topic six Flashcards
what requirements must be fulfilled for a chemical reaction to take palce
correct orientation
sufficent energy (reaching activation energy)
what is activation energy
the minimum amount of energy that colliding particles must have for a chemical reaction to occur.
what is the transition state
the highest energy state on a reaction coordinate; it indicates a point at which new bonds are being formed at the same time as old bonds are being broken.
in general, faster reactions tend to haev lower or higher activation energies
lower, whereas slower reactions tend to have higher activation energies
if activation enerfy is met will the reaction take place
if the correct orientation yes
what is the maxwell boltzmann distribution
a graph where the x axis is kinetic energy and the y axis fraction of particles
the area beneath the curve in the MB curve is directly proportional to
directly proportional to the number of molecules having a value of kinetic energy in that range.
as temp increases in the mb curve,
the particles gain ke and the curve flattens out
how is the maxwell boltzmann distribution helpful
in understanding how changes in temperature and the use of a catalyst will affect the rate of a reaction.
the rate of a chemical reaction is a measure of what of the reaction
the speed of the reaction
what can the reate of reaction be defined as
the hange in conc of a reactant or product per unit of time
rate of reaction =
change in reactant / change in time
increase in product conc / change in time
rate of reaction is not constnat over the course of reaction so you must consider it at
a particular time
how can instantaneous rate of reaction can be determined
graphically
what is avg rate of reaction
calculated as the avergae over a particualr time period or interval
what factors affect rate of reaction
temp (affects freq of collisions and energy they have)
concentration (affects freq of collisions)
pressure (affects freq of collisions)
surface area (increase as more molecules available)
when does particle size affect rate of reaction
when the solid is being reacted directly, not when it is dissolved in solution.
how do catalysts icnrease rate of reaction
providing an alternative reaction pathway so a greater proportion of reactant particles now have energy equal to, or greater than, the original activation energy for the reaction. This results in an increase in the frequency of successful collisions between reactant particles and an increase in the rate of reaction.
catalysts at the end of a reaction
remains chemically unchanged and sometimes can be reused
The factor by which the concentration of a reactant affects the rate of a reaction is known as
its order of reaction
what is the only way the order of a reaction with respect to a particular reactance can be determined
through experimental data
order of reactions can be:
zero, first or second order
what does the rate expression give
the relationship between reactant concentrations and the rate of reaction.
what is k
the rate constant
what are the superscribts in hte rate expression
the orders of reaction with respect to the letter
what is the overall order of reaction
the sum of indiviual orders of reaction
zeroth order
Rate is independent of [A]; any change in the concentration of A does not affect the rate of reaction
first order
Rate is directly proportional to [A]; if the concentration of A is doubled, the rate also doubles
The rate constant k is a constant for
for a reaction at a particular temperature (it is temperature-dependent).
second order
Rate changes as the square of [A]; if the concentration of A is tripled (3x), the rate increases by a factor of 9 (32 = 9)
the rate expression gives the relationship between the conc of the reactants and the
overall rate of reaction
k is only constant at
a particular temp
it is temp dependent
zero order units
mol dm-3 s-1
first order units
s-1
second order units
mol-1 dm3 s-1
third order units
mol-2 dm6 s-1
what type of graph can be used to find the order of a reaction with respect to a reactant
a concentration (y) time (x) graph
or a rate (y) conc (x) graph
for zero order, the ct graph is
straight line
for first order ct graph, the line is
exponential curve
for second order ct graph the line is a steeper
exponential curve
for zero order rc graph
horizontal straight line
for first order rc graph
straight line
for second order rc graph
parabolic curve
what is half life
the time it takes for the concentration of a reactant to decrease by half
a reaction that takes place in a single step is known as a
elementary reaction
Many reactions, however, do not take place in a single step and are called
non elementary reactions
what are the steps in a non elementary reaction called
elementary steps
the slowest elementary step in a reaction mechanism is the
rds
the elementary steps must add together to give
the overall balanced equation for the reaction
the rate expression deduced from the rate-determining step must be consistent with
the experimentally determined rate expression
The NO3, which appears in both steps but not in the overall equation, is known as
a reaction intermediate.
what does moelcularity tell us
the number of reactant particles in an elementary step
If only one particle is involved in the elementary step, it is
unimolecular
If two reactant particles are involved,
it is bimolecular
if three reactant particles are involved,
termolecular
if two reactant particles are involved
its bimolecular
if three reactant particels are involved
termolecular
what is an intermediate
a species that is formed from the reactants in a chemical reaction which then goes on to react further to form the products.
what is the transition state
the highest energy state on a reaction coordinate; it indicates a point at which new bonds are being formed at the same time as old bonds are being broken.
the rds has the highest
activation energy as its the slowest
why do catalysts appear in the elementary steps of a reaction mechanism and in the rate-determining step but not in the overall balanced equation for the reaction
they are chemically unchanged and can be reused
what value constant does a catalyst change
increasing the value of the rate constant k
what maeks somehting bimolecuelar
elementary steps have two reactant particels
rate constant k is temp dependent, this relatiponsh[ is shown by the arrhenius equation:
k=Ae ^−Ea/RT
what is a in the arrhenius equation
the pre-exponential factor or the frequency factor.
what is Ea in the arrhenius equation
activation energy (J/mol^-1)
what is r in the arrhenius equation
universal gas constant (8.31 J K^1mol^-1)
what is e in the arrhenius equation
eulers number
what does the arrhenius constant take into account with a
the frequency of collisions and the probability that they have the correct orientation (or geometry)
The expression (e−Ea /RT) (the exponential factor) is the fraction of molecules that have
sufficient kinetic energy to react at a certain temperature
modified version of arrhenus equation (to find ativation energy)
lnk=−Ea/RT +lnA
lnk =−Ea/RT +lnA is a form of y = mx+c
therefore, a graph of ln k against 1/T gives a straight line (Figure 1). The gradient of the line is equal to –Ea/R (where R is the universal gas constant with a value of 8.31 J K−1 mol−1). The intercept on the y-axis is equal to ln A. Once we have determined the activation energy for the reaction, we can calculate the Arrhenius constant by substituting the activation energy value into the Arrhenius equation.
Gradient=
−Ea/R
−Ea=
gradient×R
the rate constant, k, for a reaction increases ???? with increasing temperature
exponentially
