Reaction Mechanisms A 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 occurs 2.elementary step: a reaction whose rate law can be written from its molecularity 3.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 Mechanisms 5. 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 reaction 6. Catalyst: a species that is seen in the reactant and product of the overall reaction 7. Elementary Reaction: single step reaction -reactant rearranges: H 3 C-N=C → H 3 C-C=N -if there is only one reactant, what collides? -NO(g) + O 3 (g) → NO 2 (g) + O 2 (g) -what is the molecularity of this reaction?

Molecularity Why are termolecular (or higher) very rare?

Reaction Mechanisms Goal: 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 reaction 2.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

1.If the following reaction occurs in a single elementary reaction, predict its rate law: H 2 (g) + Br 2 (g) → 2HBr(g) 2. Given: 2NO(g) + Br 2 (g) → 2NOBr(g) a.write the rate law (assuming single elementary reaction) b.Is a single-step mechanism likely?

Multistep Mechanism sequence of elementary steps Reaction: NO 2 (g) + CO(g) → NO(g) + CO 2 (g) elementary steps: NO 2 (g) + NO 2 (g) → NO 3 (g) + NO(g) NO 3 (g) + CO(g) → NO 2 (g) + CO 2 (g) NO 3 is an intermediate = formed in one elementary step and consumed in another

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

Multistep Mechanisms Proposed conversion of ozone into O 2 : O 3 (g) → O 2 (g) + O(g) O 3 (g) + O(g) → 2O 2 (g) 1.describe the molecularity of each elementary reaction 2.write the equation for the overall reaction 3.identify the intermediates

Multistep Mechanisms For the reaction: Mo(CO) 6 + P(CH 3 ) 3 → Mo(CO) 5 P(CH 3 ) 3 + CO the proposed mechanism is: Mo(CO) 6 → Mo(CO) 5 + CO Mo(CO) 5 + P(CH 3 ) 3 → Mo(CO) 5 P(CH 3 ) 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 steprate-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 step o 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 Steps the 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: N 2 O(g) → N 2 (g) + O(g)(slow) N 2 O(g) + O(g) → N 2 (g) + O 2 (g) (fast) 1.write the equation of the overall reaction 2.write the rate law for the overall reaction

Rate Determining Steps Given: O 3 (g) + 2NO 2 (g) → N 2 O 5 (g) + O 2 (g) The reaction is believed to occur in two steps: O 3 (g) + NO 2 (g) → NO 3 (g) + O 2 (g) NO 3 (g) + NO 2 (g) → N 2 O 5 (g) The experimental rate law is rate = k[O 3 ][NO 2 ]. What can you say about the relative rates of the 2 steps of the mechanism?

Rate Determining Steps Choose 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)

Catalysis Catalyst: changes speed of the reaction -catalyst remains unchanged -enzymes in body homogeneous catalyst: same phase as the reactants example: H 2 O 2 (aq) → 2H 2 O(l) + O 2 (g) With bromide ion catalyst: 2Br - (aq) + H 2 O 2 (aq) + 2H + (aq) → Br 2 (aq) + 2H 2 O(l) is the bromide ion a catalyst according to the definition above? No, but the Br 2 also reacts with the H 2 O 2 : Br 2 (aq) + H 2 O 2 (aq) → 2Br - (aq) + 2H + (aq) + O 2 (g) combine both reaction and you get the original reaction of the decomposition of hydrogen peroxide