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AP Chemistry Rates of Reaction Reaction Mechanisms A balanced equation gives us overall reaction results...at the molecular level, things may happen in several steps (elementary reactions) overall: NO2 (g) + CO (g) NO (g) + CO2 (g) elementary: NO2 + NO2 NO3 + NO2 elementary: NO3 + CO NO2 + CO2 NO3 = reaction intermediate! Not in the overall reaction (must cancel out). Usually cannot be isolated, but may be proven to exist through spectrum or other analysis tool. Reaction mechanism: a set of elementary reactions that together give the net overall reaction. Carbon tetrachloride, CCl4, is obtained by chlorinating methane or an incompletely chlorinated methane, like chloroform, CHCl3. The mechanism for the gasphase chlorination of CHCl3 is: What is the net, overall reaction? Cl2 Cl + CHCl3 Cl + CCl3 Cl2 + CHCl3 13.713.9 2 Cl HCl + CCl3 CCl4 HCl + CCl4 AP Chemistry Rates of Reaction Molecularity: the number of reactant molecules in an elementary reaction. unimolecular: Cl2 bimolecular: trimolecular: 2 Cl Cl + CHCl3 HCl + CCl3 Br + Br + Ar Br2 + Ar* Rate Law for an elementary reaction: proportional to the product of the concentration of each reactant molecule Cl2 2 Cl Rate = k [Cl2] Cl + CHCl3 HCl + CCl3 Rate = k [Cl][CHCl3] Br + Cl + Ar BrCl + Ar* Rate = k [Br][Cl][Ar] *Remember, the rate law MUST be obtained experimentally for the overall reaction! 13.713.9 AP Chemistry Rates of Reaction The Rate Law and the Mechanism The mechanism of a reaction cannot be observed directly...it is proposed to explain experimental observations...like a detective's proposal to explain a crime from clues left behind...a number can be postulated, and they may be accepted, but it is understood that further experiments may lead to a more accepted mechanism...finding the dominant reaction pathway is a central activity in Chemistry...The Scientific Method in action! The mechanism and the Rate Law must agree! The Rate Law and the Mechanism 2 NO2 (g) + F2 (g) 2 NO2F (g) Experimental Rate Law: Rate = k [NO2] [F2] Assume one elementary reaction based on Balanced Equation: NO2 + NO2 + F2 NO2F + NO2F Rate = k [NO2]2 [F2] This does not agree with experiment! Predicted Rate Law: The reaction must occur in multiple steps! 13.713.9 AP Chemistry Rates of Reaction The RateDetermining Step 2 NO2 (g) + F2 (g) 2 NO2F (g) Reaction is believed to occur in the following way: NO2 + F2 NO2F + F F + NO2 NO2F (slow step) (fast step) 2 NO2 + F2 2 NO2F The ratedetermining step is the slow step in the reaction mechanism Rate = k [NO2] [F2] *matches experimentation! *Mechanisms with an Initial Fast Step Situation when the ratedetermining step follows a fast, equilibrium step 2 NO (g) + 2 H2 (g) N2 (g) + 2 H2O (g) Reaction is believed to occur in the following way: 2 NO N2O2 (fast, equilibrium) N2O2 + H2 N2O + H2O N2O + H2 N2 + H2O (slow) (fast) According to the ratedetermining step: Rate = k [N2O2] [H2] NOT POSSIBLE! N2O2 is an intermediate!!! Since: 2 NO N2O2 Then: [N2O2] = [NO]2 Rate = k [NO]2 [H2] *Confirmed experimentally! 13.713.9 AP Chemistry Rates of Reaction Catalysis! The increase in the rate of reaction resulting from the use of a catalyst: speeds up a reaction without being consumed Benefit: can run reactions at lower temperatures (lower energy costs), and catalysts are often very specific only working with certain reactions Biological catalysts: Enzymes How? catalysts lower the Ea of the reaction exponent in Arrhenius equation!) The catalyst must be involved with one step of the reaction mechanism and removed in another step Cl O3 (g) + O (g) 2 O 2 (g) O3 (g) + Cl (g) ClO (g) + O2 (g) ClO (g) + O (g) Cl (g) + O2 (g) pg. 561 13.713.9 Chlorine comes from decomp of CFCs AP Chemistry Rates of Reaction Homogeneous catalysis: The catalyst is in the same phase as the reacting species gasgasÍž aqueousaqueous Heterogeneous catalysis: The catalyst is in a different phase as the reacting species solid catalyst in gaseous or liquid solution Heterogeneous catalysis: Also known as surface catalysis occurs on the surface of the solid through adsorption catalytic hydrogenation on the surface of Platinum 13.713.9 AP Chemistry Rates of Reaction Enzymes: Most enzymes are protein molecules with molecular masses over a million amu converting a thousand or so reactant molecules to products in a second! Also, they are very specific, with each enzyme acting only on a specific substance Lactase acts on Lactose "LockandKey" Model substrate substance the enzyme catalyzes Again, the formation of the enzyme How? substrate complex provides a new reaction pathway with a lower Ea 13.713.9
