CHM 116-Chapter 14 Homework Questions

Chapter 14 Homework Questions 1. According to Collision Theory, what is/are the essential qualification(s) that must occur for a reaction to form products?  Reacting particles must have sufficient energy.  Reacting particles must have the proper orientation.  Reacting particles must collide. ***Collision theory is a model used to predict the rates of chemical reactions, particularly for gases. Collision theory is based on the assumption that, for a reaction to occur, it is necessary for the reacting species (atoms or molecules) to come together, or collide , with one another. 2. Which statement, in regards to a successful reaction as defined by Collision Theory, is false ?  Reactions that require four particles to collide are too highly improbable to be an effective reaction step.  All collisions with energy greater than the activation energy will result in products.  Collisions can be defined as unimolecular, bimolecular, or termolecular.  A successful collision must have energy equal to or greater than the activation energy.  Most collisions do not result in a reaction. 3. Categorize each of the following based on their effect on the rate constant, k . Larger k Smaller k faster reaction rate insufficient kinetic energy sufficient kinetic energy larger Eₐ smaller Eₐ slower reaction rate The larger the E a , the smaller the k . Reaction rate decreases because fewer reactants have the necessary kinetic energy to overcome E a . 4. Consider the effect changing temperature can have on a reaction. A given reaction run at a higher temperature has a larger k because there are more particles with sufficient energy to overcome the activation energy, E a .

A reaction run at a higher temperature has a faster reaction rate than the same reaction run at a lower temperature. However, this reaction has the same Eₐ as the same reaction run at the lower temperature. *** Note: E a does NOT change with temperature. 5. Which of the following statements are true regarding elementary reactions?  If the first step in a reaction mechanism is slow, the rate law for the overall reaction will be the same as the rate law for that elementary step.  Elementary reactions describe a process that occurs by a single collision (molecular event).  A reaction mechanism is a proposed sequence of elementary steps.  When the rate law for an overall reaction does not correspond to the known rate-determining (slow) step, it usually means one or more fast steps precede it in the reaction mechanism.  The combination of all elementary steps in a reaction mechanism must result in the overall reaction.  The reaction mechanism must be consistent with all experimentally- derived observations, including the experimentally-derived rate law. **An overall (or net) chemical reaction is often the result of two or more individual reactions that occur in a discrete sequence. The individual reactions that make up the overall reaction are called elementary steps (reactions) . The sequence of elementary steps that leads to product formation is the reaction mechanism . 6. Identify whether the following elementary reactions are unimolecular, bimolecular, or termolecular , and determine if the correct rate law for each elementary reaction. Cl 2 ( g ) → 2 Cl ( g ) ------  unimolecular ----  rate = k[Cl 2 ] ( correct ) 2 F ( g ) + Ne ( g ) → F 2 ( g ) + Ne ( g ) ----  termolecular -----  rate = k[F][Ne] ( incorrect ) 2 NO ( g ) → N 2 O 2 ( g ) -----  bimolecular ------  rate = k[NO] 2 ( correct ) 7. Reaction Mechanism Step 1: ClO ⎯ + H 2 O → HOCl + OH ⎯ Step 2: Br ⎯ + HOCl → HOBr + Cl ⎯ Step 3: OH ⎯ + HOBr → H 2 O + BrO ⎯ Consider the reaction mechanism.

Which mechanism is consistent with the experimentally-determined rate law seen below? rate = k[D] 2 Step 1: D + D → F + H ( slow ) Step 2: H + E → G + D ( fast ) 10. Answer the questions below about catalysts. Part A : How does a catalyst affect the rate of the forward and/or reverse reactions? Increases the rate of the forward AND reverse reactions . Part B : Which type of reaction has a faster reaction rate ? Catalyzed . Part C : Which type of reaction has a larger activation energy, E a ? uncatalyzed Part D : Which type of reaction has a larger rate constant, k ? catalyzed Part E : How does the mass of a catalyst change from before to after a reaction was complete? there is no net change in mass 11. True or False . A homogeneous catalyst can be in a different phase as the rest of the reaction as long as it is evenly distributed throughout the reaction mixture. FALSE