Chemistry Rate Versus Concentration From Experiment

TP For CH3 3CBr what is the value of 25% 25% 25% 25% 1. 2. 3. 4. ‒OCH ? 3  CH3 3COCH3 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 Br‒ CH3 3CBr ‒OCH 3 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 • Rate vs concentration from experiment • Making sense of rate vs concentration: Reaction mechanism • Making sense of rate constants: The Arrhenius relation Differential rate laws: rate vs concentration Rate versus concentration from experiment CH3 3CBr We know how to define rate. We also know that rate In liquid methanol, CH3OH, the following reaction occurs: concentration ‒OCH 3  CH3 3COCH3 Br‒ Write a rate law for this reaction. So, we need to learn just how rate depends on concentration and what determines the rate constant, . We’ll use data to learn how rate depends on concentration, the so‐called differential rate law. We’ll see that each reaction will have it own particular dependence. 1 (CH3)3CBr + OCH3  (CH3)3COCH3 + Br Differential rate laws: rate vs concentration In liquid methanol, CH3OH, the following reaction occurs: CH3 3CBr OCH3 Rate M/s 0.0001 0.0001 2.8 x 10‒5 How can we know whether the predicited rate law is correct? 0.0002 0.0001 5.6 x 10‒5 Measure rate for different combinations of reactant concentrations. 0.0001 0.0002 2.8 x 10‒5 CH3 3CBr ‒OCH 3  CH3 3COCH3 Br‒ CH3 3CBr , 1. 2. 3. 4. TP For CH3 3CBr ‒OCH3  CH3 3COCH3 what is the order in CH3 3CBr ? 1. 2. 3. 4. 1 2 Neither of the above More info needed 3 What is the order, , in CH3 3CBr? What is the order, , in ‒OCH3? What is the full differential rate law? What is the value of the forward rate constant TP For CH3 3CBr ‒OCH3  CH3 3COCH3 what is the order in ‒OCH3? Br‒ CH3 3CBr ‒OCH ‒OCH 3 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 1. 2. 3. 4. 1 2 Neither of the above More info needed ? Br‒ CH3 3CBr ‒OCH 3 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 2 TP For CH3 3CBr ‒OCH3  CH3 3COCH3 what is the full differential rate law? 25% 25% 25% 25% 1. 2. 3. 4. rate CH3 3CBr CH3 3CBr rate ‒OCH rate 3 Neither of the above ‒OCH Br‒ CH3 3CBr 3 0.0001 Quiz For CH3 3CBr what is the value of ‒OCH 3 Rate M/s 0.0001 2.8 x 10‒5 10‒5 0.0002 0.0001 5.6 x 0.0001 0.0002 2.8 x 10‒5 Making sense of rate vs concentration From data, we have found that the differential rate law for CH3 3CBr ‒OCH 3  CH3 3COCH3 Br‒ 1. 2. 3. 4. ‒OCH ? 3  CH3 3COCH3 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 Br‒ CH3 3CBr ‒OCH 3 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 Making sense of rate versus […] CH3 3CBr ‒OCH rate 3  CH3 3COCH3 Br‒ k CH3 3CBr What reaction mechanism could account for rate not depending on ‒OCH3 ? is rate CH3 3CBr Shouldn’t the rate depend on ‒OCH3 too? 3 (CH3)3CBr + ‒OCH3  (CH3)3COCH3 + Br‒ A possible mechanism that accounts for rate not depending on 1 CH3 3CBr  CH3 3C 2 CH3 3C ‒OCH 3 ‒OCH 3 Making sense of rate constants is … The mechanism is Br‒ , slow  CH3 3COCH3 , fast Show that this accounts for the observed rate law. 1 CH3 3CBr  CH3 3C 2 CH3 3C ‒OCH 3 Br‒, slow  CH3 3COCH3 , fast Reaction 1 is much slower than reaction 2 because … , is much smaller than , What determines the relative size of , and , ? Making sense of rate constants To understand what determines the relative size of rate constants, let’s begin by imagining how enthalpy changes as an exothermic reaction, such as NO2 CO  NO CO2 evolves as reactants, R, are transformed into products, P. Sketch a diagram showing how enthalpy changes vertical axis as time passes horizontal axis , starting with all reactants, R, and ending with all products, P. 4