Ch 4 Handout Answers

Ch 4 Handout Answers - Chem 350 Jasperse Ch. 4 Handouts 1...

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Unformatted text preview: Chem 350 Jasperse Ch. 4 Handouts 1 Ch. 4 The Study of Chemical Reactions 4.1 Three Factors in Every Reaction: 1. Mechanism: what is the step-by-step pathway by which old bonds break and new bonds form? 2. Thermodynamics: what are the energy changes, both for the overall reaction and for individual steps in the reaction mechanism? 3. Kinetics: How fast does a reaction occur? How do changes in reactant structure, reaction solvent, or reaction temperature speed up or slow down a reaction? 4.2 The Chlorination of Methane: A Case Study CH 4 + Cl 2 hv (photon) or ! (heat) CH 3 Cl + HCl + CH 2 Cl 2 + CHCl 3 + CCl 4 A B C D Polysubstituted Products Observations-usually a mixture of products forms, including not only mono-chlorinated product A , but also polychlorinated products B-D . 1. Light (or heat) is required to initiate the reaction (energy required) 2. Blue light, absorbed by Cl 2 , is most effective 3. High quantum yield: one photon can result in conversion of thousands of methane reactant molecules into product molecules Q: if light energy is needed, why isnt one photon needed for each reaction? ANY MECHANISM MUST BE CONSISTENT WITH EXPERIMENTAL OBSERVATIONS Chem 350 Jasperse Ch. 4 Handouts 2 4.3 The Mechanism: Radical Chain Reaction CH 4 + Cl 2 hv CH 3 Cl + HCl Balanced Reaction: The mechanism must show all bonds broken and made: Bonds Broken Bonds Made C-H Cl-Cl C-Cl H-Cl 3 Phases in Mechanism 1. Initiation (gets it started) 2. Propagation (keeps on going and going and going) 3. Termination (what happens when it sometimes stops) Initiation Cl Cl hv Cl (2 Cl ) "radical" something with an unpaired electron Cl + In a radical initiation step, two reactive radicals form from a nonradical precursor PROPAGATION H CH 3 Cl CH 3 H Cl + Cl Cl CH 3 Cl Cl CH 3 + P r o p a g a t i o n Step 1 Step 2 In each propagation step, one reactive radicals reacts with a nonradical to produce a new reactive radical and a new nonradical. Since a reactive radical is reproduced in each step, you always have another reactive radical ready to keep the chain going. The chlorine radical produced in step two is the same as the chlorine radical that reacts in step 1. Thus you can sustain a repeating chain of step 1- step 2 -step 1- step 2 - step 1- step 2 - step 1- step 2 - step 1- step 2 - step 1- step 2 - etc. -As long as there is a radical around, the chain will keep going The sum of the two propagation steps is the overall balanced reaction Termination Cl Cl Cl CH 3 H 3 C CH 3 Cl Cl + Cl + + CH 3 CH 3 H 3 C or or Chem 350 Jasperse Ch. 4 Handouts 3 Mechanism Notes: 1. Radical: Something with an unpaired electron....
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Ch 4 Handout Answers - Chem 350 Jasperse Ch. 4 Handouts 1...

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