WARM UP The rate constant for the 2nd order reaction 2NO2(g)  2NO(g) + O2(g) is 0.54/M·s at 300°C. How long (in s) would it take for the concentration of NO2 to decrease from 0.62 M to 0.28 M?

Reaction Mechanisms Chemistry II

Elementary Steps Reactions occur in a series of small steps Reaction mechanism is the sequence of these elementary steps Ex: 2 NO(g) + O2(g)  2 NO2(g) 2 NO  N2O2 N2O2 + O2  2 NO2

Elementary Steps Ex: Intermediates are formed and used (ex: N2O2) 2 NO(g) + O2(g)  2 NO2(g) 2 NO  N2O2 N2O2 + O2  2 NO2 Intermediates are formed and used (ex: N2O2) Catalysts enter and re-appear, while intermediates are produced and disappear

Elementary Steps Necessary because it is much more likely for molecules to meet 2-at-a-time in a chain, rather than 8-at-a-time all at once

Molecularity # of reacting molecules in an elementary step Unimolecular: one reactant Bimolecular: two reactants Termolecular: more than 2 (rare)

Rate Determining Step Elementary step that determines rate The slowest “car”

Rate Determining Step Concentration of reactants in rate-determining step gives us the rate law Ex: 2 NO(g) + O2(g)  2 NO2(g) 2 NO  N2O2 N2O2 + O2  2 NO2 If rate = k [NO], step 1 is rate-determining

WRAP UP If the rate law for the reaction, 2 NO(g) + O2(g)  2 NO2(g), is rate = k[NO][O2]3 , which is the rate-determining step? How do you know this? Step 1) 2 NO  N2O2 Step 2) N2O2 + O2  2 NO2 

WARM UP “It’s not the load that breaks you down – it’s the way you carry it.” Do you agree with this? Why? How does this apply to your own life?

WRAP UP Which type of elementary step is more common – bimolecular or termolecular? Why?