Understanding Reaction Mechanisms and Rate Laws

1. Reaction Mechanisms: Reaction is broken into steps with intermediates being formed. “some RXNS occur in one step, but most occur in in multiple steps.” Each Step is called an elementary step, and the number of molecules involved in each step defines the molecularity of the step. unimolecular: = 1 i.e. O3* O2 + O bimolecular: = 2 (these are the most common) i.e. HI + HI  activated complex  H2 + I2 termolecular: = 3 (rare, due to probability of orientation and energy both being correct.) i.e. O(g) + O2(g) + N2(g)  O3(g) + “energetic” N2(g)

2. The Raschig process for the preparation of hydrazine (N2H4) Overall RXN: 2NH3(g) + NaOCl(aq)  N2H4(aq) + NaCl(aq) + H2O(l) Proposed Mechanism: (Only from experiment) Step 1: NH3(aq) + OCl-(aq)  NH2Cl(aq) + OH-(aq) NH2Cl (aq) + NH3(aq)  N2H5+ + Cl-(aq) Step 2: N2H5+(aq) + OH-(aq)  N2H4(aq) + H2O(l) Step 3: “Cancel intermediates and “add steps” to give overall RXN.” 2NH3(g) + OCl-(aq)  N2H4(aq) + Cl-(aq) + H2O(l) The overall rate law, mechanism, and the total order can’t be predicted from the stoichiometry, only by experiment.

3. The following is only true for individual steps: The rate lawof an elementary step is given by the product of a rate constant and the conc. of the reactants in the step. Step Molecularityrate law uni rate = k[A] A  Product(s) rate = k[A][B] bi A + B  Product(s) rate = k[A]2 bi A + A  Product(s) rate = k[A]2[B] ter 2A + B  Product(s) The overall mechanism must match the observed rate law. Usually one STEP is assumed to be the rate determining step.

4. Example: 2NO2(g) + F2(g)  2NO2F(g) Overall RXN: Observed Experimental rate law: rate = k[NO2][F2] Question: Why does this rule out a single step RXN? Answer: rate law for single step process would be: rate = k[NO2]2[F2] “Let’s try to work out a Mechanism that matches the observed rate law.”