A work in progress that will be abandoned as soon as this course is over. I'm not very good at P-Chem, but if you have any questions, feel free to ask. [00 CHEM 308 (1) - THIS ONE]
1. PHYSICAL CHEMISTRY II
CHAPTER 9: Chemical Kinetics I
Rate of Consumption,Formation,and Reaction
Consider the reaction π΄ + 3π΅ β 2π. The numbers of moles of the species at the
beginning of the reaction (at time π‘ = 0) are π π΄0
, π π΅0
, πππ π π0
, respectively.
Assuming the reaction thereafter proceeds from left to right, the moles at some time
π‘ (π ) become π π΄0
β π, π π΅0
β 3π, πππ π π0
+ 2π, respectively, where π is the extent of
the reaction in terms of moles.
Now, we can define the moles of reactants and product after time π‘ (π ) as:
π π΄ = π π΄0
β π, π π΅ = π π΅0
β 3π, πππ π π = π π0
+ 2π, such that π π΄0
, π π΅0
, πππ π π0
are
constants and π π΄ , π π΅, πππ π π are functions of the reaction extent, i.e. π( π).
Furthermore, we may derive these expressions with respect to time and obtain:
ππ π΄
ππ‘
= 0 β
ππ
ππ‘
,
ππ π΅
ππ‘
= 0 β
3ππ
ππ‘
, πππ
ππ π
ππ‘
= 0 +
2ππ
ππ‘
Solving for
ππ
ππ‘
gives the rate of reaction in terms of moles:a
ππ
ππ‘
= β
ππ π΄
ππ‘
= β
ππ π΅
3ππ‘
= +
ππ π
2ππ‘
However, it is more typical to track concentrations than moles. So we may modify
the above expression as follows.
Let π be the volume of the reaction chamber (assuming it remains constant
throughout the reaction). Then:
ππ
πππ‘
= β
ππ π΄
πππ‘
= β
π π π΅
3πππ‘
= +
ππ π
2πππ‘
If we define the extent of the reaction in terms of concentration as π₯ =
π
π
and the
concentrations of the reactants and product as [ π΄] =
π π΄
π
, [ π΅] =
π π΅
π
, πππ [ π] =
π π
π
respectively, then the rate of reaction in terms of concentration is:
π =
ππ₯
ππ‘
= β
π[ π΄]
ππ‘
= β
π[ π΅]
3ππ‘
=
π[ π]
2ππ‘
Specifically, we can define the rate of reaction in terms of concentration with
respect to each species:
ππ΄ = β
π[ π΄]
ππ‘
= πππ‘π ππ πππ ππππππππππ ππ π΄
6. IntegrationofRate Laws
First Order Reaction:
Consider ππ΄ β πππππ’ππ‘π .
At π‘ = 0, [ π΄] = [ π΄]0 and [ πππππ’ππ‘π ] = 0.
At π‘ = πππ¦, [ π΄] = [ π΄]0 β ππ₯, where a, ax, and [A]0 are constants.
The derivative of [A] with respect to time gives us
π[ π΄]
ππ‘
= 0 β π
ππ₯
ππ‘
.
Therefore, π =
ππ₯
ππ‘
= β
π[ π΄]
π ππ‘
= π[ π΄] πΌ
, where ο‘ is the partial order of reaction with
respect to A, determined experimentally. Assuming the reaction is 1st order, πΌ = 1.
Thus, π = π[ π΄]1
.
To simplify, we can let β
π[ π΄]
π ππ‘
= π[ π΄]and let π π΄ = ππ.
Rearranging therefore gives β
π [ π¨]
[ π¨]
= π π¨ π π, the rate law in differential form.
7. If we integrate the above equation, we get the following:
β
π[ π΄]
[ π¨]
= π π΄ ππ‘ β β π₯π§[ π¨] = π π¨ π + πͺ, the rate law in integral form.
When π‘ = 0, β ln[ π΄]0 = 0 + πΆ. Therefore, πͺ = β π₯π§[ π¨] π.
Substituting gives: βln[ π΄] = π π΄ π‘ β ln[ π΄]0. Rearranging, ln[ π΄] β ln[ π΄]0 = βπ π΄ π‘ and
ln
[ π΄]
[ π΄]0
= βπ π΄ π‘.
We now define the half-life of the reaction to be the time required for [A] to drop to
half its value; that is, π = π π
π
ππππ [ π¨] =
[ π¨] π
π
.
Thus, for a 1st order reaction, at tΒ½:
ln
1
2
[ π΄]0
[ π΄]0
= βπ π΄ π‘1
2
β ln
1
2
= βπ π΄ π‘1
2
β β ln 2 β= βπ π΄ π‘1
2
β π π
π
=
π₯π§ π
π π¨
π‘1
2
is independent of concentration in 1st order reactions.
Important Integrals
β«
1
π₯
ππ₯ = ln| π₯| (1)
β« ππ₯ = π₯ (2)
β« π₯ π
ππ₯ =
π₯ π+1
π+1
(3)
β«( π + ππ₯) π
ππ₯ =
( π+ππ₯) π+1
π( π+1)
(4)
β«
1
π+ππ₯
ππ₯ =
1
π
ln(| π + ππ₯|) (5)
β«
1
( π+ππ₯) π ππ₯ = β
1
( πβ1) π( π+ππ₯) πβ1 (6)
8. Second Order Reaction:
Consider ππ΄ β πππππ’ππ‘π .
π =
ππ₯
ππ‘
= β
π[ π΄]
π ππ‘
= π[ π΄] πΌ
, where ο‘ is the partial order of reaction with respect to A,
determined experimentally. Assuming the reaction is 2nd order, πΌ = 2. Thus, π =
π[ π΄]2
.
To simplify, we can let β
π[ π΄]
π ππ‘
= π[ π΄]and let π π΄ = ππ.
Rearranging therefore gives
π [ π¨]
[ π¨] π = βπ π¨ π π, the rate law in differential form.
If we integrate the above equation, we get the following:
β
π[ π΄]
[ π¨] π = π π΄ ππ‘ β β
π
[ π¨]
= βπ π¨ π + πͺ, the rate law in integral form.
When π‘ = 0, β
1
[ π΄]
= 0 + πΆ. Therefore, πͺ = β
π
[ π¨] π
.
Substituting gives: β
1
[ π΄]
= βπ π΄ π‘ β
π
[ π¨] π
. Rearranging,
1
[ π΄]
β
π
[ π¨] π
= π π΄ π‘.
Thus, for a 2nd order reaction, at tΒ½:
1
1
2
[ π΄]0
β
1
[ π΄]0
= π π΄ π‘1
2
β
2
[ π΄]0
β
1
[ π΄]0
= π π΄ π‘1
2
β
1
[ π΄]0
= π π΄ π‘1
2
β π π
π
=
π
π π¨[ π¨] π
π‘1
2
depends on [ π΄]0 in 2nd order reactions.
Third Order Reaction:
Consider ππ΄ β πππππ’ππ‘π .
π =
ππ₯
ππ‘
= β
π[ π΄]
π ππ‘
= π[ π΄] πΌ
, where ο‘ is the partial order of reaction with respect to A,
determined experimentally. Assuming the reaction is 3rd order, πΌ = 3. Thus, π =
π[ π΄]3
.
To simplify, we can let β
π[ π΄]
π ππ‘
= π[ π΄]and let π π΄ = ππ.
Rearranging therefore gives
π [ π¨]
[ π¨] π = βπ π¨ π π, the rate law in differential form.
If we integrate the above equation, we get the following:
9. β
π[ π΄]
[ π¨] π = π π΄ ππ‘ β β
π
π[ π¨] π = βπ π¨ π + πͺ, the rate law in integral form.
When π‘ = 0, β
1
2β π΄β0
2 = 0 + πΆ. Therefore, πͺ = β
π
πβ π΄β0
2.
Substituting gives: β
π
π[ π΄]2 = βπ π΄ π‘ β
π
πβ π΄β0
2. Rearranging,
π
π[ π΄]2 β
π
πβ π΄β0
2 = π π΄ π‘.
Thus, for a 3rd order reaction, at tΒ½:
1
2 (
1
2
[ π΄]0)
2
β
1
2[ π΄]0
2
= π π΄ π‘1
2
β
1
2 (
1
4
[ π΄]2)
β
1
2[ π΄]0
2
= π π΄ π‘1
2
β
1
1
2
[ π΄]0
2
β
1
2[ π΄]0
2
= π π΄ π‘1
2
β
2
[ π΄]0
2
β
1
2[ π΄]0
2
= π π΄ π‘1
2
β
3
2[ π΄]0
2
= π π΄ π‘1
2
β π π
π
=
π
π[ π¨] π
π
π π΄
π‘1
2
depends on [ π΄]0 in 3rd order reactions.
nth Order Reaction:
Consider ππ΄ β πππππ’ππ‘π .
π =
ππ₯
ππ‘
= β
π[ π΄]
π ππ‘
= π[ π΄] πΌ
, where ο‘ is the partial order of reaction with respect to A,
determined experimentally. Assuming the reaction is nth order, πΌ = π. Thus, π =
π[ π΄] π
.
To simplify, we can let β
π[ π΄]
ππ‘
= π[ π΄] π
and let π π΄ = ππ.
Rearranging therefore gives
π [ π¨]
[ π¨] π = βπ π¨ π π, the rate law in differential form.
If we integrate the above equation, we get the following:
π[ π΄]
[ π¨] π = βπ π΄ ππ‘ β
[ π¨]βπ+π
βπ+π
= βπ π¨ π + πͺ, the rate law in integral form.
When π‘ = 0,
[ π¨] π
βπ+π
βπ+π
= 0 + πΆ. Therefore, πͺ =
[ π¨] π
βπ+π
βπ+π
.
Substituting gives:
[ π¨]βπ+π
βπ+π
= βπ π΄ π‘ +
[ π¨] π
βπ+π
βπ+π
. Rearranging,
[ π¨]βπ+π
βπ+π
β
[ π¨] π
βπ+π
βπ+π
= βπ π΄ π‘.
Thus, for a nth order reaction, at tΒ½:
10. (
1
2
[ π΄]0
βπ+1
)
βπ + 1
β
[ π΄]0
βπ+1
βπ + 1
= βπ π΄ π‘1
2
β
((
1
2
)
βπ+1
[ π΄]0
βπ+1
)
βπ + 1
β
[ π΄]0
βπ+1
βπ + 1
= βπ π΄ π‘1
2
β π π
π
=
([ π΄]0
βπ+1){(
1
2
)
βπ+1
β 1}
βπ + 1
= βπ π΄ π‘1
2
We can invert the initial concentration term to send it to the number with the index
π β 1, flip the fraction Β½ to also give it the index π β 1, and divide both sides of the
equation by βπ π΄ (to isolate tΒ½ and distribute the negative sign to the denominator,
βπ + 1). Thus:
π π
π
=
π πβπ
β π
[ π¨] π
πβπ( π β π) π π¨
π‘1
2
depends on [ π΄]0 in nth order reactions.
Example:
Consider the 1st order reaction π΄ β π΅.
At time π‘ = πππ¦, [ π΄] = [ π΄]0 β π₯ and [ π΅] = π₯.
Hence, [ π΄] + [ π΅] = [ π΄]0 and π =
ππ₯
ππ‘
= β
π[ π΄]
π‘
=
π[ π΅]
π‘
= π[ π΄] = π([ π΄]0 β π₯).
If β
π[ π΄]
π‘
π[ π΄], then [ π΄] = [ π΄]0 πβππ‘
or ln
[ π΄]0βπ₯
[ π΄]0
= βππ‘.
Therefore, ln
π0βπ₯
π0
= βππ‘.
Also,
ππ₯
ππ‘
= π( π0 β π₯)is a separable equation such that
ππ₯
π0 βπ₯
= π ππ‘.
Integrating gives βln| π0 β π₯| = ππ‘ + πΆ, where C is an integration constant.
At time π‘ = 0: π₯ = 0 and β ln| π0| = πΆ.
Then βln| π0 β π₯| = ππ‘ β ln| π0| β ln| π0 β π₯| β ln| π0| = βππ‘ β ln
π0βπ₯
π0
= βππ‘.
And if [ π΄] + [ π΅] = π0, then [ π΅] = π0 β [ π΄] = π0 β π0 πβππ‘
= π0(1 β πβππ‘).
In terms of [B]:
π[ π΅]
ππ‘
= π( π0 β π₯) = π( π0 β [ π΅]).
This is a separable equation such that:
π[ π΅]
π0β[ π΅]
= π ππ‘.
Integrating gives βln π0 β [ π΅] = ππ‘ + πΆ.
At time π‘ = 0: [ π΅] = 0 and β ln| π0| = πΆ.
Then βln| π0 β [ π΅]| = ππ‘ β ln| π0| β ln| π0 β [ π΅]| β ln| π0 | = βππ‘ β ln
π0β[ π΅]
π0
=
βππ‘, as above.
11. Example
Consider the 2nd order reaction 2π΄ πββββ π or π΄ + π΄ πββββ π.
Let β
π[ π΄]
2ππ‘
= π[ π΄] πΌ
= π[ π΄]2
. And let π π΄ = 2π.
Then rearranging gives
π[ π΄]
π[ π΄]2 = βπ π΄ ππ‘.
Integrating gives β
1
[ π΄]
= βπ π΄ π‘ + πΆ.
At time π‘ = 0: β
1
[ π΄]0
= πΆ.
Then β
1
[ π΄]
= π π΄ π‘ β
1
[ π΄]0
β
1
[ π΄]
β
1
[ π΄]0
= π π΄ π‘, where k has units
1
πππ πΏβ1 π β1.
At time π‘ = π‘1
2
:
1
1
2
[ π΄]0
β
1
[ π΄]0
= π π΄ π‘1
2
β
2
[ π΄]0
β
1
[ π΄]0
= π π΄ π‘1
2
β
1
[ π΄]0
= π π΄ π‘1
2
β π π
π
=
π
[ π¨] π π π¨
.
Consider a 2nd order reaction that has two reactants, such as π΄ + π΅ πββββ π.
We can express the rate as β
π[ π΄]
ππ‘
= π[ π΄][ π΅], but it cannot be integrated as it
contains two different variables.
We still know that at time π‘ = 0: [ π΄] = [ π΄]0, [ π΅] = [ π΅]0, and [ π] = 0.
Similarly, at time π‘ = πππ¦: [ π΄] = [ π΄]0 β π₯, [ π΅] = [ π΅]0 β π₯, and [ π] = π₯.
If we assume that [ π΄]0 = [ π΅]0, then [ π΄] = [ π΅] as well.
The rate then becomes β
π[ π΄]
ππ‘
= π[ π΄]2
, a separable equation.
Thus,
π[ π΄]
[ π΄]2 = βπ ππ‘, which can be integrated as in the example previous example.
Consider a 3rd order reaction 3π΄ πββββ π.
The rate is β
π[ π΄]
3ππ‘
= π[ π΄]3
. Let π π΄ = 3π.
Then rearranging gives
π[ π΄]
π[ π΄]3 = βπ π΄ ππ‘.
Integrating gives β
1
2[ π΄]2 = βπ π΄ π‘ + πΆ.
At time π‘ = 0: β
1
2[ π΄]0
2 = πΆ.
Then β
1
2[ π΄]2 = βπ π΄ π‘ β
1
2[ π΄]0
2 β
1
2[ π΄]2 β
1
2[ π΄]0
2 = π π΄ π‘ β
1
[ π΄]2 β
1
[ π΄]0
2 = 2π π΄ π‘.
At π‘ = π‘1
2
: GO OVER TO CLARIFY!
Example
Consider the reaction 2π΄ πββββ π or π΄ + π΄ πββββ π.
Then β
π[ π΄]
2ππ‘
= π[ π΄]2[ π΅], where [ π΄] = [ π΄]0 β π₯ and [ π΅] = [ π΅]0 β π₯ at any time t.
If we let 2[ π΄]0 β« [ π΄], then 2[ π΄]0 β 2π₯.
Then [ π΄] = 2[ π΄]0 β π₯ β [ π΄] = 2([ π΄]0 β π₯).
And since [ π΅] = [ π΄]0 β π₯, then [ π΄] = 2[ π΅] (and thus [ π΄]0 = 2[ π΅]0 and [ π΅] =
[ π΄]
2
).
12. Therefore, β
π[ π΄]
ππ‘
= 2π[ π΄]2
= 2π π΄
[ π΄]2
2
β β
π[ π΄]
ππ‘
= 2π π΄[ π΄]2
(
1
2
[ π΄]) = π[ π΄]3
β
β
π[ π΄]
ππ‘
= π[ π΄]3
.
Example
Consider a reaction π΄ + π΅ + πΆ πββββ π.
At time π‘ = 0, we assume that [ π΄] = [ π΄]0, [ π΅] = [ π΄]0, and [ πΆ] = [ π΄]0.
Likewise, at time π‘ = πππ¦, [ π΄] = [ π΄]0 β π₯, [ π΅] = [ π΄]0 β π₯, and [ πΆ] = [ π΄]0 β π₯.
The rate is given by: β
π[ π΄]
ππ‘
= π[ π΄][ π΅][ πΆ] β β
π[ π΄]
ππ‘
= π[ π΄]3
β
π[ π΄]
[ π΄]3 = βπ ππ‘.
Integrating gives β
1
[ π΄]2 = βππ‘ + πΆ β
1
[ π΄]2 = ππ‘ + πΆ.
At π‘ = 0,
1
[ π΄]0
2 = πΆ. Therefore,
1
[ π΄]2 = ππ‘ +
1
[ π΄]0
2 β
1
[ π΄]2 β
1
[ π΄]0
2 = ππ‘.
And at π‘ = π‘1
2
,
1
[ π΄]2 β
1
(
1
2
[ π΄]0)
2 = ππ‘1
2
β
1
[ π΄]2 β
1
1
4
[ π΄]0
2 = ππ‘1
2
β π π
π
=
π
π[ π¨] π β
π
π[ π¨] π
π.
HAVE HOKMABADI CHECK THIS!
Example
Consider the 2nd order reaction π΄ + π΅ πββββ π.
At time π‘ = 0, we assume that [ π΄] = [ π΄]0 and [ π΅] = [ π΅]0.
Similarly, at time π‘ = πππ¦, [ π΄] = [ π΄]0 β π₯ and [ π΅] = [ π΅]0 β π₯.
The rate is then given by:
ππ₯
ππ‘
= β
π[ π΄]
ππ‘
= π[ π΄][ π΅] = π([ π΄]0 β π₯ )([ π΅]0 β π₯).
We will denote [ π΄] = π and [ π΄]0 = π0.
Then
ππ₯
ππ‘
= π( π0 β π₯ )( π0 β π₯) β
1
( π0βπ₯ )( π0βπ₯)
ππ₯ = π ππ‘.
We use partial fractions to solve for a and b.
1 = π΄( π0 β π₯) + π΅( π0 β π₯)
π΄π0 + π΅π0 β ( π΄ + π΅) π₯ β 1 = 0
Let {
π΄ + π΅ = 0
π΄π0 + π΅π0 β 1 = 0
We can show that π΅ =
1
π0βπ0
and that π΄ =
1
π0βπ0
.
22. ReactionMechanisms
Consider the reaction 2ππ2 + π2 β 2ππ3.
The nitrogen monoxide-catalyzed oxidation of ππ2 gas gives the overall
stoichiometry of the reaction but does not tell us the process, or mechanism, by
which the reaction actually occurs. This reaction has been postulated to occur by the
following two-step process:
23. ππ‘ππ 1: [ π2 + 2ππ β 2ππ2] π1 = 1
ππ‘ππ 2: [ ππ2 + ππ2 β ππ + ππ3] π2 = 1
ππ£πππππ πππππ‘πππ: π2 + 2ππ2 β 2ππ3
The number of times a given step in the mechanism occurs for each occurrence of
the overall reaction is called the stoichiometric number S of the step.
Do not confuse the stoichiometric number S of a mechanistic step with the
stoichiometric coefficient of a chemical species.
Example
The gas-phase decomposition of π2 π5 has overall reaction:
ππ‘ππ 1: [ π2 π5 β ππ2 + ππ3] π1 = 2
ππ‘ππ 2: [ ππ2 + ππ3 β ππ + π2 + ππ2] π2 = 1
ππ‘ππ 3: [ ππ+ ππ3 β 2ππ2] π3 = 1
ππ£πππππ πππππ‘πππ: 2π2 π5 β 4ππ2 + π2
Species like ππ3 and ππ, which do not appear in the overall reaction, are called
reaction intermediates. Each step in the mechanism is called an elementary reaction.
A simple reaction consists of a single elementary step. A complex/composite
reaction consists of two or more elementary steps.
The Diels-Alder addition of ethylene to butan-1,3-diene to give cyclohexene is
believed to be simple, occurring as a simple step:
πΆπ»2 = πΆπ»2 + πΆπ»2 = πΆπ»πΆπ» = πΆπ»2 β πΆ6 π»10
β¦ π€βπππ π = π[ πΆπ»2 = πΆπ»2][ πΆπ»2 = πΆπ»πΆπ» = πΆπ»2]
Rate-Determining Step Determination
Consider the composite reaction π΄ π1
ββββββ π΅ π2
ββββββ πΆ.
If we assume that π2 β« 1, then the overall reaction becomes π΄ π1
ββββββ πΆ.
The concentration of B, [ π΅], remains very small and constant during the course of
the reaction, such that
π[ π΅]
ππ‘
= 0 = π1[ π΄] β π2[ π΅]. Hence, π1[ π΄] = π2[ π΅] and [ π΅] =
π1
π2
[ π΄].
We also know that
π[ πΆ]
ππ‘
= π2[ π΅]. Thus,
π[ πΆ]
ππ‘
= π2 (
π1
π2
[ π΄]) = π1[ π΄].
Finally,
π[ π΄]
ππ‘
= βπ1[ π΄].
27. Kinetics of Relaxation
Consider the reaction π΄ π1, πβ1
β‘ββββββββββββββββ π.
We know that:
ο [ π΄]0 = [ π΄] + [ π]
ο π₯ = [ π]
ο [ π΄] = [ π΄]0 β [ π] = π0 β π₯
Hence, the rate is given by
ππ₯
ππ‘
= π1[ π΄] β πβ1[ π].
At equilibrium:
ππ₯
ππ‘
= π1( π0 β π₯) β πβ1 π₯ = 0.
If π₯ π is the concentration of Z at equilibrium (i.e. if π₯ π = [ π] ππ), then:
ππ₯
ππ‘
= π1( π0 β π₯ π) β πβ1 π₯ π = 0
We define the change in concentration of x as: βπ₯ = π₯ = π₯ π β π₯ = βπ₯ + π₯ π. Thus:
πβπ₯
ππ‘
= π1[ π0 β (βπ₯ + π₯ π)] β πβ1(βπ₯ + π₯ π)
πβπ₯
ππ‘
= π1 π0 β π1(βπ₯ + π₯ π) β πβ1(βπ₯ + π₯ π)
πβπ₯
ππ‘
= π1 π0 β π1βπ₯ β π1 π₯ π β πβ1βπ₯ β πβ1 π₯ π
πβπ₯
ππ‘
= π1( π0 β π₯ π)β ( π1 + πβ1)βπ₯ β πβ1 π₯ π
πβπ₯
ππ‘
= β( π1 + πβ1)βπ₯ + π1( π0 β π₯ π)β πβ1 π₯ π
π βπ
π π
= β( π π + πβπ)βπ
This is the rate law in differential form.
Rearranging gives
πβπ₯
βπ₯
= β( π1 + πβ1) ππ‘.
Integrating gives: ln βπ₯ = β( π1 + πβ1 ) π‘ + πΆ.
At time π‘ = 0: lnβπ₯0 = 0 + πΆ β πΆ = ln βπ₯0. Therefore:
ln βπ₯ = β( π1 + πβ1) π‘ + ln βπ₯0
ln βπ₯ β lnβπ₯0 = β( π1 + πβ1) π‘
π₯π§
βπ
βπ π
= β( π π + πβπ) π
This is the rate in the form π¦ = ππ₯ + π, where π¦ = ln
βπ₯
βπ₯0
, π = π ππππ = β( π1 + πβ1),
and π₯ = π‘.
28. A graph of ln
βπ₯
βπ₯0
π£π . π‘ would therefore look like:
A plot of π₯ π£π . π‘, however, would give a completely different graph:
The Principle of the Temperature Jump Technique
The relaxation time ο΄ is defined as the time corresponding to
(βπ₯)0
βπ₯
= π and π‘ = π.
Therefore, ln
(βπ₯)0
βπ₯
= ( π1 + πβ1) π‘ becomes ln π = ( π1 + πβ1) π.
And so: 1 = ( π1 + πβ1) π and π =
π
( π π+πβπ)
.
Thus, the negative reciprocal of the slope (β
1
π ππππ
) of a graph of ln
βπ₯
βπ₯0
π£π . π‘ is equal
to the relaxation time ο΄.
N.B The equilibrium constant is given by πΎππ =
π1
πβ1
.
30. Integrating gives lnβπ₯ = β( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π‘ + πΆ.
At time π‘ = 0, βπ₯ = βπ₯0 and therefore lnβπ₯0 = 0 + πΆ β πΆ = ln βπ₯0.
Thus:
ln βπ₯ = β( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π‘ + ln βπ₯0
ln βπ₯ β lnβπ₯0 = β( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π‘
ln
βπ₯
βπ₯0
= β( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π‘
ln
βπ₯0
βπ₯
= ( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π‘
The relaxation time ο΄ is defined as that corresponding to
βπ₯0
βπ₯
= π.
ln π = ( π1 π0 + π1 π0 + πβ1 β 2π1 π₯ π) π β π =
π
π π π π + π π π π + πβπ β ππ π π π
The dissociation of a weak acid, π»π΄ + π»2 π β π»3 π+
+ π΄β
, can be represented as
π΄ π1, πβ1
β‘ββββββββββββββ π+ π.
The rate constants k1 and k-1 cannot be determined by conventional methods but
can be determined by the T-jump technique.
We can prove that the relaxation time is given by π =
1
π1+2πβ1 π₯ π
, where xe is the
concentration of the ions (Y and Z) at equilibrium.
ASK HOKMABADI FOR THE HW BACK IN ORDER TO SEE THE ANSWER TO THIS.
The Bohr Hydrogen or Hydrogen-like (isoelectronic to hydrogen) Atom
Consider a hydrogen bulb shining light upon a concave lens in front of a triangular
pris. If a screen is placed behind the prism, discrete bands of colored and invisible
light (see below).
31. As the capacitor plates in a hydrogen bulb accumulate charge, the negatively
charged plate emits a beam of electrons that bombards hydrogen atoms, which in
turn release photons of light as they are excited.
Compare hydrogen to some hydrogen-like atoms:
H 1 proton 1 electron
He+ 2 protons 1 electron
Li2+ 3 protons 1 electron
β¦ β¦ 1 electron
Let π = ππ‘ππππ ππ’ππππ, π π = πππ π ππ π ππππ‘ππ, π π = πππ π ππ ππ πππππ‘πππ, and
+π§π = πβππππ ππ π‘βπ ππ’ππππ’π .
π π = 1836π π
1. The attractive force between negative and positive charges (protons and
electrons) is the normal type of Coulombic attraction, such that: πΉ β
(+π§π)(β1π)
π2 ,
where β=
1
4ππ0
, π0 is the permittivity of the vacuum. Therefore:
πΉ =
βπ§π2
(4ππ0) π2
2. The electron can remain in any particular state (fixed radius) indefinitely
without radiating its energy, provided that the electronβs angular momentum is
πΏ = ππ£π (Equation 20.10), where πΏ = ππ£π =
πβ
2π
(quantum number, π =
1, 2, 3,β¦, Planckβs constant, β = 6.626 Γ 10β34
π2
ππ/π ( π½/π ), π = π π, π£ =
π£ππππππ‘π¦ ππ πππππ‘πππ, π = πππππ’π ).
Hence πΏ β π, and possible values of L are:
β
2π
,
2β
2π
=
β
π
,
3β
2π
,
4β
2π
=
2β
π
, β¦
Also, L has the same units as h: joule per second.
Note: The total energy of the atom is equal to the sum of the kinetic energy of
orbiting electrons and the potential energy of electrons attracted to protons.
32. Mechanical Stability
Mechanical stability requires that the attractive and centripetal forces cancel each
other; that is:
β
π§π2
(4ππ0) π2 +
ππ£2
π
= 0 β
π§ π2
(4ππ0) π2 =
ππ£2
π
β (4ππ0) ππ£2
π = π§π2
(Eq. 20.13)
From the quantization of Eq. 20.10 ( πΏ = ππ£π), we can write that:
π2
π£2
π2
=
π2
β2
4π2 β π£2
=
π2
β2
π2 π2 (Eq. 20.15)
β¦where β =
β
2π
and ππ£π = β.
Substitute for v2 in Eq. 20.13:
(4ππ0) π (
π2
β2
π2 π2
) π = π§π2
β π =
π2
β2 (4ππ0)
ππ§π2
=
π2
π§
(
β2 (4ππ0)
ππ2
) =
π2
π§
π0
Therefore: {π =
π π
π
π π}, where π = 1,2, 3, β¦, π0 =
β2(4ππ0)
ππ2 = 5.29 Γ 10β11
π.
Figure 1: Proof of a0.
This proves that the distance of the electron from the nucleus is quantized.
Note that for the hydrogen atom, π§ = 1 and thus ππ = π2
π0.
33. Proof that kinetic energy (T), potential energy (U), and total energy are also
quantized:
πΈπ‘ππ‘ = π + π
The force is given by: βπΉ β‘
ππ
ππ
β ππ = βπΉ ππ = β(
βπ§π2
(4ππ0) π2) ππ = (
π§π2
4ππ0
)
ππ
π2
Integrating gives: π = β (
π§π2
4ππ0
)
1
π
+ πΆ
When π β β, π = 0and πΆ = 0.
Then πΌ = β
ππ π
( ππ πΊ π) π
.
The kinetic energy is is given by: π =
1
2
ππ£2
.
Substitute Eq. 20.13: π =
1
2
ππ£2
=
1
2
(
π§π2
(4ππ0) π
). That is: π» =
π
π
(
ππ π
( ππ πΊ π) π
).
Therefore: πΈπ‘ππ‘ = π + π =
1
2
(
π§π2
(4ππ0) π
) β (
π§π2
(4ππ0) π
) = β
1
2
(
π§π2
(4ππ0) π
).
We see that Etot is negative and inversely proportional to r.
The final expression for the (total) energy is obtained by substituting the value of r
as expressed in Eq. 20.17 below:
π =
π2
β2(4ππ0)
ππ§π2
and
ππ =
π2
π0
π§
β¦where π0 =
β(4ππ0)
ππ2 5.29 Γ 10β11
π and β =
β
2π
= 0.529β«
Therefore: πΈπ‘ππ‘ = πΈ π = β
1
2
π§π2
(4ππ0)
β
1
π
= β
1
2
π§ π2
(4ππ0)
β
ππ§ π2
π2β2 (4ππ0)
= β
ππ§2
π4
2π2 β2(4ππ0)2 =
β
π§2
π2 (
ππ4
2β2 (4ππ0)2) = β
π§2
π2 π = β
π§2
π2 π π»
That is: π¬ π = β
π π
π π πΉ π―. (Eq. 20.17)
β¦where π ( π») =
ππ4
2β2 (4ππ0)2 = π π¦πππ’ππ ππππ π‘πππ‘ = 2.179 Γ 10β18
π½.
34. The Bohr Theory/Model of the Hydrogen Atom
Let π§ = 1.
Let ππ£ = ( ππππππ) ππππππ‘π’π, ππ£π = ππππ’πππ ππππππ‘π’π, and πΏ = ππ£π =
πβ
2π
,
where π = 1, 2, 3, β¦, ππ = π2
π0, and πΈ π = β
π π»
π2 .
The potential energy is π = β
π2
(4ππ0) π
= β
2π π»
π2 .
The kinetic energy is π =
1
2
π2
(4ππ0) π
=
π π»
π2 .
The total energy is πΈπ‘ππ‘ = πΈ π = β
π π»
π2
35. The energy required to move an electron from π = 1 to π = β is called its ionization
energy (IE).
πΌπΈ = πΈπ β πΈπ = β
π π»
(β)2
β
π π»
(1)2
= 0 + π π» = π π» = 2.179 Γ 10β18
π½
πΌπΈ/πππ = π π» ππ΄ = 2.179 Γ 10β18
π½(6.022 Γ 10β23
πππβ1)
= 13.11758 Γ 102
ππ½/πππ
βπΈ = βπ£0 β π£0 =
βπΈ
β
=
2.179 Γ 10β18
π½
6.626 Γ 10β34 π½π
= 3.289 Γ 1015
π»π§
The term π£0 is called the threshold frequency.
Hydrogen-like Atoms
Angular momentum is an integral multiple of
β
2π
, such that:
πΏ = ππ£π =
πβ
2π
, where π = 1, 2, 3, β¦
From this, we can get that:
i. ππ =
π2
π0
π§
π€βπππ π0 = 0.529 β« = π΅πβπ πππππ’π
ii. πΈ π = β (
π§2
π2 ) π π», π€βπππ π π» = 2.179 Γ 10β18
π½ = π π¦πππ’ππ ππππ π‘πππ‘
iii. πΏππ‘ πΈ π1
= βπ π» (
π§2
π1
2 ) πππ πΈ π2
= βπ π» (
π§2
π2
2 )
πβπππππππ:βπΈ = πΈ π2
β πΈ π1
= [βπ π» (
π§2
π2
2 )]β π π» (
π§2
π1
2 )
βπ¬ = πΉ π― π π
(
π
π π
π β
π
π π
π )
iv. βπΈ = βπ£, since this change in energy is due to absorption or emission of
photons.
For a hydrogen atom, π§ = 1. Therefore: βπ£ = π π» (
1
π1
2 β
1
π2
2 ).
π = ππ£ β π£ =
π
π
= π
1
π
, π€βπππ
1
π
= π£Μ = π€ππ£πππ’ππππ ( ππβ1)
Therefore: π£ = ππ£Μ .
And βππ£Μ = π π» (
1
π1
2 β
1
π2
2 ) β πΜ =
πΉ π―
ππ
πΉ π― (
π
π π
π β
π
π π
π)
36. If we let π1 = 1 and π2 β« 2, then: π£Μ =
π π»
βπ
(1 β
1
π2
2 ). This is the Lyman series (UV).
If we let π1 = 2 and π2 β« 3, then: π£Μ =
π π»
βπ
(
1
4
β
1
π2
2 ). This is the Balmer series (visible).
If we let π1 = 3 and π2 β« 4, then: π£Μ =
π π»
βπ
(
1
9
β
1
π2
2 ). This is the Paschen series (IR).
De Broglie Wavelength
The De Broglie wavelength is given by: π =
π
π
=
π
ππ
, where π£ = π£ππππππ‘π¦.
Since ππ£π =
πβ
2π
β ππ£ =
πβ
2ππ
, then π =
β
πβ
2ππβ
=
2ππ
π
; that is, π =
ππ π
π
.
Example
If π = 3, then π =
2ππ
3
.
In the diagram above, ππ =
π2
π0
π§
=
(32) π0
1
= 9π0.
This comes from the standing wave shown below:
Joining the ends of the string, a and b, creates the pattern and circle above.
38. The Photoelectric Effect
Consider the horizontal, parallel, clean metal plates (see below) connected by a
circuit containing an ammeter. Light of fixed intensity and varying frequency is
shone upon the lower plate at an angle. Above a given threshold frequency,
electrons jump from the lower to the upper plate, traveling through the circuit to
generate a measurable current.
In other words, a metal with a clean surface is placed in vacuum and illuminated
with light of a known frequency. If the frequency is greater than a particular
minimum, or threshold frequency, electrons are instantly liberated fron the metal
surface at a rate proportional to the light intensity (increasing the intensity does not
liberate more electrons).
The energy is given by πΈ =
1
2
ππ£2
= ππ.
The total energy is conserved; therefore, βπ£ = π + ππ
β¦where π = πππππ‘πππ πππ π , π£ = πππππ‘πππ π£ππππππ‘π¦, π = πππππ‘πππ πβππππ, π =
π ππππππ π£πππ‘πππ (1 π£πππ‘ = 1 πππ’ππ πππ πππ’ππππ; 1π = 1π½/πΆ), and π =
ππ‘π‘ππππ‘ππ£π πππ‘ππππ‘ππ ππ πππ‘ππ ππ π€πππ ππ’πππ‘πππ.
Note that ππ = 1.60206 Γ 10β19
π½ per electron.
Conservation of energy requires that:
βπ£ = π +
1
2
ππ£2
= π + ππ
βπ£0 = π
βπ£ = βπ£0 + +ππ β π£ = π£0 +
1
β
ππ
β¦where π£0 = π‘βπ π‘βπππ βπππ πππππ’ππππ¦.
π ππ = π. πππ Γ ππβππ
π± = ππππππ πππ ππππππππ and π¬ π² = ππ
39. A plot of the frequency against the stopping voltage is shown below:
The equation of the graph π£ = π£0 +
1
β
ππ is of the form π¦ = π + ππ₯.
41. Foundations of Quantum Mechanics
Two vectors π΄ and π΅β can be added or subtracted as follows:
A vector whose magnitudeis unity is called a unit vector.
SKIPPED SOME STUFF
42. Operators
An operator is a symbol that indicates that a particular operation is being performed
on what follows the operator. For example, the square root operatoris β and can
operate as follows: β4 = 2. The differential operator
π
ππ₯
operates as in
π
ππ₯
( π₯2) = 2π₯.
We denote operators in quantum mechanics by using the symbol hat, Μ , as in the
operator πΜ.
Examples
If πΜ =
π
ππ₯
and π( π₯) = π₯4
, then πΜ π( π₯) =
π
ππ₯
π( π₯) =
π
ππ₯
( π₯4) = 4π₯3
.
Then πΜ πΜ π( π₯) = πΜ[πΜ π( π₯)] = πΜ[4π₯3] = 12π₯2
And πΜ πΜ πΜ π( π₯) = πΜ[12π₯2] = 24π₯
Example
If π Μ = π₯2
, then πΜ πΜ[ π( π₯)] = πΜ πΜ[ π( π₯)]
Reactions Having No Order
Not all reactions behave in the manner aforementioned, and the term order should
not be used for those that do not. Instead, such reactions are usually enzyme-
catalyzed and frequently follow a law of the form:
π =
π[ π]
πΎ π + [ π]
In the above equation, V and Km are constants, while [S] is a variable known as the
substrate concentration. This equation does not correspond to a simple order, but
under two limiting conditions, an order may be assigned:
i. If the substrate concentration is sufficiently low, so that [ π] βͺ πΎ π, then the law
becomes π =
π[ π]
πΎ π
, and the reaction is then 1st order with respect to S; or
ii. If [S] is sufficiently large, so that [ π] β« πΎ π, then the law becomes π = π, and
the reaction is said to be 0th order (meaning the rate is independent of [S]).
Consider a reaction π΄ πββββ π.
Then β
π[ π΄]
ππ‘
= π[ π΄] πΌ
= π[ π΄]0
= π.
This is separable such that π [ π¨] = βπ π π, the rate law in differential form.
Integrating gives [ π΄] = βππ‘ + πΆ.
At time π‘ = 0: [ π΄]0 = πΆ.
43. Then [ π΄] = βππ‘ + [ π΄]0 β [ π¨]β [ π¨] π = βππ, the rate law in integral form.
At π‘ = π‘1
2
:
1
2
[ π΄]0 β [ π΄]0 = βππ‘1
2
β β
1
2
[ π΄]0 = βππ‘1
2
β π π
π
=
[ π¨] π
ππ
, where k has units
πππ πΏβ1
π β1
.
Rate Constants and Rate Coefficients
The constant k used in such rate equations is known as the rate constant or rate
coefficient, depending on whether the reaction is believed to be elementary or to
occur in more than one stage (respectively).
The units of k vary with the order of the reaction, as shown in the table below.
Order
Rate Equation
Units of k Half-life, tΒ½
Differential Form Integrated Form
0
ππ₯
ππ‘
= π π =
π₯
π‘
πππ ππβ3 π β1
π0
2π
1
ππ₯
ππ‘
= π( π0 β π₯) π =
1
π‘
ln (
π0
π0 β π₯
) π β1 ln 2
π
2
ππ₯
ππ‘
= π( π0 β π₯)2 π =
1
π‘
ln(
π₯
π0( π0 β π₯)
) ππ3 πππβ1 π β1
1
ππ0
2
ππ₯
ππ‘
= π( π0 β π₯)( π0 β π₯)
for reactants with
different concentrations
π =
1
π‘( π0 β π0)
ln (
π0( π0 β π₯)
π0( π0 β π₯)
) ππ3 πππβ1 π β1
β
n
ππ₯
ππ‘
= π( π0 β π₯) π π =
1
π‘( π β 1)
[
1
( π0 β π₯) πβ1 β
1
π0
πβ1
] πππ1βπ ππ3πβ3 π β1 2 πβ1 β 1
π( π β 1) π0
πβ1