Reaction Rates: 2 NO2  2 NO + O2 change in conc. 1. slope = change in time 1. slope = 2. consumption of reactants per time –[NO2] t concentration (M) [NO2] 3. production of products per time +[NO] t t 4. Are equalized to a stoich. coefficient of 1 2 NO –[NO2] 2 t = [NO] 2 t = [O2] t 1 O2 5. Slow down during rxn (reactant collisions decrease) time (t)

minimum E required to start reaction Collision Model reactant bonds break, then product bonds form successful reactants Collision products Reaction rates depend on collisions between reactant particles by: collision frequency collision energy proper orientation activation energy: minimum E required to start reaction (Eact ) unsuccessful

Energy Profile Diagrams ENDOthermic EXOthermic transition state transition state Ea P R Ea DH DH (or DE) R P (or DE)

4 Factors That Affect Reaction Rates: Concentration Temperature (exposed) Surface Area (particle size) Catalyst ↑Conc. , ↑collision Frequency ↑Temp , ↑k ↑collision Frequency ↑collision Energy Eact (more particles over Eact) ↓size, ↑surface area, ↑collision Frequency ↑ rate by ↓Eact by… Catalyzed …changing the mechanism energy (intermediate) consumed, then produced

Reaction Mechanisms mechanism: Step 1: NO2 + NO2  NO3 + NO (slow) Step 2: NO3 + CO  NO2 + CO2 (fast) NO2 (g) + CO (g)  NO (g) + CO2 (g) Stoich of mech. and overall rxn must be consistent The overall reaction occurs only as fast as the slowest, rate-determining step. (RDS) Rate law depends on reactants in the slow step: rate = k [NO2]2 A reactant NOT in the slow RDS is 0th order [CO]0 Intermediate (NO3) is produced then consumed (must be substituted out of rate law) 5

Orders in Rate Laws …only found experimentally (from data). …do NOT come from the coefficients of reactants of an overall reaction. …represent the number of reactant particles (coefficients) in the RDS of the mechanism. …zero order reactants have no effect on rate b/c they do not appear in the RDS of the mechanism (coefficient of 0 in RDS). …typically 0, 1, 2, but can be any # or fraction

Half-life (t1/2): time at which half of initial amount remains. [A]t = 0.5 [A]0 initial concentration at time, t = 0 given on exam concentration at time, t = t1/2 0.693 k Half-life (t1/2) is constant for 1st order only. 7

Units of k M∙s–1 s–1 M–1∙s–1 Summary 0th Order 1st Order 2nd Order [A] Rate Law Rate = k Rate = k[A] Rate = k[A]2 Integrated Rate Law [A] = –kt + [A]0 ln[A] = –kt + ln[A]0 Linear plot [A] ln[A] slope of line Slope = rate Slope = –k Slope = k M∙s–1 s–1 M–1∙s–1 Half-Life depends on [A]0 y = mx + b y = mx + b 1 [A] t t t M = ? s M = ?∙M s M = ?∙M2 s Units of k