Complete Rate vs Concentration From Experiment

[TP] When 25% 25% 25% 25% 1. 2. 3. 4. is increased, the rate of every chemical reaction must... • • • • increase stay the same decrease More information needed Complete: Rate vs concentration from experiment Making sense of rate vs concentration: Reaction mechanism Making sense of rate constants: The Arrhenius relation and . Rate versus temperature: 1 [TP] For (CH3)3CBr + ‒OCH3 → (CH3)3COCH3 + Br‒ what is the order in ‒OCH3? 1. 2. 3. 4. 1 2 Neither of the above More info needed [TP] For (CH3)3CBr + ‒OCH3 → (CH3)3COCH3 + Br‒ what is the full differential rate law? [(CH3)3CBr] [‒OCH3] Rate (M/ ) 0.0001 0.0001 2.8 x 10‒5 0.0002 0.0001 5.6 x 10‒5 0.0001 0.0002 2.8 x 10‒5 11 1. 2. 3. 4. rate = [(CH3)3CBr] [‒OCH3] [(CH3)3CBr] rate = [‒OCH3] rate = Neither of the above [(CH3)3CBr] [‒OCH3] 0.0001 0.0001 Rate (M/s) 2.8 x 10‒5 0.0002 0.0001 5.6 x 10‒5 0.0001 0.0002 2.8 x 10‒5 12 1 [Quiz] For (CH3)3CBr + ‒OCH3 → (CH3)3COCH3 + Br‒ what is the value of ? 1. 2. 3. 4. 2.8 x 10‒5 M s‒1 2.8 x 10‒1 s‒1 2.8 x 10‒1 M s‒1 None of the above Making sense of rate vs concentration [(CH3)3CBr] [‒OCH3] Rate (M/s) 0.0001 0.0001 2.8 x 10‒5 0.0002 0.0001 5.6 x 10‒5 0.0001 0.0002 2.8 x 10‒5 From data, we have found that the differential rate law for (CH3)3CBr + ‒OCH3 → (CH3)3COCH3 + Br‒ is rate = [(CH3)3CBr] Shouldn’t the rate depend on [‒OCH3] too? 13 15 (CH3)3CBr + ‒OCH3 → (CH3)3COCH3 + Br‒ Making sense of rate constants A possible mechanism that accounts for rate not depending on [‒OCH3] is … The mechanism is Step 1: (CH3)3CBr → (CH3)3 C+ + Br‒ Step 1: (CH3)3CBr → (CH3)3C+ + Br‒, slow , slow Step 2: (CH3)3C+ + ‒OCH3 → (CH3)3COCH3 , fast Step 2: (CH3)3C+ + ‒OCH3 → (CH3)3COCH3 , fast Step 1 is much slower than step 2 because … Show that this accounts for the observed rate law. , is much smaller than , What determines the relative size of 17 , and , ? 27 2 Making sense of rate constants Making sense of rate constants To understand what determines the relative size of rate constants, let’s consider a different, gas-phase exothermic reaction Sketch a two-dimensional diagram showing how enthalpy changes (vertical axis) as time passes (horizontal axis), starting with all reactants, R, on the eft, and ending with all products, P, on the right. NO2( ) + CO( ) → NO( ) + CO2( ) Take into account that sinxw the reaction involves both bond breaking in reactants and bond making in products, energy must be supplied for both R → P and P → R. 28 29 NO2(g) + CO(g) → NO(g) + CO2(g) NO2(g) + CO(g) → NO(g) + CO2(g) Where does the energy come from for R → P? Where does the energy come from for R → P? Kinetic energy of reactant molecules. Kinetic energy of reactant molecules. 30 31 3 NO2(g) + CO(g) → NO(g) + CO2(g) NO2(g) + CO(g) → NO(g) + CO2(g) Only molecules with kinetic energy greater than the activation energy, react. , can Only molecules with kinetic energy greater than can react Raising temperature increases the number with at least kinetic energy 32 33 NO2(g) + CO(g) → NO(g) + CO2(g) NO2(g) + CO(g) → NO(g) + CO2(g) Only molecules with kinetic energy greater than the activation energy can react. What is fraction exp − exp − Answer: 4 × 10 =… Fraction of molecules with minimum kinetic energy reaction to take place (always less than 1) for = 100 kJ/mol and = 300 K? (really small!) required for the 34 35 4 NO2(g) + CO(g) → NO(g) + CO2(g) NO2(g) + CO(g) → NO(g) + CO2(g) What is fraction exp − What role does the relative orientation of the colliding molecules (collision geometry) play? for = 100 kJ/mol and = 5000 K? Answer: 0.09 (much bigger!) 36 NO2(g) + CO(g) → NO(g) + CO2(g) 38 Putting geometry and temperature together Collisions must have the correct geometry: = exp − =… frequency of collisions × … probability that collisions have an appropriate collision geometry exp − =… Fraction of molecules with minimum kinetic energy (always less than 1) 39 for reaction 40 5 [TP] When Rate versus temperature: 0% 0% 0% 0% and 1. 2. 3. 4. is increased, the rate of every chemical reaction must... increase stay the same decrease More information needed 41 [TP] Rate constants depend on The value of at = 0 is ... 0% 1. 0 0% 2. /( ) 0% 3. 0% 4. ∞ as = exp[− /( )]. 42 [Quiz] Rate constants depend on The value of at = ∞ is ... 0% 1. 0 0% 2. /( ) 0% 3. 0% 4. ∞ 44 as = exp[− /( )]. 45 6