Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

2.2 Kinetics of irreversible first-order reactions 17
I
Figure 2.5: Kinetics of first-order reactions.
I Time constant: The rate coefficient has the dimension of an inverse time. Thus,
= 1
(2.39)
hasdimensionoftimeandisthetimeafterwhich[A] has dropped to 1
its initial value:
[A] =
=[A] 0
− = [A] 0
≈ [A] 0
2718
(=368 %) of
(2.40)
I Half lifetime: The half lifetime 12 is the time after which [A] has dropped to 1 2 of
its initial value (Fig. 2.5):
y
£
A
¡
= 12
¢¤
=[A]0 − 12
= [A] 0
2
− 12
= 1 2
(2.41)
(2.42)
y
12 =ln2 (2.43)
12 = ln 2
(2.44)