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Reaction mechanism

Reaction mechanism

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Reaction mechanism

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  1. Reaction Mechanism UNIS 2020

  2. There is a sequence of elementary steps from reactants to products. The collection of these steps is know as mechanism. Mechanism is a suggested or proposed and usually can not be proven but it can be rejected if: it does not account for the overall stoichiometry of the reaction. And or 2) Does not agree with the experimental rate law.

  3. The slowest elementary step is know as rate determining step/ RDS. RDS has the highest Ea, the smallest rate constant (k) and takes the longest to react. RDS's rate expression should be the same as the experimental rate.

  4. For the following reaction: H2 (g) + 2ICl (g)  I2 (g) + 2HCl (g) Via experiment, the rate law is found to be = k [H2][ICl] A proposed mechanism is the following two elementary steps: step 1: H2 +ICl  HI + HCl ratestep 1 = k [H2][ICl] step 2: HI + ICl  I2 + HCl ratestep 2 = k [HI][ICl] overall : H2 (g) + 2ICl (g)  I2 (g) + 2HCl (g) step 1 is the rate determining step since it’s rate agrees with the experiment. It is a good proposal: the RDS’s rate agrees with the rate law and secondly the overall reaction is same as the starting equation. HI is called an intermediate; it is produced in one step and removed in the next step.

  5. Intermediate: Exist for a while occurs at lower Potentialenergy Formed in one step and consumed in the next. Activated complex/ transition state: Exist in transient State Occurs at maximum potential energy. Exist through every step involving bond breaking/ forming.

  6. molecularity = is the number of reactants involved in the RDS; unimolecular, bimolecular and or trimolecular/termolecular. However, this type of reaction is very uncommon as all Three reactants must simultaneously collide with each other with sufficient energy and correct orientation, in order to produce a reaction.

  7. Another example: The following reaction was studied in the lab and the experimental rate law was determined: CO (g) + NO2 (g)  CO2 (g) + NO (g) ; Rateexp. = k[NO2]2 To propose a mechanism we should deduce from the rate law that elementary step should only involve the collision of two particles of NO2 and it’s bimolecular. So one plausible two step mechanism can be: NO2 + NO2 NO +NO3 step 1 ; RDS ; and rate = K[N2O]2 NO3 + CO  NO + CO2 This step is faster and the intermediate, [NO3]will be consumed. The above proposal can not be rejected since the RDS’s rate law agrees with the experimental rate law, And the sum of two elementary steps adds up to be the overall reaction/equation.

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