Unit 2.5 Flashcards
reaction rate
change in concentration of a reactant or product per unit time
rate as time goes on
should decrease and be as close to 0 as possible (reaction is “finished”)
instantaneous rate vs. average rate
average rate: just the slope of the change in reactant over time
instantaneous rate: shows rate at a certain point in time
rate law
shows how the rate depends on the concentrations of reactants
why don’t products appear in the rate law?
the concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall rate
integrated rate law
shows how concentration depend on time
how to measure the rate of reaction of a gas
- water displacement measures the volume of H2/ the rate that the volume changes is equal to the rate that the moles change which can give you the concentration/time
- with concentration measuring probes
- can measure the rate that the mass of a solution goes down by a scale
how to measure rate of reaction when solid is the limiting reagent
measures the mass/time which can be moles/ time
how to measure rate of reaction when acid/base
measure pH to translate to concentration of H+. as the concentration of H+ shows how much more acidic/basic a solution is getting
how to measure rate of reaction when coloured compound
measure the rate that the colour disappears/appears (qualitative) or measure using a colorimeter
how to measure rate of reaction when opaque
this is also relative using the disappearing cross experiment
rate of appearance/disappearance
when calculating the rate of change of the reactants disappearing, a negative sign is added to make the rate positive
how does rate of reaction make sense
10 mol/s means 10 moles of reactions occur each second, not that 10 moles of chemical actually react. 6.02 x 10^23 times molecules number of reactions
k
contains al the factors such as temperature, steric factor, reactivity etc. and the units can vary
order
shows how much of an effect changing the concentration of one variable has on the rate
ex. order 2 is doubled
= quadrupled reaction
overall order
sum of the exponents. a higher total order means more dependent/sensitive reaction to concentration
when given the concentration of a reactant and the eqn
don’t multiply the concentration by the coefficient!
initial rate
the initial rate at which the reactants combine
experimentally: measure the instantaneous rate at t=0
nuclear decay
first order reaction
collision theory
molecules must collide to react however majority of collisions do not lead to a reaction
collision theory concentration
rate increases as the concentration of reactants increase
transition state/activated complex
bonds go through a transition state. the species at the transition state is called the activated complex. this is very unstable
activation energy
reactants must collide with enough energy to form the activated complex
temperature
at high temperature means that the molecules have a greater average kinetic energy which means the collision will have more energy to overcome the activation energy
steric factor
A, a number that represents the likelihood that a collision occurs with the right geometry.
decreasing the size, increases the rate of reaction
increases the surface area available for collisions to take place
reaction mechanism
the process by which a reaction occurs
each collision in a mechanism is called
elementary step
elementary steps
- can be uni or bimolecular (1 or 2 molecules reacting)
rate determining step
the slowest elementary step determines the rate of the overall reaction
- the order is the coefficient since each reactant is the coefficient
a valid mechanism
- sum of elementary steps must give the overall reaction
- the rate law based on the RDS must be consistent with the experimental rate law
- no elementary step includes more than 2 reactants
intermediate
generated in a reaction and reactant in a subsequent one (c)
A+B –> C
C + D –> E +
catalyst
reactant of a reaction and regenerated in a subsequent one
A+B –> C
C + D –> E + A
phase
one phase: no distinguishable substance
two phases: looks like two things mixed together
Heterogeneous catalysis
This involves the use of a catalyst in a different phase from the reactants. Typical examples involve a solid catalyst with the reactants as either liquids or gases.
One or more of the reactants are adsorbed on to the surface of the catalyst at active sites.
An active site is a part of the surface which is particularly good at adsorbing things and helping them to react.
enzyme weakens the bonds or interacts with the surface
then the molecule is desorbed
adsorption
Adsorption is where something sticks to a surface
desorbtion
Desorption simply means that the product molecules break away. This leaves the active site available for a new set of molecules to attach to and react.
good Heterogeneous catalysis
adsorbs pretty well but not strongly enough to not be able to desorb
catalytic converters
Catalytic converters change poisonous molecules like carbon monoxide and various nitrogen oxides in car exhausts into more harmless molecules like carbon dioxide and nitrogen using expensive metals. can be affected by catalyst poisoning
catalyst poisoning
this happens when something which isn’t a part of the reaction gets very strongly adsorbed onto the surface of the catalyst, preventing the normal reactants from reaching it.
Homogeneous catalysis
catalyst is in the same phase as the reactants. Typically everything will be present as a gas or contained in a single liquid phase.
homogeneous catalysis examples
The destruction of atmospheric ozone by CFCs is an example of homogeneous catalysis.
each collision is called
elementary step. sum of this equals the overall reaction
elementary steps and rate law
you can use coefficients of the rate determining step since the order for that is the coefficients.
the rate of reaction is equal to rate of RDS
valid mechanism
1) sum of steps must equal overall rxn
2) rate law based on RDS must equal experimental law
3) no elementary steps can include more than 2 reactants (can’t be more than bimolecular)
intermediate
generated in reaction and is a reactant in another one
catalyst
used in beginning and comes out at the end: not in overall
intermediate and reaction mechanisms
if there is an intermediate in the RDS, you must substitute it in the rate law with what it depends on to be vlalid
for reaction to occur
molecules must collide with the right reactants, orientation and energy greater than the activation energy
rate is proportional to the # of collisions times the % of successful collisions
high temperature
high # of collisions as more kinetic energy therefore more successful collisions (ratio is same) strongest effect on rate
catalyst
high % of successful collisions, lowers activation energy which means high number of collisions with Energy greater than activation energy
high surface area
more # of collisions since more reactant is exposed
chemical nature of elements
a higher K means a higher rate than others. this K means that some chemicals are more reactive than others and will react faster
high reactant concentration
of collisions is high since more moles = more collisions between reaction states
activated complex
species in transition state that is unstable (has all bonds formed)
smaller size of particles
higher rate since its higher surface area for collisions to occur
maxwell botlzman
higher temperature means that there is a lower peak on the distribution
a greater spread/ variance means overall greater average energy
Ea is a point on the x-axis and the area under the distribution past the Ea is the proportion of molecules that will react
unimolecular vs bimolecular
uni is 1H2
bimo is 2H2
uni has no intermediate
bio has 1 intermediate and 2 transition states
a good catalyst
adsorbs well but not too well that it can’t desorb
draw out the reaction progress with a positive delta H
this should have a high p and low r
raw out the reaction progress with a negative delta H
this should have a low R and a high P
high order
means more sensitive to concentration
how to tell order based on uniter
look at K
