rate equations (physical) Flashcards
define the term rate of reaction
change in concentration (of any product or reactant) per unit time. state what is being monitored (usually production of a product)
at a given instant, how do you calculate rate of reaction
change in conc/change in time
how could you measure the rate of reaction experimentally? (different methods)
use a colorimeter at different intervals if there is a colour change.
if gas is evolved, use a gas syringe to collect volume of gas evolved or measure change of mass in the reaction mixture
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 concentration
monitors reactions that occur between 10-12s
how can you determine rate expression and rate constant for a reaction?
only experimentally
what factors effect the value of the rate reaction for a given reaction?
temperature, nothing else
write a generic rate expression and explain what each term means
rate = k[X]x[Y]y;
k= rate constant for the reaction
[X] and [Y] = concentrations for X and Y
x and y = the orders of the reaction wrt 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 (catalysts) may be included
define the term order of reaction with respect to a given product
the power to which a species’ concentration is raised in the rate equation
define the term overall order of reaction
the sum of the orders of reaction of all species in the rate expression (e.g. total order = x+y)
how would you calculate the units of the rate constant?
units of rate are moldm-3s-1 and units of conc are moldm-3
rearrange the equation to get k=
sub in units then cancel them out
how would you draw a rate concentration graph?
plot [A] against time, draw tangents at different values -> draw a secondary graph of rate against [A]
describe what a rate concentration graph would look like for a zero order reactant
straight horizontal line towards the top
describe what a rate concentration graph would look like for a first order reaction
straight diagonal line from 0
describe what a rate concentration graph would look like for a second order reactant
gradual curve upwards from 0
(you cant tell if it is 2nd order or an order greater than 2)
how could you find the rate expression using the initial rate method?
do a series of experiments where you vary concentrations, so the concentration of 1 reactant changes each time.
plot a graph of concentration against time for each reactant and use a tangent t=0 to find the initial rate of the reaction.
compare rates and concentrations between each experiment to find order of reactants and overall rate equation
what must you add to react with the I2 as it is produced for an iodine clock reaction (equation)
known moles of sodium thiosulfate and a little starch; reacts with I₂ in 1:2 ratio
I₂+ 2S₂O₃²⁻ -> S₄O₆²⁻ + 2I⁻
When does the starch turn a blue-black colour in an iodine clock and why?
When all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch and causes 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. se rate = 1/t . this effectively is the initial rate
what is true of the half life of a first order reactant (conc against time graph)
half life is constant
what is arrhenius equation and what does each term mean
k = Ae -ea/RT
k=rate constant for reaction
A=pre-exponential factor (number of collisions between reactant molecules)
e= mathematical quantity
R= gas constant
T= temperature
Ea = activation energy
how can you convert the arrhenius equation into a useful form for plotting a graph
k=-Ea/RT + lnA
graph of lnk against 1/T is a straight line: gradient = -Ea/R and y-intercept is lnA
what is the rate determining step
the slowest step in a mechanism, which determines the overall rate of reaction
how does the rate determining step link to the species involved in the rate equation?
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
