Reaction MechanismsA balanced equation tells us the reactants and products, but does not tell us how the reactants became products.Vocabulary:1. Reaction mechanism: a series of elementary steps by which a reaction

occurs2. elementary step: a reaction whose rate law can be written from its

molecularity3. molecularity: the number of species that must collide to produce the

reaction indicated by that step (unimolecular, bimolecular, termolecular)4. rate determining step: one step in the reaction mechanism that is much

slower than all the other steps. This it determines the rate of reaction

Reaction Mechanisms5. intermediate: a species that is formed in one step and then is a reactant in the next step. It is never seen as a product in the overall reaction6. Catalyst: a species that is seen in the reactant and product of the overall reaction7. Elementary Reaction: single step reaction

-reactant rearranges: H3C-N=C → H3C-C=N-if there is only one reactant, what collides?

-NO(g) + O3(g) → NO2(g) + O2(g)-what is the molecularity of this reaction?

MolecularityWhy are termolecular (or higher) very rare?

Reaction MechanismsGoal: choose a plausible reaction mechanism from 2 or 3 possibilities

-the correct reaction mechanism will satisfy 2 requirements:1. the sum of the elementary steps must give the overall balanced equation

for the reaction2. the mechanism must agree with the experimentally determined rate law

a. the rate determining step (the slow step) will have the molecularity of the rate law

b. to check for the slow step, assume one is slow and then check to see if it matches the rate law

Rate Laws for Elementary Reactions

-rate laws are not simply coefficients-may be multiple steps; each step with it’s own rate law and relative

speed-can’t tell just by looking at a reaction if it has one or more elementary

steps-if a reaction is elementary, it’s rate law is based on molecularity

example: A → products Rate = k[A] unimolecular-as the number of A molecules increases, the rate increases

A + B → products Rate = k[A][B] bimolecular-rate is first order in both [A] and [B], second order overall

Rate Laws for Elementary Reactions

Rate Laws for Elementary Reactions

1. If the following reaction occurs in a single elementary reaction, predict its rate law:

H2(g) + Br2(g) → 2HBr(g)

2. Given: 2NO(g) + Br2(g) → 2NOBr(g)a. write the rate law (assuming single elementary reaction)b. Is a single-step mechanism likely?

Multistep Mechanismsequence of elementary steps

Reaction: NO2(g) + CO(g) → NO(g) + CO2(g)

elementary steps:NO2(g) + NO2(g) → NO3(g) + NO(g) NO3(g) + CO(g) → NO2(g) + CO2(g)

NO3 is an intermediate = formed in one elementary step and consumed in another

Multistep Mechanismsintermediates can be stable (isolated and identified) unlike transition state

Multistep MechanismsProposed conversion of ozone into O2: O3(g) → O2(g) + O(g)O3(g) + O(g) → 2O2(g)

1. describe the molecularity of each elementary reaction2. write the equation for the overall reaction3. identify the intermediates

Multistep MechanismsFor the reaction:

Mo(CO)6 + P(CH3)3 → Mo(CO)5P(CH3)3 + COthe proposed mechanism is:

Mo(CO)6 → Mo(CO)5 + COMo(CO)5 + P(CH3)3 → Mo(CO)5P(CH3)3

1. is the proposed mechanism consistent with the equation for the overall reaction?

2. What is the molecularity of each step of the mechanism?3. Identify the intermediate(s)

Rate Determining Steps● aka rate-limiting step● slowest intermediate step

o the rate of a fast step that follows the RDS does NOT speed up the overall rate the products of the slow step are immediately consumed in the

fast stepo if the slow step is not first, the faster preceding steps produce

intermediate products that build up before being consumed the intermediate is a reactant in the slow step

● Why can’t the rate law for a reaction generally be deduced from the balanced equation?

Rate Determining Stepsthe overall rate of a reaction is determined by the molecularity of the slow step

The decomposition of nitrous oxide is believed to occur in a 2-step mechanism: N2O(g) → N2(g) + O(g) (slow)N2O(g) + O(g) → N2(g) + O2(g) (fast)

1. write the equation of the overall reaction2. write the rate law for the overall reaction

Rate Determining StepsGiven: O3(g) + 2NO2(g) → N2O5(g) + O2(g)

The reaction is believed to occur in two steps: O3(g) + NO2(g) → NO3(g) + O2(g)NO3(g) + NO2(g) → N2O5(g)

The experimental rate law is rate = k[O3][NO2]. What can you say about the relative rates of the 2 steps of the mechanism?

Rate Determining StepsChoose the reaction mechanism for 2X + Y → Z; Rate = k[Y]1. X+Y → M (slow)

X+M → Z (fast)2. X+X ←> M (fast) equilibrium Y+M → Z (slow)3. Y → M (slow) M+X → N (fast) N+X → Z (fast)

CatalysisCatalyst: changes speed of the reaction

-catalyst remains unchanged-enzymes in body

homogeneous catalyst: same phase as the reactantsexample: H2O2(aq) → 2H2O(l) + O2(g)With bromide ion catalyst: 2Br-(aq) + H2O2(aq) + 2H+(aq) → Br2(aq) + 2H2O(l)

is the bromide ion a catalyst according to the definition above?

No, but the Br2 also reacts with the H2O2:Br2(aq) + H2O2(aq) → 2Br-(aq) + 2H+(aq) + O2(g)

combine both reaction and you get the original reaction of the decomposition of hydrogen peroxide