CHEM 108 1st Edition Lecture 13 Current LectureChapter 14: Chemical Kinetics Continued:Reaction Mechanisms:- Reaction Mechanism: proposed set of steps that describe how bonds in reactant molecules break and bonds in the product molecules form. - Elementary Step: molecular-level view of a single process in a chemical reaction. - Intermediate: a species produced in one step of a reaction mechanism and consumed in a later step- Molecularity: the number of reactant ions, atoms, or molecules involved in an elementary step.- Consider the reaction: 2 NO2 (g) → 2 NO(g) + O2 (g) - Proposed mechanism: o Elementary Step 1: 2 NO2 (g) → NO(g) + NO3 (g) o Elementary Step 2: NO3 (g) → NO(g) + O2 (g) o Adding these two equations should yield to the overall reaction: 2 NO2 (g) → 2 NO(g) + O2 (g)o Any mechanism for a reaction must be consistent with the Rate law of that reaction. o Rate1 = k1[NO2]2o Rate2 = k2[NO3]2o Rate law depends on rate determining step (r.s.d.), the slowest elementary step in the mechanism.oo Rate law for the overall reaction: Rate = koverall[NO]2[O2]These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. koverall= k2(kfoward/kreverse)- Multiple step mechanisms NO2 (g) + NO2 (g) NO3 (g) + NO(g) NO3 (g) + CO(g) NO2 (g) + CO2 (g) - Add steps in mechanism to get overall rxn NO2 (g) + CO(g) NO(g) + CO2 (g)- Criteria for valid mechanisms:o Sum of elementary steps must be consistent with the stoichiometry of the overall reaction. o Rate law based on molecularity of rate determining step must be consistent with rate law derived from experimental data.Catalyst: - Is a substance added to a reaction that increases the rate of the reaction but it is not consumed. - Catalyst does not appear in overall reaction it appears as reactant and product, not as intermediate - Forward and reverse rates are increased in the same proportion and concentrations at equilibrium are not affected - Homogeneous catalyst: Same phase as reacting species. - Heterogeneous catalyst: Different phase than reacting species.- Ex: mechanism for depletion of ozone in atmosphere: o Cl + O3 → ClO + O2 ClO + O3 → Cl + 2O2 o Chlorine atom is homogenous catalyst and Chlorine atoms appear as both reactant and product in reaction mechanism, but not in overall reaction- Mechanism of Catalysis o Catalysts act by: a decrease the activation energy o An increase the number of effective collisions o Catalysts do not affect the initial energy of reactants or the final energy of products.Enzymes: - Biological catalysts. Most enzymes are protein molecules with large molar masses (104 to 106 g/mol), and mostly catalyze very specific reactions- Substrate: reactant that binds to enzyme o Active site: place on enzyme that substrate reacts o Lock and key mechanism substrate fits into active site like key in lock o Reaction catalyzed at active site products move away from active
View Full Document
Unlocking...