book kinetics Flashcards
rate of reaction definition
rate at which reactants are converted into products
what is easured when measuring rate of a reaction
measure formation of products
consumption of reactants
what is a kinetics profile
graph of conc against time that shows how fast conc changes as the reaction proceeds
what do we plot when looking at rate of reaction
concentration against time
rate of reaction at a particular time
find the gradient (y/x)
rate must be positive
initial rate is the
rate at 0 time
tangent on 0 + gradient at 0
units for rate of reaction if conc is in moldm-3
mol dm-3 s-1
units for rate of reaction of a gas whrn units are Pa
Pa s-1
gradient of line or rate at particular time in terms of differentials
d[a] / dt
what affects the rate of reaction
conc
temp
catalyst
surface area of solid
radiation intensity
solvent nature
what is rate constant k
constant that relates the speed of the reaction to a certain condition, normally temp.
does rate law / rate equations use reactant conc or product conc
reactant conc
rate law / rate equation example
k [rea] [rea]
why do we need integrated rate equations
rates of reaction cannot be measured directly.
exp data usually consists of pro and rea conc measurements at different times.
integrated rate laws give an expression that shows how conc varies with time.
IRL of conc of rea A at time t
[A]t
IRL of conc of rea A at the start of the rea
[A]0
IR for first order reaction:
A -> B
ln [A]t = ln[A]0 - kt
if k and ln[A]0 are known, conc of A at any time can be found
what do u plot on the graph for 1st order rea
ln[A] against time
gradient = -k
IRL straight line edition for 1st order
ln[A]t = -kt + ln[A]0
y = mx + c
IRL for second order rea
1/ln[A]t = 1/ln[A]0 + 2kt
IRL straight order version for 2nd order
1/ln[A]t = 2kt + 1/ln[A]0
what do we plot for a 2nd order rea
1/[A] against time
2k = gradient
half life definition
t 1/2
time taken for conc of reactant to fall to half its initial value
half life equation first order
t1/2 = ln(2) / k
half life equation 2nd order
t1/2 = 1 / 2k [A]0
how must a rate equation be determined
experiments
to see how rate depends on conc of each reactant. in order to find the order.
if the conc change of a reactant has no effect on the rate it isss
0th order
doesnt appear in the overall rate equation
what is the reactions mechanism
series of elementary steps that make up the whole reaction (complex reactions, reactions with more than one step)
isolation method
put one reactant in excess so its initial conc is the same as its conc throughout the reaction.
find the effect of 1 rea conc on rate,
then switch and repeat for the other reactant.
these will give us pseudo rate constants.
initial rates method
gradient is found for t=0 - ror at start of reaction
initial rate
repeat for different starting concentrations
conc is plotted against time
conc ust be monitored throught and plotted in order to get a graph to find t=0
each elementary step has its own
rate constant
the rate determining step is the
slowest step
slowest mechanism
what is a concerted reaction
when rea turns into pro in a single step
if a rate constant has a larger value that means the reaction is
faster
k is …. for a given temp
constant
rate is …. for a given temp
not constant
usually changes during the reaction.
what does the term e -Ea/RT mean
fraction of collisions that have the required energy
what happens to e -Ea/RT as temp increases
the value gets smaller, it gets closer to 0
e0 = 1
1 means all collisions have enough energy.
so k = A
what is A in arrhenius
pre exponential factor
same units as k
rate constant value if all the collisions have enough energy to overcome the energy barrier aka the activation energy.
what is the arrhenius equation
k = Ae -Ea/RT
equation that describes how the rate constant varies with temperature
what is Ea
activation energy
Jmol-1
energy barrier reactants need to overcome in order to form products
how does rate constant increase with temp
a greater fraction of the reactant molecules have enough energy to pass over the energy barrier.
more reaction molecules have or have above the Ea value.
what happens in the collision theory
used to find approx value of A.
for a reaction to occur, 2 molecules must collide with a certain minimum kinetic energy.
the Ke must be equal to or larger than the Ea.
what is kinetics
study of the rates of chemical reactions
