BASICS PART 10: REACTION MECHANISM How does reaction mechanism affect reaction rate?

���� � �. �� � ����� → ����� → �����

�� =? 226Ra-Activity: 30 Bq What are the kinetic parameters of the simple reaction � → �?

����� → ��� = ��� + ��� (�� − 5.3) � � = �. �� [���� ]� � � � �.�� � [�����] = [�����]� � �

�� ���.� ��� �� � � = �. �� �� � �� � (�� − 5.7) #������� #������������������� #�������������� How do chemical reactions work at an atomic level? ������ � ������ �������� + ��� → (���)����

��������� � ������� �

� + � → � − �

� − � → � + �

� − � → �

�� �� � + � ⇄ �� → � + �

�� #������������������ #��������� What is an elementary reaction in ?

� + � → � − � � �� � = = � � � �� �

� − � → � + � � �� −�′ = = −� �� �� �

� − � → � � �� −� = = � �� �� �

#������������ #������������� (�� − �. 27) True or False?

The dimerization of two polymer chain radicals proceeds as an elementary step of radical polymerization according to a ...

A: B: 2th order 1st order kinetics. kinetics

C: D: 1.5nd order 2nd order kinetics kinetics Which mechanisms can we combine from 2 elementary reactions?

� → � � � ⇄ � � → � � (�� − 8.1)

� → � � �′ � → � → � Max Bodenstein

* 1871 � → � † 1942 (�� − 9.1)

#������������������ � → � � �′ � ← � → � #������������������� � → � (�� − 10.1) #���������������� � What is the mechanism of a reversible reaction? � ⇄ �

� � ∝ −� − ������� ⇄ � − � − �������

� (�� − 8.2)

� ∝ � = − � = � [∝] �� → (�� − 8.3)

� ∝ � = � = � [�] �� ← (�� − 8.4) #��������������� #��������������� � How to model a reversible reaction? � ⇄ �

#���� #������������ � How do we determine the rate law for a reversible reaction? � ⇄ �

� � � � = � = � [�] �� ←

� � = � � − � [�] (�� − 8.5) ��

� � #������ � = − � = � [�] �� → #���� � How do concentrations change over time? � ⇄ � � � � � � (�� − 8.21) � � = � + � � � + �

� = � � − �

� [�]��

� ��

#����������������� � Thermodynamics and kinetics of a reversible reaction � ⇄ �

�� − �� = ∆�� (�� − 8.28)

� �� � � = = � [�] = � [�] �� � #������������������ �� �� �� � (�� − 8.26) (�� − 8.27) True or False? At dynamic chemical equilibium... A: …the rate of B: … …the rate the forward constant of the reaction is equal forward reaction is to the rate of the equal to the rate reverse reaction constant of the reverse reaction C: …the activation energy of the forward reaction is equal to the activation energy of the reverse reaction How does temperature affect a reversible reaction?

�� + �� ⇄ � ��

� � ��� Max Bodenstein �� � �� � 1871 - 1942 � �. �� �� �. �� �� 129 ��� � ��� � � � � � � �. �� �� �. �� �� 77.8 ��� � ��� � � � � � � �. �� �� �. � �� 54.9 ��� � ��� � (�� − ���. 8.1)

#����������������� Max Bodenstein * Magdeburg 1871 #��� ����† �Berlin��������������� 1942 How does temperature affect rate constants and equilibrium constant?

Hinreaktion : k(298 K) = 3,7 1/h; EA=9.86 kJ/mol; Rückreaktion: k(298 K) = 2,7 1/h; EA=19.7 kJ/mol;

�� − ��. �� − (�� − ��. ��) � �′ � → � → � What is the mechanism of a consecutive reaction?

�� ��′ ′ �� � →→ ��� → → � �� (�� − 9.1)

� � � = � = � [�] �� � (�� − 9.2) #������������ #��������������� � � � = − � = �´ [�] �� #����������� �� (�� − 9.3) � �′ How to model a concecutive reaction? � → � → �

#���� #������������ � �′ How do we determine the rate law for a consecutive reaction? � → � → �

� � � = � = � [�] �� �

� � = � � − �´ [�] �� (�� − 9.5)

#������ � � � = − � = �´ [�] �� #���� �� � �′ How do concentrations change over time? � → � → �

� � (�� − 9.10) � = � � �

� � � � ´ � (�� − 9.19) � = � � � − � � ´ − �

� ´ � � � � ´ � � = � � � − � + � (�� − 9.21) � ´ − � � ´ − �

� �

#����������������� How reactive is the intermediate?

Max Bodenstein 1871 - 1942

� � = � � − �´ [�] ≈ � #������������������������ �� (�� − 9.24) [�] ≈ � #������������������� (�� − 9.25) Which is rate-limiting step in a consecutive reaction?

� � � � ´ � � = � � � − � � ´ − �

#��� − �������������������� #�������������������� True or False?

A: The rate B: The rate determining step determining step is the slowest is the fastest step in a step in a consecutive consecutive reaction reaction mechanism mechanism

#���������� True or False? Steady state approximation means:

A: The B: The concentration of concentration of a reactive a reactive intermediate is intermediate does very low. not change with time. C: The rate of formation of a E: The rate of reactive formation of a intermediate reactive is equal to its intermediate rate of decay. is slower tha its rate of decay. How much radon gas is emitted from concrete? (226Ra-Activity: 30 Bq)

� �′ ����� → ����� → �����

�� � = � [��] = � ��

�� � �� � � = = �´ = = ���� ∗ ��� �. �� � � �. �� ��� �. �� ��� � � � � �. �� ��� = � = �. �� ��� �� = �´ � �. �� ��� #���������� � �´ � � ← � → � What is the mechanism of a parallel reaction?

� � � = − � = � [�] �� �

� � � = − � = �´ [�] #������������������ �� �� �´ � How to model a parallel reaction? � ← � → �

#���� #������������ �´ � How do we determine the rate law for a parallel reaction? � ← � → �

� � � = − � = � [�] �� � � � = − � � − �´ [�] �� (�� − 10.3)

#������ � � � = − � = �´ [�] �� #���� �� How do concentrations change over time?

(� ´� ) � � = � � � (�� − 10.7)

� � = � � − �(� ´� ) � Rudolf � � ´ + � (�� − 10.13) Wegscheider 1859 -1935 � ´ (� ´� ) � � = � � � − � � ´ + � (�� − 10.17) � � = � � ´ (�� − 10.18) � �

#����������� � #����������������� Which product dominates in a parallel reaction? �´ � � ← � → �

� � � ��� = = � � ´ � �′ �� #���������������������� � ↓; � ↓; ��� � ↑; � ↑ #��������������������������� #������������������� ��� + ��� �� True or False? ��� = ��� →

(�� − �. 39) To get the kinetic product...

A: …use higher B: …lengthen the temperatures. reaction time.

C: …use a catalyst. Which product dominates in a parallel reaction?

�� − ��. �� − (�� − ��. ��)

(�� − ���. 10.2) How is the total resistance related to the individual resistances?

� � � = + ���� �� ��

#������������� ���� = �� + �� #��������������� REFERENCES: PHYSICAL CHEMISTRY BASICS PART 10: REACTION MECHANISM

� � ⇄ � � �� �� � ��� = = � �� ��

�� − �� = ∆�� � �′ � → � → �

� � = � � − �´ [�] ≈ � WHITEBOARD COPIES: �� � �

�´ � � ← � → � � � = � � ��´ Why does kinetics change from 0th order to 1st order?

� � → �

��� � �� � �

��� � �� � What elementary reactions are involved in enzyme ? ������ � ������ �������� + � ��� → (���)����

��������� � ������� �

� + � → � − � � − � → � + � � − � → �

�� �� � + � ⇄ �� → � + �

�� #������������������������ � �′ � + � ⇄ �� → � What are the elementary reactions in enzyme kinetics?

� � + � → � − � � �� � = = � � � �� �

� − � → � + � � �� −�′ = = −� �� �� �

� − � → � � �� −� = = � �� �� �

� �′ We use steady state approximation for the enzyme-substrate intermediate � + � ⇄ �� → �

� � � ��

�� � �� � �� � �� � �� = + + �� �� �� �� � � �

� �� = � � � − � �� − � �� �� � � �

� = �� � � − �� �� − �� �� � �′ How do we calculate the intermediate concentration? � + � ⇄ �� → � � � �� = � � � − � �� − � �� ��

� = [�]�−[��] � �� = � � − �� � − � �� − � �� �� � �� = 0 ��

� � �� = � � + � � + � � �� #����������������������� � �′ What is the reaction rate in enzyme kinetics? � + � ⇄ �� → � � � � � + � �� = � = � + � � + � �

� � �� = � + �

� � � = = � �� �� � � � = � � � = � � + �

� � = ���� � + �� #����������������������� � �′ How do we analyse the Michaelis-Menten equation to get kinetic paramters? � + � ⇄ �� → �

� ���� � � � = � + ��

[�] [�]

� � � 1 = + � � � �

�/�

#LINEW EAVER−BURK���� �/[�] � �′ At what substrate concentration has r dropped to half of r ? � + � ⇄ �� → � max

� � � � = ��� � + ��

��� � �� � �

��� � �� � � �′ � + � ⇄ �� → � True or False?

ES = enzyme substrate complex

A: The decay B: The larger the reaction of ES MICHAELIS-MENTEN-

is 1st order constante KMM, the larger is [ES]

C: Steady state approximation applies to ES