CI - kinetics *02 Flashcards
what is rate of reaction
the reaction rate is the change in the amount of reactants per unit time (normally per second)
what is the rate constant - and what does it tell us
k
units vary so have to work them out
the bigger it is the faster the rate of reaction
what are orders of reaction?
tells you how the reactant concentration affects the rate
rate is proportional to [A]
- need order of reaction with respect to reactant
- reaction also has an overall order which is the sum of the orders of the different reactants
how do you find the order w.r.t a reactant?
look at how each reactant affects the rate one by one
if [A] changes and rate stays the same: 0 order w.r.t [A]
if [A] doubles and rate double (proportional): 1st order w.r.t [A]
if [A] double and the rate is 2^2 faster (triples is 3^2 faster): 2nd order w.r.t [A]
order of reaction from concentration-time graphs
zero = straight line down
1st = slope down
2nd = steeper slope down
order of reaction from rate-concentration graphs
zero = horizontal line
1st = straight line through the origin (proportional)
2nd = curve upwards
rate equation
rate = k [A]^m [B]^n
m+n = order w.r.t reactant
does temperature affect the rate constant?
yes
how does temperature affect the rate constant?
if you increase temperature you will increase the rate of reaction
- more kinetic energy
- particle speed up and are more likely to collide
- have the right activation energy to react so greater proportion will have enough energy to react
at a higher temperature the reaction will have a higher rate constant
what is half life
the time taken for a reactant to halve in quanitiy
how to you calculate half-life - from graphs
pick values and see how long it takes to half
eg.
0.1
0.05
0.25
and so on
if time taken to half similar then half life is constant
orders of reaction and half-life graphs
zero - if rate doesn’t change as concentration falls (graph is a straight line)
1st - the rate decreases but t1/2 is constant (curved graph)
2nd - the half-life increases as reaction goes on (steeper curve)
how can you find rate constant from the half-life of a 1st order reaction?
k = ln2 / t1/2
units = no units / s = s-1
find half-life from the graph
how can you work out reaction rate from concentration-time graph?
draw a tangent
like the main curve bit
work out gradient = change in y / change in x
how can you estimate the initial rate from a concentration-time graph
draw a tangent at the start!!
find gradient
how do you work of the initial rate of a reaction?
time how long it takes to make a set amount of product to form at the beginning of the reaction
- conc. of other reactants isn’t changing
- temperature constant
- reaction hasn’t proceeded to far
initial rate = amount of reactant used or product form / time
how can to follow the rate of reaction - experimental methods
pH measurement
gas volume
loss of mass
colour change
titration
example of an initial rate experiment
clock reactions
what are clock reactions
measure how much time takes for a set amount of product to form changes as you vary concentrations of one of the reactants
has an easily observable end point - a colour change - that tells you when desired product is formed
the quicker the clock reaction finishes the faster the initial rate
clock reactions - iodine clock reaction
H2O2 + 2I- + 2H+ —–> 2H2O2 + I2
- a small amount of sodium thiosulfate and starch are added to an excess of hydrogen peroxide and iodide ions in an acid solution
- the sodium thiosulfate that is added to the reaction mixture reacts instantaneously with any iodine that forms: 2S2O3 2- + I2 —–> 2I- + S4O6 2-
- to begin with all the iodine that forms in the first reaction is used up straight away in the second reaction. But once all the sodium thiosulfate is used up, any more iodine that forms will stay in solution so the starch indicator will suddenly turn the solution blue-black - this is the end point of the reaction
- varying concentrations of the iodide ions or hydrogen peroxide will give different times for the colour change
what does the arrhenius equation link?
an equation that links the rate constant and activation enthalpy
that is the arrhenius equation
K = A e^-Ea/RT
K = rate constant Ea = activation enthalpy (Jmol-1) T = temp. (k) R = gas constant A = the pre-exponential factor (another constant)
e = ex button on calculator
arrhenius equation - as the activation energy gets bigger…..
…K gets smaller
arrhenius equation - a large Ea means….
…a slow rate
not many particles have enough energy to react
so only a few collisions will result in a reaction
arrhenius equation - as temperature rises….
…K increases
arrhenius equation - logarithmic form
ln K = -Ea/R x 1/T + ln A
ln button on calculator
arrhenius equation - gradient
-Ea/R
arrhenius equation - y intercept
ln A
arrhenius equation - arrhenius plot
plot ln K against 1/T
arrhenius plot - finding activation enthalpy
find gradient of graph
gradient = -Ea/R
then -Ea = R x gradient
x -1 for Ea
arrhenius plot - finding A (the exponential factor)
the y-intercept
then A = e^ln A
what is the rate determining step?
the slowest step in a multistep reaction
the overall rate is determined by this
what is a multistep reaction
reaction mechanisms can have a series of steps
how can you use the rate equation to determine the rate determining step?
if a reactant appears in the equation, it must affect the rate
- so this reactant or something made in it must be in the step determining step
if a reactant doesn’t appear in the rate equation, then it isn’t involved in the rate determining step
does that rate determining step need to be the first step in a mechanism
no
can the reaction mechanisms be predicted from JUST the chemical equation
not usually
what does the order w.r.t a reactant show about the rate determining step
shows the no. of molecules of that reactant that are involved in the rate-determining step
