Unit 6 Reactions Mechanism Flashcards
Reaction mechanism definition?
Step by step sequence of an elementary reaction
Each step in reaction mechanism involves
Small amount of bond breaking and making
Elementary reaction
one or more chemical species react directly to form products in a single reaction step
Rate limiting step
The slowest step
reaction intermediate
the product of one step and the reaction of the other step
molecularity
classifying an elementary reaction based on the number of reactant molecules
unimolecular reaction
type of elementary reaction involving only one reactant molecule
bimolecular reaction
elementary reaction resulting from the collision of two reactant molecules
termolecular reaction
elementary reaction involving three reactant molecules
rare
why are termolecular reaction rare?
the simultaneous collision of three molecules in the correct orientation is unlikely
how do you predict the rate law?
for a question like this, you would be given two elementary reactions and the rate constant
- find the rate constant of the slower reaction and see if it matches the rate constant given
- if they do not match, set up an equilibrium expression from the fast reaction which is the concentration of the products over the reactants
- solve for the concentration of the products
- plug the reactant side in the for the same molecule in the slow step
- now see if it matches the known rate law
how to write the rate law for an elementary step? units?
rate with units molarity over seconds equals the k the rate constant times the concentration of the reactants
what are factors that can affect the reaction rate?
increasing the concentration of reactants
molecules being closer together higher chance of molecules colliding
increase in temperature
collision theory
if molecules are not at the right angle a reaction will not occur where one molecule must touch a different atom in another molecule
collision theory for bimolecular molecule
for a bimolecular reaction to take place. the two molecules must collide at the correct orientation and larger energy than the activation energy in order for the reaction to occur
what is the purpose of a catalyst??
speed up a chemical reaction
what does k in the collision theory equation depend on?
the temperature
collision theory equation? what does each variable mean?
k=Zpf
Z is the number of collisions
p is the number of correct orientations
f is the number of molecules with sufficient energy
does temperature affect the orientation of molecules?
no
Does the temperature affect the amount of energy? why?
temperature increases the kinetic energy meaning more molecules will collide at a faster rate and more energy will be produced
what is the Arrhenius equation? what does each variable represent? units of each?
k=Ae^(-Ea/RT)
A is the frequency factor and has the same units as k
Ea is the activation energy
R is the gas constant 0.0083145 KJ/mol
and T is the temperature in kelvin
what does the Arrhenius equation tell us?
how the rate constant k changes based on activation energy and temperature
as activation energy in inversely proportional to rate constant
temperature is proportional to rate constant
what is the formula to solve for Ea? units for Ea?
ln(k1/k2)=Ea/R(1/T2-1/T1)?
what is the Arrhenius equation in the form y=mx+b
ln(k)=(-Ea/R)(1/T) + lnA
y is ln(k)
m is -Ea/R
x is 1/T
b is lnA
given a table of data solve for the activation energy?
- solve for m using y2-y1/x2-x1
- -Ea/R set equal to the slope solve and isolate for -Ea
transition state
the unstable group of atoms representing the highest energy
along the pathway from the reactants to the products
activation energy
minimum energy needed to activate a successful reaction
how does sufficient energy in k=Zpf behave when temperature increases
the sufficient energy increases exponentially
what does the TS theory stand for and what is the Ts theory?
also known as the transition state theory
explains the reaction rates of elementary chemical reactions
what is the sufficient energy formula?
f=e^-Ea/RT
how does higher T affect Ea/Rt in the arrhenius equation
decreases it
how does higher T affect e^x in the arrhenius equation?
decreases it
how does higher T affect k in the Arrhenius equation?
increases it
how does higher T affect rate in the arrhenius equation
increase it
factors affecting the reaction rate
concentration, temperature and catalyst
how does concentration affect the reaction rate?
their reaction rate increases when moving molecules are closer together because it increases they likelihood that they will collide
how does temperature affect the reaction rate?
Increasing the temperature will increase the rate of the reaction because temperature increases the kinetic energy and molecules with higher energy than the activation energy will collide more often
where are catalysts involved?
in the rate-limiting step to speed up the reaction
are catalysts consumed in the reaction?
no
do catalyzed reaction have the same endo/exothermicity as the uncatalyzed reaction?
yes