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Physical Chemistry Basics 10: How Does Reaction Mechanism Affect

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Physical Chemistry Basics 10: How Does Reaction Mechanism Affect PHYSICAL CHEMISTRY 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) � � = �. �� 8 [���� ]� � � � @�.�� 8 � [�����] = [�����]� � � �� ���.� ��� �� � @ � = �. �� 8 �� 8 � �� � (�� − 5.7) #������� #������������������� #�������������� How do chemical reactions work at an atomic level? ������ � ������ �������� + ��� → (���)���� ��������� � ������� � � + � → � − � � − � → � + � � − � → � �� �� � + � ⇄ �� → � + � �� #������������������ #��������� What is an elementary reaction in chemical kinetics? � + � → � − � � �� � = = � 8 � 8 � �� � � − � → � + � w � �� −�′ = = −� 8 �� �� � � − � → � ww � �� −�ww = = � 8 �� �� � #������������ #������������� (�� − �. 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 * Magdeburg 1871 � → � † Berlin 1942 (�� − 9.1) #������������������ � → � � �′ � ← � → � #������������������� � → � (�� − 10.1) #���������������� � What is the mechanism of a reversible reaction? � ⇄ � � � ∝ −� − ������� ⇄ � − � − ������� � (�� − 8.2) � ∝ � = − � = � 8 [∝] �� → (�� − 8.3) � ∝ � = � = � [�] �� ← (�� − 8.4) #��������������� #��������������� � How to model a reversible reaction? � ⇄ � � #���� #������������ � How do we determine the rate law for a reversible reaction? � ⇄ � � � � � = � = � [�] �� ← � � = � � − � 8 [�] (�� − 8.5) �� � � #������ � = − � = � 8 [�] �� → #���� � How do concentrations change over time? � ⇄ � � � @ � ‹� � (�� − 8.21) � � = � + � 8 � � + � � = � � − � � [�]�� � �� � #����������������� � Thermodynamics and kinetics of a reversible reaction � ⇄ � � �� − �� = ∆�� (�� − 8.28) � �� � � = = � [�] = � 8 [�] �� � #������������������ �� �� �� � (�� − 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 �“ �. �� 8 �� �. �� 8 �� 129 ��� � ��� � ” ” @� � @� � �• �. �� 8 �� �. �� 8 �� 77.8 ��� � ��� � ” ” @� � @� � �” �. �� 8 �� �. � 8 �� 54.9 ��� � ��� � (�� − ���. 8.1) #����������������� Max Bodenstein w * Magdeburgw 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) � � � = � = � 8 [�] �� � (�� − 9.2) #������������ #��������������� � � � = − � = �´ [�] �� #����������� �� (�� − 9.3) � �′ How to model a concecutive reaction? � → � → � #���� #������������ � �′ How do we determine the rate law for a consecutive reaction? � → � → � � � � = � = � 8 [�] �� � � � = � � − �´ 8 [�] �� (�� − 9.5) #������ � � � = − � = �´ [�] �� #���� �� � �′ How do concentrations change over time? � → � → � @� � (�� − 9.10) � = � � � � @� � @� ´ � (�� − 9.19) � = 8 � � � − � � ´ − � � ´ � @� � @� ´ � � = � � � − � + � (�� − 9.21) � ´ − � � ´ − � � � � #����������������� How reactive is the intermediate? Max Bodenstein 1871 - 1942 � � = � � − �´ [�] ≈ � #������������������������ �� (�� − 9.24) [�] ≈ � #������������������� (�� − 9.25) Which is rate-limiting step in a consecutive reaction? � @� � @� ´ � � = 8 � � � − � � ´ − � #��� − �������������������� #�������������������� 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) � �′ ����� → ����� → ����� �� � = � [��] = � �� �� � �� � � = = �´ = = ���� ∗ ��� �. �� � � �. �� 8 ��@� �. �� 8 ��@� � � � � �. �� 8 ��� = � = �. �� 8 ��� �� = �´ � �. �� 8 ��@� #���������� � �´ � � ← � → � What is the mechanism of a parallel reaction? � � � = − � = � 8 [�] �� � � � � = − � = �´ [�] #������������������ �� �� �´ � How to model a parallel reaction? � ← � → � #���� #������������ �´ � How do we determine the rate law for a parallel reaction? � ← � → � � � � = − � = � 8 [�] �� � � � = − � � − �´ 8 [�] �� (�� − 10.3) #������ � � � = − � = �´ [�] �� #���� �� How do concentrations change over time? @(� ´‹� ) � � = � � � (�� − 10.7) � � = 8 � � − �@(� ´‹� ) � Rudolf � � ´ + � (�� − 10.13) Wegscheider 1859 -1935 � ´ @(� ´‹� ) � � = 8 � � � − � � ´ + � (�� − 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 � � ⇄ � � �� �� � ��� = = � �� �� �� − �� = ∆�� � �′ � → � → � � � = � � − �´ 8 [�] ≈ � WHITEBOARD COPIES: �� � � �´ � � ← � → � � � = � � ��´ Why does kinetics change from 0th order to 1st order? � � → � ��� � �� � � ��� � �� � What elementary reactions are involved in enzyme catalysis? ������ � ������ �������� + � ��� → (���)���� ��������� � ������� � � + � → � − � � − � → � + � � − � → � �� �� � + � ⇄ �� → � + � �� #������������������������ � �′ � + � ⇄ �� → � What are the elementary reactions in enzyme kinetics? � � + � → � − � � �� � = = � 8 � 8 � �� � � − � → � + � w � �� −�′ = = −� 8 �� �� � � − � → � ww � �� −�ww = = � 8 �� �� � � �′ We use steady state approximation for the enzyme-substrate intermediate � + � ⇄ �� → � � � � �� �� � �� � �� � �� � �� = + + �� �� �� �� � � � � �� = � 8 � 8 � − � 8 �� − � 8 �� �� � � � � = �� 8 � 8 � − �� 8 �� − �� 8 �� � �′ How do we calculate the intermediate concentration? � + � ⇄ �� → � � � �� = � 8 � 8 � − � 8 �� − � 8 �� �� • “ “ � = [�]�−[��] � �� = � � − �� � − � �� − � �� �� • © “ “ � �� = 0 �� � � �� = � � + � � + � � �� #����������������������� � �′ What is the reaction rate in enzyme kinetics? � + � ⇄ �� → � � � © � � + � �� = � = “ “ � + � « � + “ “ �• �• � � �� = © � + �« � � � = = � 8 �� �� “ � � © � = � 8 � � = �“ 8 ¬•® “ © � + �« � � = ���� 8 � + �� #����������������������� � �′ How do we analyse the Michaelis-Menten equation to get kinetic paramters? � + � ⇄ �� → � � ���� 8 � � � = � + �� [�] [�] � � � 1 = « 8 + � �¬•® � �¬•® �/� #LINEW EAVER−BURK���� �/[�] � �′ At what substrate concentration has r dropped to half of r ? � + � ⇄ �� → � max � � 8 � � = ��� � + �� ��� � �� � � ��� � �� � � �′ � + � ⇄ �� → � 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.
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