Reaction Pathways

Reaction mechanism Most reactions occur in a series of steps. Most you don’t see. The reaction mechanism is these series of steps. Won’t be responsible for knowing these steps

Reaction Mechanism Elementary Steps a reaction where the rate law can be written from its molecularity molecularity = total # of atoms, ions, or molecules that must collide to produce a reaction; the chance of having 3 collide at the same time is not likely.

Elementary Steps Look at steps to determine overall reaction. Look at overall order of reaction. Unimolecular-one species: A -- > Products Overall first order rate = k[A]

Elementary Steps Look at steps to determine overall reaction. Look at overall order of reaction. Bimolecular-two different species or two of same species could be A + B  Products or 2A  products or 2B  products Overall second order rate = k[A][B] or rate =k[A]2 or rate = k[B]2

Elementary Steps Look at overall order of reaction. Termolecular- Look at steps to determine overall reaction. Look at overall order of reaction. Termolecular- three different species (A + B + C  products) or 2A + one species (2A + B  products) or 3A (3A  products) or …… Overall third order rate = k[A][B][C] or rate =k[A]2[B] or rate=k[A]3

Reaction mechanism It was found to be rate = k[NO2]2 EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 As usual, the rate law was determined experimentally It was found to be rate = k[NO2]2 What do you notice?

Reaction mechanism EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 No CO? Must be zero order. Don’t include it in rate law.

Reaction mechanism EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 The rate law is based on the slowest step in the reaction! This is called the rate determining step. Always the case.

Reaction mechanism EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 The sum of the steps must equal the overall balanced equation. (Need to justify your answer – Part ONE of TWO)

Reaction mechanism EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 The slowest step (rate determining step) must agree with the experimentally proven rate law – the order of the reaction must equal the sum of coefficients in the rate determining step. (Need to justify your answer – Part TWO of TWO)

Reaction mechanism 5) Vocabulary: EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 5) Vocabulary: a) NO3? It was produced in reaction 1 and then consumed in reaction 2. This is called an intermediate.

Reaction mechanism 5) Vocabulary: EX: NO2 + NO2 --k1--> NO3 + NO slow NO3 + CO --k2--> NO2 + CO2 fast NO2 + CO ---> NO + CO2 5) Vocabulary: b) NO2? Need it for the reaction in exp 1 but then is not consumed in exp 2. By definition this is a catalyst.

Mechanism with an equilibrium step Can write the slow step by substituting for the substance that is also present in the equilibrium step So possible mechanism is rate = k [O2] [NO]2

Drawing an energy diagram for a mechanism

AP Problems 2 NO + Br2 ---> 2NOBr was found experimentally. Rate = k[NO]2[Br2] The following mechanism was proposed for the reaction Br2 + NO ---> NOBr2 (slow) NOBr2 + NO ---> 2 NOBr (fast)   Is this mechanism consistent with the given experimental observations? Justify your answer. Does it fulfill all the requirements?

Other AP problems Consider the reaction O3 + NO --> O2 + NO2 Rate = k [O3][NO] The following three step mechanism is proposed for the reaction. Identify the step that must be the slowest. Explain   Step 1: O3 + NO ---> O + NO3 Step 2: O + O3 ----> 2 O2 Step 3: NO3 + NO ---> 2 NO2

2002 #7 The proposed mechanism for the depletion of O3 in the upper atmosphere is shown below: Step 1 O3 + Cl ----> O2 + ClO Step 2 ClO + O ---> Cl + O2 a) Write a balanced equation for the overall reaction represented by Step 1 and 2 b) Identify the catalyst in the mechanism above. Justify your answer c) Clearly identify the intermediate in the mechanism above. Justify your answer. d) If the rate law for the overall reaction is found to be, with rate in mol/L*s: rate = k[O3][Cl]   d1. The overall order of the reaction d2. Appropriate units for the rate constant, k d3. The rate determining step of the reaction-Justify