for the data listed in Table 16.1
PROBLEM:
(a) Balance the following equation and express the reaction rate
in terms of the change in concentration with time for each
substance: NO(g) + O2(g) → N2O3(g). (b) How fast is [O2]
decreasing when [NO] is decreasing at a rate of 1.60x10-4 M/s?
The Rate Law
• The rate law governs the progress of a
given rxn.
• For a general rxn: aA + bB → cC + dD
– The rate law is given by the equation below:
– Rate = k[A]x[B]y,
• k – rate constant; x & y are rxn orders wrt A & B
• All components of a rxn’s rate law must be
determined experimentally
– Measure physical quantity that you can relate to the
concentration of a reactant either @ a specific instant
(initial rate method) or over time (integrated rate law)
Determining Rate Laws
• The Initial rate method
– Used to determine rxn orders experimentally
– Measure initial rate of rxn @ different reactant
concentrations
– Data is listed in a table
– Ratio data, in general rate law form, from 2
lines in the table to determine order of each
reactant in rxn
• Choose lines where conc. reactant in question
changes and conc. of all other reactants stays the
same
Practice
2NO(g) + O2(g)
Experiment
2NO2(g)
Initial Reactant Concentrations
(mol/L)
O2
NO
Initial Rate
(mol/L*s)
1
1.10x10-2
1.30x10-2
3.21x10-3
2
2.20x10-2
1.30x10-2
6.40x10-3
3
1.10x10-2
2.60x10-2
12.8x10-3
4
3.30x10-2
1.30x10-2
9.60x10-3
5
1.10x10-2
3.90x10-2
28.8x10-3
1. Determine the general rate law for the rxn.
2. Calculate the rate constant for experiment 2.
The Rate Constant
• Specific for a particular rxn at a particular
temperature, within experimental error
• Units for k tell you the overall rxn order
– Remember units for rate are M/time
– Units for [A]x are Mx
– For a rxn with an overall order R, the unit for k can be
found by
The Integrated Rate Law
• Can be used for 2 reasons
1. Determine reactant concentration after an
elapsed time--- must know order of reactant,
rate constant, correct formula
2. Determine rxn order for a specific reactant--must graph different quantities vs. time and
see which gives most linear plot
• Can only be used for 0, 1st, and 2nd order
rates
Integrated Rate Law Formulas & Plots
Practice
PROBLEM: At 250C, hydrogen iodide (HI) breaks down ver slowly to
hydrogen and iodine: rate = k[HI]2.The rate constant @
250C is 2.4x10-21 L/mols. It 0.0100 mol of HI is place in a
1.00L container, how long will it take for the concentration
of HI to reach 0.00900M (10% reacted)?
PROBLEM:
Determine the rxn order for N2O5 using the
graphical data given
Reaction Half-Life
• Time required for reactant concentration to
reach ½ its initial value
The Arrhenius Equation
• Describes the relationship between temperature and rxn rate
where k is the kinetic rate constant at T
Ea is the activation energy
R is the energy gas constant
T is the Kelvin temperature
A is the collision frequency factor
•
•
•
•
Higher T  Larger k  Increased/faster rate
Smaller Ea  Larger k  Increased/faster rate
Lower Ea (or T)  Smaller k  Decreased/slower rate
A is related to both the collision frequency an orientation probability
factor (dependent on structural complexity)
Activation Energy
• Energy that must be overcome for
reactants to form products
– All rxns regardless of initial and final energies
have Ea > 0
– Some bonds must break and new bonds must
form
– Reactant molecules gain this energy through
collisions with one another
– Increasing temperature increases rate as #
collisions and energy of collisions increase
Practice
Reaction Energy Diagrams
• Used to depict changes reactant molecules
undergo to form products
Reaction Mechanism
• Sequence of single rxn steps that sum to the overall rxn
• It is impossible to prove rxn mechanism experimentally
• Rxn energy diagrams can elucidate # steps in a
mechanism
• Steps in a mechanism for an overall rxn are elementary
steps in which the coefficients of each reactant denote
the reaction rate order wrt the reactant
• The sum of all reactant coefficients in an elementary
step denote the molecularity of the step
• The higher the molecularity of an elementary step, the
slower its rate
2NO2(g) + F2(g) → 2NO2F(g)
• Experimental rate law
determined:
– rate = k[NO2][F2]
• Accepted mechanism:
– NO2(g)+ F2(g) → NO2F(g)+ F(g)
– NO2(g) + F(g) → NO2F(g)
Correlating Mechanism w/ Rate Law
• For a mechanism to be reasonable, its
elementary steps must meet 3 criteria:
1. Elementary steps must add up to overall balanced
eqtn
2. Elementary steps must be physically reasonable
(usu. bi- or lower molecularity)
3. Mechanism must correlate with the rate law
•
•
Overall rate law is usually equivalent to the slowest step’s
(the rate limiting step, RLS) rate law
RLS can be picked out in a rxn energy diagram and
predicted in a mechanism
Practice
•
The rxn and rate law for the decomposition of dinitrogen pentoxide are
2N2O5(g)→ 4NO2(g) + O2(g)
rate = k[N2O5]2
and the rxn energy diagram is given above. Which of the following
mechanisms is most likely?
A. One-step collision
C. 2N2O5(g) → N4O10(g) [fast]
N4O10(g) → 4NO2(g) + O2(g) [slow]
B. 2N2O5(g) → N4O10(g)
[slow]
N4O10(g) → 4NO2(g) + O2(g) [fast]
Catalysis
• Increasing rxn rate by
adding a catalyst
• Catalyst function:
– Lowers Ea increases k without
being consumed or changing
product amount
• Usually lowers Ea by
providing a different
mechanism