** half-life** (t1/2) - time needed for concentration of reactant to drop to 1/2 or original value

- fast reaction >> short half-life
- t1/2 = -ln (1/2) / k = 0.693 / k for 1st-order reactions (no dependence on initial concentration)
- t1/2 = 1 / k[A]0 for 2nd-order reactions (dependence on initial concentration)

Find the half-life of a substance that decomposes by 20% after 5 years.

- 0.8 = (1)(1/2)5/x
- ln(0.8) = 5/x ln(1/2)
- ln(0.8) / ln(1/2) = 5/x
- x = 5 ln(1/2) / ln(0.8)
- 15.5 years

Find the age of a piece of wood whose carbon-14 count is 35/min, when a new piece of wood has a count of 125/min.

- Given:
- half life of carbon-14 = 5715 years
- ln[A]t = -kt + ln[A]0
- [A]t = 35
- [A]0 = 125
- t1/2 = -ln (1/2) / k

- 5715 = -ln(1/2) / k
- k = -ln(1/2) / 5715 = 0.00012
- ln(35) = -(0.00012)t + ln(125)
- ln(35) - ln(125) = -(0.00012)t
- t = (ln35 - ln125) / -0.00012
- 10608 years

Find the half life of a substance if 95% of it disappears after 10 years.

- 0.05 = (1/2)10/x
- ln (0.05) = 10/x (ln(1/2))
- ln (0.05) / ln (1/2) = 10 / x
- x = 10 ln(1/2) / ln(0.05)
- 2.3 years

** collision model** - based on kinetic-molecular theory

- shows effects of both temperature/concentration on molecular level
- assumes that molecules must collide to react w/ each other
- not all collisions lead to reactions
**orientation factor**- molecules need to be in a certain position to react when colliding