13.9
Reaktionsmechanismen
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Q1: What is the difference between a reaction mechanism and an overall chemical equation?
An overall chemical equation shows only reactants and products, but a reaction mechanism reveals the step-by-step process at the molecular level. The mechanism consists of multiple elementary reactions occurring in sequence that sum to produce the balanced overall equation. This detailed view explains how bonds actually break and form during the reaction.
Q2: What are reaction intermediates and why don't they appear in the overall equation?
Reaction intermediates are low-energy species produced in one elementary step and consumed in a subsequent step. They are short-lived and absent from the final product mixture. When elementary steps are combined to form the overall equation, intermediates cancel out because they are both produced and consumed, leaving only the true reactants and products.
Q3: How does the rate-limiting step control the overall reaction rate?
The rate-limiting step is the slowest elementary reaction in a multistep mechanism. Since a reaction cannot proceed faster than its slowest step, this step determines the overall reaction rate. Even if other elementary steps occur quickly, the entire reaction is constrained by the speed of the rate-determining step.
Q4: What is molecularity and how does it relate to reaction order in elementary reactions?
Molecularity is the number of reactant molecules in an elementary reaction. Unlike overall reactions, the rate law for an elementary reaction can be predicted directly from its stoichiometric coefficients. Unimolecular reactions are first-order, bimolecular reactions are second-order, and termolecular reactions are third-order.
Q5: Why are termolecular elementary reactions so rare?
Termolecular reactions involve the simultaneous collision of three atoms, molecules, or ions. The probability of three particles colliding at the same time and place is extremely low compared to unimolecular or bimolecular collisions. Although rare, a few established termolecular reactions exist, such as certain reactions involving nitric oxide.
Q6: How do you determine the rate law for an elementary reaction versus an overall reaction?
For elementary reactions, the rate law can be predicted directly from the balanced equation using stoichiometric coefficients. For overall reactions, the rate law must be determined experimentally because the mechanism is unknown. This distinction is crucial because elementary reaction equations represent actual molecular interactions, while overall equations do not.
Q7: What is the difference between reaction intermediates and activated complexes?
Reaction intermediates are low-energy species produced and consumed during elementary steps, appearing in the mechanism but not the overall equation. Activated complexes are high-energy transition states that exist only momentarily during the transformation of reactants to products. Both are temporary, but intermediates are stable enough to participate in subsequent steps.