Elementary Reactions - AP Chemistry

1. Which species cancels when you add elementary steps to obtain the overall reaction?

   Intermediates (and catalysts) cancel from the net equation. Species produced and consumed in mechanism don't appear in net equation.

2. Identify the units of $k$ for an elementary step with $\text{rate}=k[A][B]$.

   $\text{M}^{-1}\text{s}^{-1}$. Second-order overall requires one inverse concentration unit.

3. What is the rate law for a unimolecular elementary step $A \rightarrow$ products?

   $\text{rate}=k[A]$. One molecule decomposes, so rate depends on [A] only.

4. What is the rate law for the elementary step $A+2B \rightarrow$ products?

   $\text{rate}=k[A][B]^2$. Coefficient 2 for B becomes exponent 2 in elementary steps.

5. What is the molecularity of an elementary step with three reactant particles?

   Termolecular (rare). Three-body collisions are statistically unlikely.

6. Which option is correct: Can the overall reaction stoichiometry be used to write the overall rate law?

   No; only an elementary step directly determines its rate law. Overall rate laws must be determined experimentally.

7. What is the rate law for the bimolecular elementary step $2A \rightarrow$ products?

   $\text{rate}=k[A]^2$. Coefficient 2 becomes exponent 2 in elementary rate laws.

8. What is the rate law for the termolecular elementary step $2A+B \rightarrow$ products?

   $\text{rate}=k[A]^2[B]$. Exponents match stoichiometric coefficients for elementary steps.

9. Find the overall reaction for: $A \rightarrow B$ and $B+C \rightarrow D$.

   $A+C \rightarrow D$. Add steps; B cancels as intermediate.

10. What is the rate law for a termolecular elementary step $A+B+C \rightarrow$ products?

    $\text{rate}=k[A][B][C]$. Three molecules must collide simultaneously.

11. What is the rate law for the unimolecular elementary step $A \rightarrow$ products?

    $\text{rate}=k[A]$. For elementary steps, rate law exponents equal stoichiometric coefficients.

12. What is the relationship between the RDS and the observed rate law in many mechanisms?

    Observed rate law often matches the RDS rate law. The RDS controls the overall reaction rate.

13. What is the predicted rate law if the slow elementary step is $A+B \rightarrow$ products?

    $\text{rate}=k[A][B]$. Elementary step rate laws directly reflect stoichiometry.

14. What is the definition of the rate-determining step (RDS) in a mechanism?

    The slowest step that limits the overall rate. All other steps must wait for the slowest step to complete.

15. What is the rate-determining step in a multistep mechanism?

    The slowest elementary step that limits the overall rate. Acts as bottleneck; determines overall reaction rate.

16. What is the definition of an elementary reaction step in a mechanism?

    A single-step molecular event with no intermediates. Elementary steps occur in one collision without breaking into smaller steps.

17. What is the rate law for a bimolecular elementary step $A+B \rightarrow$ products?

    $\text{rate}=k[A][B]$. Two different molecules collide, each contributes to rate.

18. What is the overall reaction obtained by adding these steps: $A+B \rightarrow C$ and $C+D \rightarrow E$?

    $A+B+D \rightarrow E$. Add steps and cancel intermediate C on both sides.

19. What is the definition of a catalyst in a reaction mechanism?

    Consumed early and regenerated later; not in net equation. Catalysts appear in mechanism but not in overall equation.

20. Identify the units of $k$ for an elementary step with $\text{rate}=k[A]$.

    $\text{s}^{-1}$. First-order reactions have units of inverse time.

21. Identify the units of $k$ for an elementary step with $\text{rate}=k[A]^2[B]$.

    $\text{M}^{-2}\text{s}^{-1}$. Third-order overall requires two inverse concentration units.

22. What is the molecularity of an elementary step with one reactant particle?

    Unimolecular. One molecule participates in the elementary step.

23. Find the net reaction for: $A+B \rightarrow I$; $I+C \rightarrow D$.

    $A+B+C \rightarrow D$. Add steps and cancel intermediate $I$.

24. What is the rate law for the bimolecular elementary step $A+B \rightarrow$ products?

    $\text{rate}=k[A][B]$. Each reactant appears to the first power in bimolecular steps.

25. Identify the molecularity of the elementary step $A+2B \rightarrow$ products.

    Termolecular. Three particles total: one A and two B molecules.

26. What is the overall reaction order for an elementary step with rate law $\text{rate}=k[A]^2[B]$?

    $3$. Sum all exponents: $2 + 1 = 3$.

27. What is the molecularity of an elementary step with two reactant particles?

    Bimolecular. Two molecules collide in the elementary step.

28. Identify the intermediate from the mechanism: $A+B \rightarrow I$; $I+C \rightarrow$ products.

    $I$. Produced in step 1, consumed in step 2.

29. What is the rate law for a termolecular elementary step $2A+B \rightarrow$ products?

    $\text{rate}=k[A]^2[B]$. Two A's and one B collide; A contributes squared term.

30. What is an elementary reaction in a reaction mechanism?

    A single-step molecular event with one transition state. Cannot be broken down into simpler steps.

31. Identify the molecularity of the elementary step $2A \rightarrow$ products.

    Bimolecular. Two particles collide (both are A molecules).

32. Identify the catalyst from these steps: $X+A \rightarrow Y$ and $Y \rightarrow X+B$.

    $X$. Consumed in step 1, regenerated in step 2.

33. What is the molecularity of an elementary step?

    The number of reactant species that collide in that step. Counts molecules/atoms participating in the elementary step.

34. What are the only possible molecularities for an elementary step in AP Chemistry?

    Unimolecular, bimolecular, or termolecular. 1, 2, or 3 molecules colliding; higher is too improbable.

35. What is a catalyst in a reaction mechanism (as identified from steps)?

    A species consumed early and regenerated in a later step. Appears in overall reaction unchanged; speeds up reaction.

36. What is the rate law for a bimolecular elementary step $2A \rightarrow$ products?

    $\text{rate}=k[A]^2$. Two A molecules collide, so [A] appears squared.

37. Identify the intermediate when these steps are added: $A+B \rightarrow C$ and $C \rightarrow D$.

    $C$. Produced in step 1, consumed in step 2.

38. What is the key rule linking stoichiometric coefficients to rate-law exponents for an elementary step?

    For an elementary step, exponents equal reactant coefficients. Unlike overall reactions, elementary steps follow this rule.

39. What is the definition of an intermediate in a reaction mechanism?

    Formed in one step and consumed in a later step. Intermediates don't appear in reactants or products of overall reaction.

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