Chemical Kinetics

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Chemical Kinetics

• Sorry not all reactions are instantaneous!

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What is Chemical Kinetics?
Kinetics examines the rates at which chemical
reactions occur. Consider the decomposition
of 2NI3(s) ? N2(g) and 3I2(s) This reaction
is SOOO rapid that we say it occurs
instantaneously.
NI3 is very unstable with respect to its
elements, it is sensitive to light and vibration.
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What about a non-instantaneous reaction?
Na2S2O3(aq) 2HCl ? S(s) H2SO3(aq) 2NaCl(aq)
We saw that the rate which S(s) was formed, so
the rate of the reaction could be altered by
adjusting the concentration of S2O32- or by
varying the temperature.
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What do we mean by rate of reaction?
In general terms rate means how many of something
can be done per unit of time. (ie km/h, m/s etc)
In chemistry though we refer to rates of reaction
based on how much reactant was consumed or how
much product was produced per unit of time
(usually seconds). Consider our experiment . . .
Na2S2O3(aq) 2HCl ? S(s) H2SO3(aq) 2NaCl(aq)
We agreed that the reaction was complete when
0.25mmol of sulfur was present. (Amount of
product formed)
Rate of reaction (Afinal Ainitial) /
(tfinal tinitial)
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The 5 Factors That Can Affect the Rate of Reaction

1. The rate of reaction can (generally) be increased
   by increasing the temperature.
2. Increasing reactant concentrations typically
   increases reaction rate.
3. An increase in surface are will increase the
   reaction rate.
4. The nature of the reactants (ie metal react
   quickly with strong acids)
5. The use of a catalyst will increase the rate of
   reaction (if the catalyst can interact with the
   reactants).

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What causes molecules to interact?

• Kinetic Molecular Theory (KMT) or Collision
  Theory are often used to describe this process.
• KMT maintains that
• all particles of matter are in constant motion
  (they possess kinetic energy)
• there are spaces between the particles, how big
  the space and the speed determine the state of
  matter (solid, liquid or gas)
• as we increase heat, the particles move faster
  increasing theyre kinetic energy

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Collision Theory
This theory is literally based around the notion
that in order for two molecules, atoms or ions to
react they must be able to successfully bump
into each other. These collisions are increased
at higher temperatures, with greater
concentrations, if the reactants are ionic rather
that molecular, with greater surface area. A
catalysts helps bring together the two reacting
pieces thereby facilitating a collision.
Read pages 469 473 for general information. 481
482 Section Review Questions 1, 2, 7, 11 (484)
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So what do we measure?
Consider the following reaction 2H2O2(aq) ?
2H2O O2(g) We can say the rate of this reaction
is equal to the disappearance of H2O2 or the
appearance of O2. Rate -DH2O2
Rate DO2

.
Dt
Dt But for every H2O2 that decomposes only 0.5 O2
is formed, therefore Rate -0.5DH2O2 DO2


. Dt
Dt Units mol/L/s or M/s or M s-1
We call this the average rate of reaction.
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What about the Instantaneous Rate?
2H2O2(aq) ? 2H2O O2(g)
Time (s) Accumulated Mass O2 (g) H2O2 (M)
0 0 0.882
60 2.960 0.697
120 5.056 0.566
180 6.784 0.458
240 8.160 0.372
300 9.344 0.298
360 10.336 0.236
420 11.104 0.188
480 11.680 0.152
540 12.192 0.120
600 12.608 0.094
Initial rate 3.21 x 10-3 M s-1
In order to get the rate of reaction at any given
instant we need to find the negative slope of the
tangent line . . . Huh? The derivative for those
of you in calculus.
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Ok well what about a different set of conditions?
We know from our experiments with the 5 factors
that affect the rate of reaction that generally
if we increase the concentration of the reactants
the reaction will occur faster. The problem with
our experimental determination of rates on the
previous slide is that it works only for an
initial concentration of H2O2 0.882 M. Enter
the Rate Law.
The rate law for a chemical reaction relates the
rate of reaction to concentration of reactants so
it can be used to describe any concentration. Cons
ider a A b B ? c C d D Rate
-(1/a)(DA/t) -(1/b)(DB/t) (Negative b/c
disappearance of reactants) Rate kAmBn
(Rate Law) (m and n are determined experimentally)
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Rate Law eh . . . ?
a A b B ? c C d D Rate kAmBn
k is a proportionality constant called the rate
constant. The numerical values of k depends on
the reaction, the temperature and the catalyst
(if any). The units of k depend on m and n.
The values of the exponents determine the order
of the reaction. If m 1, the reaction is first
order in A. If n 2 the reaction is second
order in B and the reaction is third order
overall.
In general, the higher the order the more complex
the mathematical expression describing the
reaction. Therefore if you have a choice to
treat a reaction using either reactant, choose
the one with the lower order.
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Back to H2O2
2H2O2(aq) ? 2H2O O2(g)
The reaction is 1st order in H2O2, therefore the
rate law is as follows Rate kH2O21
kH2O2 Remember the exponents are determined
experimentally NOT from the balanced chemical
equation.
If we have determined the rates experimentally we
can determine k mathematically. k Rate/H2O2
3.21 x 10-3 M s-1 / 0.882M 3.64 x 10-3
s-1 Once we have the value of k, we can use it
to determine the initial rate for any initial
concentration of H2O2 (at the given temperature).
If I tell you that a reaction is zero order with
respect to A, what is the rate law . . . ?
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Determining the Rate Law The Method of Initial
Rates for Determining m and n
2NO(g) Cl2(g) ? 2NOCl(g)
Experiment Initial NO Initial Cl2 Initial Rate, M s-1
1 0.0125 0.0255 2.27E-05
2 0.0125 0.0510 4.55E-05
3 0.025 0.0255 9.08E-05
(Initial rate)3 kNOmCl2n (2 x 0.0125)m
2m x (0.0125)m 2m (Initial rate)1
kNOmCl2n (0.0125)m (0.0125)m
(Initial rate)3 9.08 x 10-5 M s-1 4 (Initial
rate)1 2.27 x 10-5 M s-1
4 2m 22, therefore the reaction is second
order in NO
If the reaction is first order, when you double
the concentration of a reactant, the rate doubles.
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Applications of the Rate Law Determining
Concentrations at a Later Time
First Order Integrated Rate Law
Time (s) H2O2 (M) lnH2O2
0 0.882 -0.12556
60 0.697 -0.36097
120 0.566 -0.56916
180 0.458 -0.78089
At A0
ln
or lnAt lnA0 -kt lnAt (-k)t lnA0
y m x b
-kt
By plotting these points we get a straight line,
this confirms that the reaction is first order.
Given the table you could solve for k.
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Applications of the Rate Law Half Life
What is half life? ? the amount of time it takes
for the concentration of something to decrease
to ½ its initial amount
½ A0 A0
t½ 0.693 / k
ln
-kt ln (½) -kt½
If you have the need for any diagnostic imaging
you may be required to ingest some radioactive
dye. This dye traces your insides so the image
can be enhanced by X-ray. Biological dyes
usually have a very short half-life and are
nearly completely excreted within a few days.