rate equations Flashcards
what does the rate of reaction show?
links rate with concentration of reaction
what is the rate equation?
rate=k[A]^a[B]^b
k=constant
A,B= concentration of subtances
^a,^b= order of reaction
what are the rates of order and its types?
tells us how the concentratino of a substance affects rate
zero order=change in concentration has no effect on rate
1st order=change in concentratino is proportional to change on rate
if [A] doubles rate doubles
2nd order=change in concentration has a squared proportional change to rate
if [A] doubles rate quadruples
how does k in the rate equation change under temperature?
k is only constant for a temperature
if temperature changes k changes
this makes sense as increase in temp causes increase in kinetic energy and hence a faster reaction
k is the only variable that can change as [A][B] are constant so therefore k increase
how to calculate units of k
rate =k[A]^2
k=rate/[A]^2
k=moldm-3s-1/[moldm-3][moldm-3]
cancel it allout
so mol-1dm3s-1
how to calculate rate within initial rate
do reaction but change concentration one at a time for each reactant
measure initial rate using tangent and gradient
work out rates of order by keeping one reactant constant and comparing one to the initial rate
work out the other the same way
work out constant k by subbing in conc and rates of order and initial rate
how do you calculate the rate of reaction if their are not the same conc for each reactant
work out the rate of order for one of the reactants
use that one one rate of order to work out the reaction rate (without using the second reactant)
use this reaction rate to compare with the other reaction rate to work out the rate of order for the second react
describe the iodine clock experiment?
react sodium thiosulfate iodine and starch (indicator )
the na2so4 orignially only reacts with the iodine however once that is finished it reacts with the starch giving a deeb blue black colour
you can vary the reactants conc to work out the rates of order
what are other ways of measuing rate of reaction?
measuing pH change by H+ used/produced .Then calculate the H+ conc
measure the mass lost of gas then calculating the moles of gas lost
measuring gas produced over time
what is a calibration curve
large range of known concentrations of reactant and plottng it agains absorbance which can be worked out using a colirimeter
what are rate- concentration graph
graphs the plot rate agains conc can be used to work out rate of order as they plot the change in gradient of the conc-time graph
e.g horionzontal line = 0 rate of order
diagonal line = 1 rate of order
curved line = 2 rate of order
what is the rate determining step
most reactions happen in multiple steps one of these is the rate determining step
substances that dont appear in the rate equation cannot be part of the rate determining step
catalysts can be part of the rate equation
how to calculate the rate determining step/mechanism?
check the rate equation for the all rate determining molecules
count of every molecules
the step all have been counted for is the rate determining step
for the mechanism see the rate equation reactants can only occur on rate determining mechanism
what is the arrhenius equation
links activation energy and temperature to the rate constant
k=Ae^(-Ea/RT) k - rate constant A= arrhenius constant Ea = activation energy R= gas constant (8.31) T = temperature kelvin
what are arrhenius plots and what can they be used for
can be used to find out the activation energy
rearraging arrheinus equation to ln(k)= ln(A) - Ea/RT
plotting graph against x axis 1/t y axis ln(k)
so gradient = -Ea/R
R is 8.31 so u can rearrange to work out activation energy
you can also work out Arrhenius constant by using lnk=-Ea/RT + lnA as y=mx+C
subbing in values and solving for A
