Rate of reaction is changeinconcentrationofareactantorproductperunit time
At a given instant you can calculate the rate of reaction by
rate = changeinconcentrationofreactantsorproducts / time
if the order is 0 with respect to a reactant changing the concentration of the reactant has no effect on the rate.
in a first order reaction, rate is directly proportional to the concentration
in a second order reaction, change in rate is equal to the change in concentration squared.
Generic rate equation:
rate = k[A]x[B]y
in a generic rate equation, k is the rate constant for the reaction
in a generic rate equation, [A] and [B] are the concentrations of the reactantsA and B
in a generic rate equation x and y are the orders of reactionwithrespecttoA and B
Zero order reactants dont appear on rate equations because they have no effect on the rate
the overall order of a reaction is calculated by the sum of individual orders.
to calculate units for the rate constant, use the units for rate ( moldm-3s-1) and the units for concentration ( moldm-3)
rearrange the equation to get k = … then cancel out any repeated units.
If the overall order is 3, the units for rate constant are
dm6mol-2s-1
If the overall order is 2, the units for rate constant are
dm3mol-1s-1
if the overall order is 1, the units for rate constant are
s-1
if the overall order is 0, the units for rate constant are
moldm-3s-1
To measure the rate of a reaction experimentally:
use a colorimeter at suitable intervals if there is a colour change
if gas is evolved, use a gas syringe to collect volume of gas evolved or measure the change in mass of the reactionmixture
Half life is the timetaken for concentration of a reactant to decrease by half.
The symbol for half life is t1/2
concentration-time graph for a zero order reactant
concentration-time graph for a first order reactant
first order reactants have constant half lives
The equation used to determine rate constant using half life is:
k = ln(2) / t 1/2
To draw a rate concentration graph plot [A] against time. Draw tangents at different values to calculate the rate and then draw a secondaryGraph of rate against [A].
Rate is directly proportional to 1/t
Rate concentration graph for a zero order reactant
Rate concentration graph for a first order reactant
Rate concentration graph of a second order reactant
To determine the rate constant from a rate concentration graph of first order:
k = rate / concentration
The rate determining step is thesloweststepinamultistepmechanism
Any species involved in the ratedetermining step appear in the rate equation. Species only involved after the ratedetermining step do not appear in the rate equation.
For a reactant in the rate equation, the order of the reactant indicates how many molecules of that reactant are involved in the ratedetermining step.
the rate equation of a reaction is rate = k[NO]2
therefore there will be 2 molecules of NO present in the rate determining step.
Temperature affects the value of the rate constant for a given reaction
The rate of reaction is roughly doubled for a 10 degree temperatureincrease
The Arrhenius equation is
k = Ae (-Ea/RT)
k: rateconstant
A: pre-exponentialfactor
R: gasconstant
T: temperature (in kelvin)
Ea: activationenergy for reaction (in joules)
The Arrhenius equation can be converted into a useful form for plotting a graph which is:
lnK = -Ea/RT + lnA
graph of lnK against 1/T is a straightline where the gradient is -Ea/R and the y intercept is lnA