Reaction Mechanisms (College Board AP® Chemistry): Flashcards
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Define reaction mechanism.
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Cards in this collection (25)
Define reaction mechanism.
A step-by-step description of the sequence of elementary reactions by which reactants are converted to products at the molecular level.
True or False?
The elementary steps in a proposed reaction mechanism must add up to give the overall balanced equation.
True.
This is a fundamental requirement for any valid proposed mechanism. If the elementary steps do not sum to the overall equation, the mechanism must be rejected.
Define reaction intermediate.
A species produced in one elementary step of a reaction mechanism and consumed in a subsequent step, so it does not appear in the overall balanced equation.
What two criteria must a proposed reaction mechanism satisfy to be considered valid?
The elementary steps must combine to give the overall balanced equation for the reaction.
The mechanism must be consistent with the experimentally determined rate law.
A reaction intermediate appears as a .......... in one elementary step and as a reactant in the next, so it cancels out of the overall equation.
A reaction intermediate appears as a product in one elementary step and as a reactant in the next, so it cancels out of the overall equation.
How can a reaction intermediate be identified on an energy profile diagram for a multistep reaction?
Reaction intermediates appear as troughs (local energy minima) between peaks on the energy profile diagram. Unlike activated complexes at the peaks, intermediates have finite stability and can sometimes be isolated.
True or False?
Reaction mechanisms can be directly observed in the laboratory and do not require experimental evidence to confirm them.
False.
Reaction mechanisms are theoretical proposals that cannot be directly observed. Experimental evidence — particularly agreement with the experimentally determined rate law — is required to support a proposed mechanism.
Define rate-determining step.
The slowest elementary step in a reaction mechanism; it has the highest activation energy and lowest rate constant, and determines the overall rate and rate law of the reaction.
Why must the rate law derived from a proposed mechanism agree with the experimental rate law?
Rate laws are determined experimentally and reflect the actual concentration dependence of the reaction rate. If a proposed mechanism generates a different rate law, it cannot be the correct mechanism and must be discarded.
True or False?
The overall rate law for a multistep reaction is determined by the rate law of the slowest (rate-determining) elementary step.
True.
The rate-determining step is the bottleneck of the reaction. Its rate law, written using the reactants of that step (not intermediates), determines the overall rate law.
The rate law for an elementary step is written using the .......... of the reactants in that step as the exponents.
The rate law for an elementary step is written using the stoichiometric coefficients of the reactants in that step as the exponents.
The slow step of a mechanism is: NO2 (g) + F2 (g) → NO2F2 (g) + F. What is the rate law for the overall reaction?
Rate = k[NO2][F2]
The rate law is derived from the slow (rate-determining) step: one molecule each of NO2 and F2 are involved, giving first order dependence on each. F is a product of this step, not a reactant, so it does not appear in the rate law.
True or False?
A reaction intermediate may appear in the rate law derived directly from the rate-determining step if it is a reactant in that step.
True.
If an intermediate is a reactant in the rate-determining step, it appears in the initial rate law expression. However, it must then be eliminated by substituting an expression from the fast equilibrium step(s) that preceded it, so the final rate law contains only reactant concentrations.
Define pre-equilibrium approximation.
An assumption applied when the first step of a mechanism is fast and reversible: the forward and reverse rates of that step are equal, allowing the concentration of any intermediate formed to be expressed in terms of the original reactant concentrations.
Why is the pre-equilibrium approximation needed when deriving a rate law from a mechanism where the slow step follows a fast reversible step?
When the rate-determining step involves an intermediate from the earlier fast step, the rate law initially contains the intermediate's concentration, which cannot be measured directly. The pre-equilibrium approximation uses the fast equilibrium to express the intermediate's concentration in terms of original reactants, giving a rate law containing only measurable quantities.
In the pre-equilibrium approximation, the concentration of the intermediate is expressed as [intermediate] = (........../kb) × [reactants], where kf is the rate constant for the forward fast step.
In the pre-equilibrium approximation, the concentration of the intermediate is expressed as [intermediate] = (k_f/kb) × [reactants], where kf is the rate constant for the forward fast step.
True or False?
The pre-equilibrium approximation is applied when the rate-determining step is the first step of the mechanism.
False.
The pre-equilibrium approximation is applied when the rate-determining step is preceded by a fast reversible step. If the first step is rate-determining, the rate law can be read directly from it without any approximation.
For the mechanism: NO + Br2 ⇋ NOBr2 (fast); NOBr2 + NO → 2NOBr (slow) — what is the final rate law after applying the pre-equilibrium approximation?
The slow step gives Rate = k[NOBr2][NO]. Applying the pre-equilibrium approximation to the fast step: [NOBr2] = (kf/kb)[NO][Br2]. Substituting: Rate = k(kf/kb)[NO]2[Br2], which simplifies to Rate = k′[NO]2[Br2].
True or False?
After applying the pre-equilibrium approximation, the final rate law must be expressed in terms of reactant concentrations only, not intermediates.
True.
The purpose of the pre-equilibrium approximation is to eliminate the intermediate from the rate law by substituting an equivalent expression in terms of reactants. A rate law containing an intermediate concentration is not acceptable as a final answer.
On an energy profile diagram for a multistep reaction, what features indicate the number of elementary steps and the number of intermediates?
The number of peaks (local maxima) equals the number of elementary steps — each peak represents an activated complex. The number of troughs (local minima) between peaks equals the number of intermediates.
True or False?
The rate-determining step in a multistep reaction corresponds to the elementary step with the highest peak on the energy profile diagram.
False.
The rate-determining step corresponds to the step with the highest activation energy, which is the largest energy difference between a trough (or the reactants) and the following peak — not necessarily the highest peak in absolute energy terms.
On an energy profile, the overall enthalpy change (ΔH) is the energy difference between the .......... and the reactants.
On an energy profile, the overall enthalpy change (ΔH) is the energy difference between the products and the reactants.
How does an energy profile diagram distinguish between an activated complex (transition state) and a reaction intermediate?
Activated complexes appear at the peaks of the energy profile — they are inherently unstable and cannot be isolated. Reaction intermediates appear at the troughs between peaks — they have finite stability, can sometimes be isolated, and always lie at a higher energy than the reactants or products.
True or False?
If the products on an energy profile diagram are at a higher energy than the reactants, the overall reaction is endothermic.
True.
An endothermic reaction absorbs energy from the surroundings, so the products have greater energy than the reactants. On the energy profile this is shown by the product energy level sitting above the reactant energy level.
The activation energy for each elementary step is measured as the energy difference between the preceding .......... (or reactant energy) and the peak of that step.
The activation energy for each elementary step is measured as the energy difference between the preceding trough (or reactant energy) and the peak of that step.
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