Section E: kinetics Flashcards
what is the rate of reaction for a reaction aA —> bB ?
rate = - 1/a dA/dt = 1/b dB/dt
what is the relationship between the stoichiometry of the reaction and the order?
there is no connection between the two
what is k and what does is depend on?
k is the rate constant and it depends on temperature
zero order reaction
rate = - d[A]/dt = k
integrating, ∫d[A] = - k ∫dt
[A] - [A]0 = - kt
therefore [A] = [A]0 - kt
first order reaction
rate = - d[A]/dt = k[A] integrating, ∫1/[A] d[A] = - k ∫dt ln[A] - ln[A]0 = - kt therefore ln[A] = ln[A]0 - kt and [A] = [A]0 e^-kt
second order reaction
rate = - 1/2 d[A]/dt = k[A]^2
integrating, ∫1/[A]^2 d[A] = - 2k ∫dt
- 1/[A] + 1/[A]0 = -2kt
therefore 1/[A] = 1/[A]0 + 2kt
how would you use graphs to determine the order of reaction?
[A] versus t linear? zero order ln[A] versus t linear? first order 1/[A] versus t linear? second order if not, more complex system
what is t1/2?
t1/2 is the half life = the time required for the concentration to drop to half its original value
how can t1/2 be calculated?
t1/2 = ln2 / k
what is the equation of radioactive decay?
N = N0 e^-kt
therefore N = N0 e^-tln2/t1/2
how can you measure the rate of reaction?
- start the reaction t=0
- measure a property as a function of time
- convert the measurement into concentration
- analyse the data
which methods can be used to measure the rate of reaction?
- spectrophotometric methods
- NMR
- polarimetry
- conductivity
- electrochemical/pH detection
- changes in pressure
how can spectrophotometric methods be used to measure the rate of reaction?
- UV/vis or IR absorbance ∝ [concentration]
- fluorescence
- stopped flow method (fast reaction)
- flash photolysis (very fast reaction)
how can NMR be used to measure the rate of reaction?
NMR integration ∝ [concentration]
how can polarimetry be used to measure the rate of reaction?
- measure changes in optical rotation for chiral molecules
- circular dichroism
how can conductivity be used to measure the rate of reaction?
changes in the number of ions (concentration) result in changes in conductivity
explain the initial rates/differential method
A + B —> products
rate = [A]^a [B]^b
- isolate variation in [A]
- let [B]»_space; [A] so that [B] is effectively constant (pseudo-order conditions)
- run the reaction at several different [A]0
- determine the initial rate for each curve by drawing tangents
- the ratio of any two rates is equal to the ratio of initial concentrations to the power a:
rate1/rate2 = ( [A]01/[A]02 )^a
- in general, rate = k(obs) [A]^a
- taking logs, log(rate) = log(k(obs)) + a log( [A] )
- this will produce a straight line graph with a slope equal to the order of the reaction
what is k(obs)?
- the observed rate constant
- the real rate constant must be determined properly once the order is known
