5.1.1 How fast? Flashcards
rate of reaction
change in the concentration of reactants or products per unit time (moldm-3s-1)
half-life
the time taken for half of a reactant to be used up
rate-determining step
the slowest step in a multi-step mechanism
rate equation
rate=k[A]^m[B]^n
zero order means
concentration of reactant has no effect on rate
first order means
concentration of reactant is proportional to rate
second order means
concentration of reactant squared is proportional to rate
what kind of graph do you use to determine half life?
concentration time
for a first order reaction, what are the two methods of calculating rate constant (k)?
- from the gradient of a tangent
- from the half life, using k=ln2/t 1/2
what does a conc/time graph of zero order look like?
what’s the half life?
straight line with negative gradient
half life is decreasing
what does a conc/time graph of first order look like?
what’s the half life?
exponential decay line
constant half life
what does a conc/time graph of second order look like?
what’s the half life?
even steeper than first order exponential decay line
increasing half life
what does a rate/conc graph of zero order look like?
how do you determine k from the graph?
a horizontal line
k=rate
what does a rate/conc graph of first order look like?
how do you determine k from the graph?
a straight line through the origin
k=gradient
what does a rate/conc graph of second order look like?
how do you determine k from the graph?
upward curve with increasing gradient
k can’t be determined straight from graph, but if you draw rate^2/conc, k=gradient
3 methods of continuous monitoring
- gas collection
- mass loss
- colorimetry
what does a colorimeter measure
the amount of light absorbed by a coloured solution.
Absorbance is recorded, and is directly proportional to concentration of solution. from this you can draw conc/time graphs
how do you do an iodine clock procedure? (initial rates method)
- measure the time t from the start of experiment for the blue-black colour of iodine and starch to form
- it can be assumed that the average rate of reaction over this time is the same as the initial ROR
- initial rate ∝ 1/t
- plot a graph of 1/t / concentration (rate/conc) to determine order, then write rate eq. and calculate k.
- repeat for many different concentrations of reactant
how does increasing the temperature affect the rate and therefore rate constant?
increasing temp increases rate constant exponentially
- Boltzmann distribution shifts to the right so a greater proportion of particles exceed Ea
- More KE so particles move faster and collide more frequently
- -change in rate is mostly determined by Ea
- -The Arrhenius equation is used to calculate how k changes with T
What is the Arrhenius equation used to calculate?
how k changes with T
Arrhenius equation and logarithmic form
k = Ae^-Ea/RT lnk = -Ea/RT + lnA
What do you plot to determine Ea and A graphically (arrhenius)
Plot lnk/ 1/T
gradient = -Ea/R
intercept = lnA
