Weekly Quiz 3

Chemistry 232
Weekly Quiz (?) #3
G. Marangoni February 9, 2012.
Answer the following questions in the time allotted. There are no tricks on this quiz, so
don’t look for any. Marks are indicated in the [ ]
Name: ID# :
1. A first order reaction has a rate constant of 0.0136/s. Calculate the half-life of the
reaction. [3 marks]
0.693
t 1
2 k
0.693
t 0.0136 s
1 1
2
51.0 s
2. For the reaction
CH3Cl (g) + H2O (g) CH3OH (g) + HCl (g)
the rate of reaction was found to double when [CH3Cl] was doubled, and the reaction
was determined to be first order in H2O (g). [7 marks]
a. Obtain the rate law and the overall reaction order.
b. If the rate of disappearance of CH3Cl (g) was 0.472 M/s, calculate the rate
constant for the reaction when [CH3Cl] = [H2O] = 0.0125 M.
c. Assume the amount of H2O is extremely large relative to the amount of the
CH3Cl. How will that affect the rate law? Explain briefly.
a) Rate = k [CH3Cl] [H2O]
Note since rate doubles as [CH3Cl] doubles
Rate CH3Cl 2 2
ln ln20.693ln ln2
Rate1
CH3Cl 1
1 = 1 (Given)
Hence vr = k [CH3Cl] 1 [H2O] 1
Overall Order
1+1 = 2
b) Note since vr = k [CH3Cl] 1 [H2O] 1
d CH Cl
dt
3 1 1
1
vr k CH3ClH2O0.472Ms

1 1
vr 0.472 M s
k 3.0210 M s
1 1 1 1
CH Cl H O
0.0125 0.0125 3 2
NOTE THE UNITS!!!!
3 11 c) In the presence of a large amount of H2O (g), we would expect the reaction to be a
pseudo first order reaction. The rate law would reduce to the following.
2
1
v k ' H O
r
where k '
k CH Cl
3