Reaction Mechanisms and Rate Law - AP Chemistry

1. How do you find the overall order of a reaction?

   Sum the exponents in the rate law. Add all concentration exponents in rate law.

2. What is the overall reaction order if Rate = $k[A]^0[B]^1$?

   1. Sum exponents: 0 + 1 = 1.

3. What is the rate law expression for a reaction $aA + bB \rightarrow cC$?

   Rate = $k[A]^m[B]^n$. General form where m and n are determined experimentally, not from coefficients.

4. State the units of the rate constant $k$ for a first-order reaction.

   $s^{-1}$. First-order reactions have units of reciprocal time.

5. Define the term 'reaction mechanism'.

   A series of elementary steps that describe the pathway from reactants to products. Shows how reactants transform through specific elementary steps.

6. What is a termolecular reaction?

   A reaction involving three reacting species in one step. Rare due to low probability of three-body collisions.

7. What is the order of reaction with respect to $A$ if Rate = $k[A]^2$?

   2. The exponent of $[A]$ indicates second order with respect to $A$.

8. Predict the rate law for the elementary reaction $A_2 + B \rightarrow A_2B$.

   Rate = $k[A_2][B]$. Stoichiometry directly gives rate law for elementary steps.

9. For a reaction Rate = $k[A]^3$, what is the overall reaction order?

   3. Single exponent gives overall order directly.

10. What is a reaction intermediate?

    A species formed and consumed during the reaction mechanism. Temporary species that doesn't appear in overall equation.

11. What is the term for a species that appears in the mechanism but not in the overall reaction?

    Intermediate. Formed during mechanism but cancels out overall.

12. For a reaction Rate = $k[A]^1[B]^2$, what is the reaction order with respect to $B$?

    2. The exponent of [B] in the rate law.

13. What is the role of the rate-determining step in a reaction mechanism?

    Controls the overall rate of the reaction. Determines how fast products form overall.

14. Define activation energy in the context of a reaction mechanism.

    The minimum energy required for a reaction to occur. Energy barrier that must be overcome for reaction.

15. How does a catalyst affect the rate-determining step?

    It lowers the activation energy of the rate-determining step. Reduces energy barrier of the limiting step.

16. For Rate = $k[A]^2[B]^0$, what is the order with respect to $B$?

    0. Zero exponent means B doesn't affect rate.

17. What is the difference between an intermediate and a catalyst in a reaction mechanism?

    Intermediates are produced and consumed; catalysts are not consumed. Key distinction: intermediates appear then disappear in the mechanism.

18. What is an elementary reaction?

    A reaction that occurs in a single step. Proceeds directly without intermediate steps.

19. What is the relationship between rate law and reaction mechanism?

    The rate law can provide insights into the mechanism, particularly the rate-determining step. Rate law reflects the slowest step's kinetics.

20. State the units of the rate constant $k$ for a second-order reaction.

    $M^{-1}s^{-1}$. Second-order reactions have units of reciprocal concentration per time.

21. What is the molecularity of a single elementary step involving three molecules?

    Termolecular. Three molecules participate in a single elementary step.

22. Identify the term for a reaction step that involves a single reactant.

    Unimolecular. Only one molecule undergoes reaction in the step.

23. What is a catalyst?

    A substance that increases the reaction rate without being consumed. Speeds up reaction but remains unchanged overall.

24. Which step in a reaction mechanism is the rate-determining step?

    The slowest step. This step controls the overall reaction rate.

25. In a reaction mechanism, what does the term 'elementary step' imply?

    Each step represents a single molecular event. Each step occurs as written without sub-steps.

26. What is the rate law for the elementary reaction $2A \rightarrow A_2$?

    Rate = $k[A]^2$. Stoichiometry gives exponents for elementary reactions.

27. What is meant by the term 'elementary step' in a reaction mechanism?

    A single reaction step with a single transition state. Cannot be broken down into simpler mechanistic steps.

28. What is a termolecular reaction?

    A reaction involving three reacting species in one step. Rare due to low probability of three-body collisions.

29. What is the first step in writing a reaction mechanism?

    Identify the overall balanced chemical equation. Foundation for proposing elementary steps.

30. What is the purpose of a reaction mechanism?

    To provide a detailed step-by-step description of a reaction. Explains the molecular pathway from reactants to products.

31. State the rate law for a bimolecular elementary reaction involving $A$ and $B$.

    Rate = $k[A][B]$. Elementary steps follow stoichiometry in rate laws.

32. What units does the rate constant $k$ have for a zero-order reaction?

    $M\cdot s^{-1}$. Zero-order has units of concentration per time.

33. Find and state the overall order of reaction for Rate = $k[A][B]^2$.

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

34. What are the units of the rate constant $k$ for a third-order reaction?

    $M^{-2}s^{-1}$. Third-order has units $M^{-2}s^{-1}$.

35. What is a unimolecular reaction?

    A reaction involving one reactant molecule in a single step. Single molecule breaks down or rearranges.

36. What is the role of a catalyst in a reaction mechanism?

    It lowers the activation energy, increasing reaction rate. Provides alternative pathway with lower energy barrier.

37. Determine the rate law for the elementary reaction $2NO + O_2 \rightarrow 2NO_2$.

    Rate = $k[NO]^2[O_2]$. For elementary reactions, stoichiometry equals rate law exponents.

38. Identify the molecularity of a reaction with the elementary step $A + B \rightarrow C$.

    Bimolecular. Two different molecules react in one step.

39. Identify the molecularity of a single-step reaction involving two reactants.

    Bimolecular. Two reactants means molecularity equals 2.

40. Determine the rate law for a unimolecular reaction involving $A$.

    Rate = $k[A]$. Unimolecular means first order in the single reactant.

41. What is the molecularity of an elementary step $A + 2B \rightarrow AB_2$?

    Termolecular. Count total molecules: 1 A + 2 B = 3 molecules.

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