Chemical Kinetics
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Factors that Affect Reaction rates
•
Reaction Rates
•
Concentration and Rate
•
The Change of Concentration with Time
•
Temperature and Rate
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Reactions Mechanisms
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Catalysis
Chemical Kinetics
Is the study of the rate at which reactions occur and also gives us
information on how the reaction occurs (the Reaction Mechanism)
Factors that Affect Reaction Rates
Reaction rates depend on several factors
Physical State of the Reactants and Mixing
The concentration of the Reactants
The Temperature and Pressure at which the
reaction occurs
Catalysts
On a molecular level Reaction rates depend on the frequency with which
molecules collide
The greater the frequency of collisions (with enough energy to break
bonds), the faster the rate of the reaction
Reaction Rates
A quantitative definition of the rate of a chemical reaction is defined in
terms of product(s) forming and reactant(s) disappearing per unit time
N2(g) + 3H2(g) → 2NH3(g)
Rate of ammonia formation can be expressed as
[NH3]t2 - [NH3]t1
t2 - t1
=
Δ[NH3]
Δt
= 0.50M - 0M
25s - 0s
= 0.50M / 25s = 0.02 M/s
(Rates are expressed as positive quantities, units: M or mol/L per second)
This is the average rate, it doesn’t gives us an actual rate at a given moment
in time
Instantaneous Rate
Gives information on the rate at a particular moment, for this we plot the
concentration of product or reactant with time and determine the slope at our
time of interest
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
Reaction Rates and Stoichiometry
2HI(g) → H2(g) + I2(g)
In this case the rate of appearance (formation) of H2 and I2 is equal, but 2 mols
of HI are consumed for every 1 mol of H2 and I2 formed, so we can express this
as:
Rate = - 1 Δ[HI]= Δ[H2] =
Δ[I2]
2 Δt
Δt
Δt
The rate of HI disappearance is twice the rate of H2 and I2 appearance
This leads to the generalization that in a given reaction
aA + bB → cC + dD
Rate = -1 Δ[A]
a Δt
= -1 Δ[B] = 1 Δ[C] = 1 Δ[D]
b Δt
c Δt
d Δt
In the reaction 2O3(g) → 3O2(g). O2 is formed at 2.0 x 10-5 M/s at a given
instant, at what rate is O3 disappearing at this instant
Concentration and Rate Rate = k[A]m[B]n
NH4+(aq) + NO2−(aq)
N2(g) + 2 H2O(l)
The rate is proportional to the concentration of both reactants, doubling either
the concentration of NH4+ or NO2- doubles the rate of the reaction, so we say
that the reaction order for NH4+ and NO2- is 1 : we can express the rate law as
Rate = k [NH4+][NO2-]
m and n are both 1
For the Reaction: A + B → C + D
The experimentally data was tabulated as shown, Write the rate law for the
reaction
Expt
[A] M
[B] M
Init.Rate M s-1
1
0.1
0.1
0.001
2
0.1
0.2
0.002
3
0.2
0.1
0.004
r=k [A]m[B]n
What are the reaction orders with respect to reactants A and B and what is
the order of the reaction overall?
The reaction orders can be found with the generalized formula:
Reaction order
=
{Log (rate 2/rate 1)} / {Log (concn 2/concn1)}
e.g. For NO2- = Log (2) / Log(2) = 1
END MIDTERM 1 MATERIAL
12.4 The Change in concentration with Time
From a rate law we can calculate the rate of reaction using the rate constant and
initial reactant concentrations. We now need an equation that allows us to
determine the concentration of reactants and products at any particular time
Zeroth Order Reactions
Consider the reaction:
Rate = - Δ [A]
Δt
= k [A]o
A → products
=k
Differential Rate Law
Intergrating this differential rate law gives:
[A]t - [A]o = -kt
Ao is the initial concentration of A (at t = 0)
At is the concentration of A at any time t after
12.4 The Change in concentration with Time
From a rate law we can calculate the rate of reaction using the rate constant and
initial reactant concentrations. We now need an equation that allows us to
determine the concentration of reactants and products at any particular time
First Order Reactions
Consider the reaction:
Rate = - Δ [A]
Δt
= k [A]
A → products
Differential Rate Law
Intergrating this differential rate law gives:
ln [A]t = -kt
[A]o
Ao is the initial concentration of A (at t = 0)
At is the concentration of A at any time t after
Since ln[A]t/[A]o =
ln[A]t - ln[A]o
We can get this equation in the form y = mx +c by re-arranging:
ln[A]t = -kt + ln[A]o
Plotting ln[A]t against t should give a straight line with slope = -k,
and intercept = ln[A]o
Second Order Reactions
Again Consider the reaction:
Rate = - Δ [A]
=k [A]2
Δt
1
= kt + 1
[A]t
[A]o
A → products
Intergrate to give:
So plotting 1/[A]t vs. t will be linear for a second order reaction