Introduction to Reaction Mechanisms - AP Chemistry

1. Define termolecular reaction.

   An elementary reaction involving three molecules. Very rare due to low probability of three molecules colliding simultaneously.

2. What is a unimolecular reaction?

   A reaction involving a single molecule undergoing change. Has molecularity of one and follows first-order kinetics.

3. What is a reaction intermediate?

   Species formed in one step of a mechanism and consumed in another. Temporary species that appear in mechanism but not in overall equation.

4. What is the significance of the rate-determining step?

   It limits the overall reaction rate. Slowest step controls overall reaction speed like bottleneck.

5. What is the steady-state approximation?

   An assumption that the concentration of intermediates remains constant. Rate of formation equals rate of consumption for intermediates.

6. What is the transition state theory?

   A theory that explains reaction rates by considering the transition state. Links thermodynamics and kinetics through activated complex formation.

7. What does the term 'rate law' refer to?

   An equation that relates the rate of reaction to the concentration of reactants. Mathematical expression relating reaction rate to concentrations.

8. What is the effect of temperature on the rate constant $k$?

   Typically, $k$ increases with increasing temperature. Exponential relationship described by Arrhenius equation.

9. What is the half-life ($t_{1/2}$) of a first-order reaction?

   $t_{1/2} = \frac{0.693}{k}$. Time required for concentration to decrease by half.

10. Identify the role of a catalyst in a reaction mechanism.

    A substance that increases reaction rate without being consumed. Lowers activation energy by providing alternative pathway.

11. Define 'reaction profile'.

    A graph showing the energy changes during a reaction. Visualizes energy barriers and intermediates along reaction pathway.

12. How does temperature affect reaction rate according to the Arrhenius equation?

    Increasing temperature increases the reaction rate. Higher temperature increases molecular energy and collision frequency.

13. Identify the units of a rate constant for a second-order reaction.

    $M^{-1}s^{-1}$. Units account for concentration dependence in second-order kinetics.

14. Identify the transition state of a reaction.

    A high-energy, unstable arrangement of atoms at the peak of a reaction coordinate. Maximum energy point along reaction coordinate diagram.

15. Define 'reaction coordinate'.

    A hypothetical path that represents the progress of a reaction. X-axis in energy diagrams showing reaction progress.

16. What is the difference between a catalyst and an intermediate?

    Catalysts are not consumed; intermediates are formed and consumed. Both help reaction proceed but have different consumption patterns.

17. What does the symbol $Ea$ represent in the Arrhenius equation?

    Activation energy. Energy barrier parameter in temperature-dependent rate equation.

18. What is a reaction mechanism?

    A step-by-step sequence of elementary reactions by which an overall chemical change occurs. Describes the detailed pathway showing how reactants become products.

19. What is the rate-determining step?

    The slowest step in a reaction mechanism that determines the overall rate. Acts as bottleneck controlling overall reaction speed.

20. What is the Lindemann mechanism?

    A mechanism describing unimolecular reactions through a two-step process. Explains pressure dependence of unimolecular reaction rates.

21. What is the effect of a catalyst on activation energy?

    A catalyst lowers the activation energy of a reaction. Provides alternative pathway requiring less energy input.

22. How does molecular orientation affect reaction rate?

    Proper orientation during collisions is necessary for reaction. Molecules must collide with correct geometry for reaction.

23. Define 'elementary reaction'.

    A single step reaction with a single transition state and no intermediate. Cannot be broken down into simpler reaction steps.

24. What does 'R' represent in the Arrhenius equation?

    The universal gas constant. Fundamental constant linking energy and temperature scales.

25. What does a reaction mechanism describe?

    The detailed steps of how a reaction proceeds from reactants to products. Shows elementary steps connecting reactants to products.

26. State the formula for the Arrhenius equation.

    $k = Ae^{-Ea/RT}$. Shows exponential dependence of rate constant on temperature.

27. What is the purpose of using the steady-state approximation?

    To simplify the analysis of complex reaction mechanisms. Eliminates need to track rapidly changing intermediate concentrations.

28. Identify the units of a rate constant for a first-order reaction.

    $s^{-1}$. Units reflect inverse time for first-order processes.

29. What is the effect of concentration on reaction rate?

    Increased concentration generally increases reaction rate. More reactant molecules lead to more frequent collisions.

30. How does a catalyst affect a reaction mechanism?

    It provides an alternative pathway with lower activation energy. Reduces activation energy without changing thermodynamics.

31. What is the typical unit for the universal gas constant ($R$)?

    $8.314 J mol^{-1} K^{-1}$. Standard SI units for gas constant in energy calculations.

32. What characterizes a bimolecular reaction?

    A reaction involving two reacting species. Has molecularity of two, most common elementary reaction type.

33. What is the role of activation energy in a reaction mechanism?

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

34. Define 'molecularity' in reaction mechanisms.

    The number of molecules that participate in an elementary reaction. Applies only to elementary reactions, not overall mechanisms.

35. Identify the units of a rate constant for a zero-order reaction.

    $M s^{-1}$. Units reflect constant rate independent of concentration.

36. What is the role of collision frequency in reaction rate?

    Higher collision frequency can increase the reaction rate. More collisions per unit time increases reaction probability.

37. What does the term 'reaction order' refer to?

    The sum of the exponents in the rate law expression. Determines how concentration changes affect overall rate.

38. What does the symbol $k$ represent in kinetics?

    The rate constant of a reaction. Proportionality constant in rate law expressions.

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