rate equations Flashcards
definition of rate of reaction
change in concentration per unit time
why does higher temp increase rate of reaction
higher temp - many more particles have energy greater than Ea
more frequent succesful collisions
why does a higher conc/pressure increase rate of reaction
more particles in a given volume
more frequent successful collisions
why does breaking a solid into smaller particles increase the rate
surface area increases
more particles available for collisions
more frequent successful collisions
what is a catalyst and how does it work
a substance that increases the rate of reaction without being used up
provides an alternate reaction route with a lower activation energy
key points of a maxwell distribution curve
-x axis - kinetic energy
-y axis - number of particles with ke
-curve must start at the origin(0 particles with 0 energy)
-peak of curve - most probable energy
-area under curve - total number of particles
-Ea - activation energy
- asymptote to x axis at the end
- only particles that have ke larger than activation energy have a successful collision
maxwell distribution with an increase in temperature
curve shifts to the rhs and lower peak
at higher temp many more particles have energy greater than Ea
more frequent successful collisions
maxwell distribution with an decrease in temperature
curve shifts to the left hand side and peak is higher
at lower temp fewer particles have energy greater than Ea
less frequent successful collisions
maxwell distribution with an increase in conc or pressure
curve higher original curve
more particles in a given volume
more frequent successful collisions
maxwell distribution with a catalyst
same curve
activation energy is lower
more particles have energy greater than Ea
more frequent successful collisions
zero order
changing the concentration of a reactant has no effect on the rate of reaction
[x]^0
horizontal graph
first order
as the conc increases the rate increases by the same amount
directly proportional
[X]
second order
as conc increases rate increases by that amount squared
exponential curve
[X]^2
using a large excess of a reactant
will mean that the order of reaction is 0 with respect to that reactant as the reactants concentration is effectively constant
rate constant k
only temp changes the value
rate of reaction when all concentrations are 1moldm-3
units always end in s-1
continuous monitoring method
CMM
following the course of a single reaction
2 ways to measure the progress of a reaction
1-by taking samples at regular intervals
2-by using a physical indicator such as gas volume
problem with CMM
1-taking samples at regular intervals
problem - the reaction mixture is still reacting
to solve the problem - to stop (quench) the reaction add a large volume of cold distilled water
this will both cool and dilute the reaction decreasing the rate
problem with CMM
2 - by using a physical indicator
problem - this records the amount of product produced
the order of reaction is about reactants
the volume recorded would need converting to reactant concentration
using the results from CMM
plot a concentration time graph
rate of reaction = gradient of the tangent
to work out order - take 2 different tangents at different concentrations and compare the scale factor of concentration and rate of reaction
if the graph is straight - zero order
initial rates method
series of experiments where concentration changes in each one with all other factors are constant
time it takes to get to a specific point in reaction is recorded eg colour change
IRM
What do u measure for
time recorded when observation occurs
rate = 1/time
concentration does not need to be known as total volume is constant so the volume of reactant is proportional to its concentration.
to carry out an iodine clock initial rates practical
-measure out known volumes of reactants
keep them separate so the reaction doesnt start
-start the stopwatch when the last of the reactant is added to a conical flask.
-stop the timer when the observation occurs and record
-repeat the experiment using the same total volume altering the concentration
rate = 1/time
plot a graph of rate by concentration
Arrhenius equation
k=Ae^-Ea/RT
k=rate constant
A=Arrhenius constant (same units as k)
e=math constant
Ea=activation energy
r=molar gas constant - 8.31
t=temp in kelvin
gradient of a line = -Ea/R
rate determining step
slowest step
RDS contains the same species as the rate equation in the same amounts
