2. First lab: Measuring kinetics of Methyl Orange/Sn(II) reaction Sn 2+ + 4H + + MeO - > Sn 4+ + other products orange colorless Tricks: reduce the rxn order by performing it with large excess of H + and Sn 2+ -> determine m:
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4. Reaction mechanisms: How does it happen? Why rate laws are what they are? Mechanism: a series of elementary rxns Elementary rxn: a reaction that occurs in one step
5. Example: hydrogen combustion 2H 2 + O 2 -> 2H 2 O H= -242 kJ/mol Exotermic rxn, releases a lot of energy. Also, has inherit tendency to go, i.e., spontaneous (thermodynamcis) Is it fast at 300 K? Mechanism: chain reaction Initialization: H 2 - > 2H O 2 - > 2O Propagation: H+O 2 - > OH + O OH+H 2 -> H 2 O + H O+H 2 - > OH + H ........... Termination: H+OH - > H 2 O H+H -> H 2 ............ New concepts: radicals, rxn intermediates
6. Reaction mechanisms: How does it happen? Why rate laws are what they are? Mechanism: a series of elementary rxns Elementary rxn: a reaction that occurs in one step Examples: (a) O 3 + NO - > NO 2 + O 2 (b) CO + NO 2 -> NO +CO 2 WHY????
7. It happens in two steps: NO 2 +NO 2 -> NO 3 + NO slow NO 3 +CO -> NO 2 + CO 2 fast Overall: CO + NO 2 -> NO +CO 2 NO 3 is produced slowly, and immediately consumed. This is an intermediate. Rate determining step: the slowest rxn
8. Rate-determining step: the slowest rxn Auto-plaza analogy: a) Bay bridge model: b) San Diego toll freeway #73: pay & get ticket drive trough the bridge pay on exit get in and drive
9. Molecularity of an elementary rxn: smallest number of species that must collide to produce a product Unimolecular: 1 e.g., decomposition H 2 CO - > H 2 + CO Bimolecular: 2 e.g., NO + O 3 - > NO 2 +O 2 Termolecular: 3 e.g., C + 2CO -> C 3 O 2 Order of an elementary rxn - same as its molecularity Rate law for an elementary rxn - from stoichiometry
13. Example: O + NO 2 -> NO + O 2 fast NO + CO 2 -> NO 2 + CO slow Overall: O + CO 2 -> O 2 + CO Molecularity and the rate expressions? Rate determining step? What's NO 2 ? What's NO?
