Examine the reaction mechanism found below:
A + B2 → AB2 (slow)
AB2 + C → AB2C (fast)
AB2C + C → B2C2 + A (fast)
B2C2 → D + E (fast)
a) Write all the reaction intermediates
b) Write all the species that can be considered a catalyst.
c) provide the overall equation for this process.
d) provide the rate law based on this mechanism.
(Note: this is thermochemistry)
I tried out a and b (I don't know if they're right, please tell me if they are) and I'm also confused for c and d.
My answers:
a) reaction intermediates : AB2, AB2C, and B2C2
b) catalyst : A
I agree with your answers for a) and b).
for c) I did this in my head so check me out thoroughly. What you want to do is to add the equations, cancel those materials on both sides. What you're left with is the final overal aequation.
I came up with this:
B2 + 2C ==> D + E
d) The rate law is based on the slow step.
rate = k(B2)
I have a question about the rate law, I understand that it is based on the slow step, but I don't understand why is it rate = k [B2] instead of rate = k [A] [B2]? Also my teacher said that I needed to show my steps but I don't know how to since this mechanism is based on variable, how do I show my steps?
Your answers for (a) and (b) are correct. The reaction intermediates in this mechanism are AB2, AB2C, and B2C2. A catalyst is a species that speeds up a reaction without being consumed in the process. In this case, species A is indeed a catalyst because it appears in the reaction mechanism but is not consumed in any of the elementary steps.
Now let's move on to (c) and (d):
c) To write the overall equation for this process, we need to cancel out any species that appear as both reactants and products. Looking at the mechanism, we can see that AB2 appears as both a reactant and a product, as does C. Canceling them out, we are left with:
A + B2 → B2C2 + D + E
This is the overall equation for the process based on the given reaction mechanism.
d) To determine the rate law based on this mechanism, we need to consider the slow step, as it is typically the rate-determining step. According to the mechanism, the slow step is the first step:
A + B2 → AB2
Since this is the slow step, it determines the rate of the overall reaction. The rate law for this step can be written as:
Rate = k[A][B2]
where k is the rate constant and [A] and [B2] represent the concentrations of A and B2, respectively.
Therefore, the rate law for the overall process based on this mechanism is:
Rate = k[A][B2]