A proposed mechanism for a reaction is:
(i) A + B2 = AB2 Ea1 = 12 kJ/mol ƒ´H1 = 3 kJ/mol
(ii) AB2 + C2 = ABC + BC Ea2 = 30 kJ/mol ƒ´H2 = 5.2 kJ/mol
(iii) ABC + B2 = AB2 + BC Ea3 = 10 kJ/mol ƒ´H3 = -7.8
kJ/mol
a) Which is the rate determining step? Explain why. (2 marks)
b) What is the overall equation for this reaction (1 marks)
c) Name a reaction intermediate (1 mark)
d) Draw a completely labeled energy diagram to scale for the overall reaction. Clearly show deltaHr
and Ea (3 marks)
a) The rate determining step is the slowest step in a reaction mechanism. To determine the rate determining step, we compare the activation energies (Ea) of the individual steps. In this case, the step with the highest activation energy is step (ii): AB2 + C2 = ABC + BC (Ea2 = 30 kJ/mol). Therefore, the rate determining step is (ii).
b) The overall equation for this reaction can be obtained by combining the individual steps. The net reaction is:
A + B2 + C2 = AB2 + BC + ABC
c) A reaction intermediate is a species that is formed in one step of the reaction and consumed in a subsequent step. In this mechanism, the intermediate is ABC, which is formed in step (ii) and consumed in step (iii).
d) Unfortunately, as a text-based AI, I am unable to provide visual illustrations. However, I can describe the energy diagram for the overall reaction.
On the y-axis, we have the energy level and on the x-axis, we have the progress of the reaction. The energy diagram should have three steps, representing the three steps in the reaction mechanism.
Starting from the reactants side, the first step (i: A + B2 = AB2) should show an energy increase of 3 kJ/mol due to the change in enthalpy (ƒ´H1 = 3 kJ/mol). The energy level then descends by 12 kJ/mol to represent the activation energy (Ea1 = 12 kJ/mol).
Next, for the second step (ii: AB2 + C2 = ABC + BC), the energy level should rise by 5.2 kJ/mol due to the change in enthalpy (ƒ´H2 = 5.2 kJ/mol). The energy peak would represent the activation energy of 30 kJ/mol (Ea2 = 30 kJ/mol).
Finally, for the third step (iii: ABC + B2 = AB2 + BC), the energy level should drop by 7.8 kJ/mol due to the negative change in enthalpy (ƒ´H3 = -7.8 kJ/mol). The energy peak would represent the activation energy of 10 kJ/mol (Ea3 = 10 kJ/mol).
The overall energy diagram should show the change in enthalpy (deltaHr) at the end of the reaction and should also label the activation energies (Ea) for each step accordingly.
a) To determine the rate determining step, we need to identify the step with the highest activation energy (Ea). In this case, Ea1 is 12 kJ/mol, Ea2 is 30 kJ/mol, and Ea3 is 10 kJ/mol. The highest activation energy is Ea2 = 30 kJ/mol in the second step (ii). Therefore, the rate determining step is (ii) AB2 + C2 = ABC + BC.
b) To find the overall equation for this reaction, we can combine the individual steps:
(i) A + B2 = AB2
(ii) AB2 + C2 = ABC + BC
(iii) ABC + B2 = AB2 + BC
By canceling out AB2 on both sides of step (ii) and BC on both sides of step (iii), we can write the overall equation as follows:
A + B2 + C2 = ABC
c) A reaction intermediate is a species formed in one step but consumed in a subsequent step. In this reaction, the reaction intermediate is AB2. It is formed in step (i) and consumed in step (ii).
d) To draw the energy diagram, we need to represent the energy changes (∆H) and activation energies (Ea) for each step. Here's a labeled energy diagram:
Ea1
A + B2 ➝ AB2
∆H1 = 3 kJ/mol
Ea2
AB2 + C2 ➝ ABC + BC
∆H2 = 5.2 kJ/mol
Ea3
ABC + B2 ➝ AB2 + BC
∆H3 = -7.8 kJ/mol
Overall reaction:
A + B2 + C2 ➝ ABC
∆Hr = ∆H1 + ∆H2 + ∆H3 = 3 kJ/mol + 5.2 kJ/mol - 7.8 kJ/mol
On the energy diagram, the reactants are placed on the left, the products on the right, and the transition states (highest points on the diagram) represent the activation energies for each step. The vertical axis represents energy levels, and the horizontal axis represents the progression of the reaction.