kinetics 2 p2 Flashcards
rate of reaction
change in concentration (of any reactant or product) per unit time
state what is being monitored (usually production of a product)
rate of reaction equation
rate of reaction = change in concentration / change in time
how could you measure the rate of reaction experimentally?
colorimeter at suitable intervals if there’s a colour change
use a gas syringe to collect volume of gas evolved, or measure change in mass
titration to time how long it takes for the end-point to occur
how would you measure reaction rate for really fast reactions?
use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species; this can show its concentration
can monitor reactions that occur in times as fast as 10-12 seconds
how can you determine the rate constant and rate expression for a reaction?
only experimentally
what affects the value of the rate constant for a given reaction?
temperature, nothing else
generic rate expression?
rate = k[X]x[Y]y
k = rate constant
[X] and [Y] are concs of species X and Y
x and y are the orders of reaction with respect to X and Y
do species need to be in the chemical equation to be in the rate expression?
no - species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts, may be included
overall order of reaction?
the sum of the orders of reaction of all species in the rate expression
how do you calculate the units of the rate constant?
units of rate are mol dm-3s-1 and units of concentration are mol dm-3
rearrange rate equation to get k =
sub in units and cancel them out
how would you draw a rate concentration graph?
plot [A] (y) against time (x), draw tangents at different values —> draw a secondary graph of rate (y) against [A] (x)
what does a rate concentration graph look like for a zero order reactant?
straight horizontal line
what does a rate concentration graph look like for a first order reactant?
x=y
directly proportionate
what does a rate concentration graph look like for a second order reactant?
kind of like half a parabola starting from origin
what does a concentration time graph look like for a zero order reactant?
negative gradient
directly proportionate
what does a concentration time graph look like for a first order reactant?
1/x vibe (only + quadrant)
above x axis
what does a concentration time graph look like for a second order reactant?
steeper version of the first order
tell the difference since the conc time graph for first order reactant has a constant half life
how can you find the rate expression using the initial rate method?
series of experiments, during which you vary concentrations, so the conc of just one reactant changes each time
(usually a graph question - work out order based on results)
what must you add to react with the I2 as it is produced for an iodine clock reaction?
known moles of sodium thiosulfate and a little starch
reacts with I2 in 1:2 ratio
I2 + 2S2O3 2- —> S4O6 2- + 2I-
when does the starch turn a blue-black colour in an iodine clock reaction and why?
when all the Na2S2O3 has been used up and so I2 is produced which reacts with starch leading to a blue black colour
how can you calculate the rate of reaction from the data from an iodine clock reaction?
record time taken for colour change to occur
use rate = 1/t
effectively the initial rate
what is the effect of a 10K temperature increase on the rate of reaction, roughly?
doubles rate of reaction
what is the Arrhenius equation? what does each term mean?
k = Ae ^ -Ea/RT
k - rate constant
A - number of collisions between reactant molecules
R - gas constant
Ea - activation energy in J
how can you convert the Arrhenius equation into a useful form for plotting a graph?
ln k = -Ea/RT + ln A
graph of lnk (y) against 1/T (x) is a straight line
gradient = -Ea/R and y intercept is lnA
what is the rate determining step?
the slowest step in a reaction mechanism, which determines the overall rate of reaction
how does the rate determining step link to the species involved in the rate expression?
any species involved in the rate determining step appear in the rate expression
species only involved after the rate determining step do not appear in the rate expression
what is an intermediate?
substance formed during a middle step of a chemical reaction
usually extremely reactive and short-lived
