Over the next few days, I'll be doing a homework assignment from my
textbook that is 27 problems long. I've already solved four of today's
nine questions, but these last five have me slightly confused.
Chemical kinetics really isn't my forte. Any help at all would be
1. The gas-phase reaction Cl(g) + HBr(g) ---> HCl(g) + Br(g) has an
overall enthalpy change of -66kJ. The activation energy for the
reaction is 7 kJ. What is the activation energy for the reverse reaction?
(My textbook gives me three equations:
A. ln k=-E_a/RT + ln A
B. ln k_1 - ln k_2 = (-E_a/RT_1 + ln A) - (-E_a/RT_2 + ln A)
C. ln k_1/k_2 = E_a/R(1/T_2 - 1/T_1)
Which of these, if any, would I use to find the activation energy?
Also, are there any additional steps to solving this, or would I be
able to stop after plugging and chugging into one of those equations?)
2. Consider the following reaction: 2NO(g) + 2H_2(g) ---> N_2(g) +
a) The rate law for this reaction is first order in H_2 and second
order in NO. Write the rate law.
(Since Rate=k[reactant 1]^m[reactant]^n, would my rate law be 3?)
b) If the rate constant for this reaction at 1000 K is 6.0*10^4
M^-2s^-1, what is the reaction rate when [NO]=0.025 M and [H_2]=0.015 M?
c) What is the reaction rate at 1000 K when the concentration of NO is
increased to 0.10 M, while the concentration of H_2 is 0.010 M?
3. You have studied the gas-phase oxidation of HBr by O_2: 4HBr(g) +
O_2(g) ---> 2H_2O(g) + 2Br(g)
You find the reaction to be first order with respect to HBr and first
order with respect to O_2. You propose the following mechanism:
HBr(g) + O_2(g) ---> HOOBr(g)
HOOBr(g) + HBr(g) ---> 2HOBr(g)
HOBr(g) + HBr(g) ---> H_2O(g) + Br_2
a) Indicate how the elementary reactions add to give the overall
(I'm guessing the coefficients for one of the equations will need to be multiplied by 2?)
b) Based on the rate law, which step is rate determining?
c) What are the intermediates in this mechanism?
d) If you are unable to detect HOBr or HOOBr among the products, does
this disprove your mechanism?
(Mechanisms were taught today in my class, and none of it made any
sense at all, so this is probably the one I need the most help on.)
4. The reaction 2NO_2 ---> 2NO + O_2 has the rate constant
k=0.63M^-1s^-1. If the initial concentration of NO_2 is 0.100M, how
would you determine how long it would take for the concentration to
decrease to 0.025 M?
5. The following mechanism has been proposed for the gas-phase
reaction of chloroform (CHCl_3) and chlorine:
Step 1: Cl_2(g) <---> 2Cl(g) (fast) [Note: k_1 is atop the
arrows, k_-1 is below the arrows]
Step 2: Cl(g) + CHCl_3(g) ---> HCl(g) + CCl_3(g) (slow)
[Note: k_2 is atop the arrow)
Step 3: Cl(g) + CCl_3(g) ---> CCl_4(g) (fast) [Note: k_3
is atop the arrow)
What is the rate law predicted by this mechanism?
(I'm aware this one isn't supposed to be an integer, but what is IS supposed to be, exactly, is what puzzles me.)