Reaction Mechanisms Reaction is broken into steps with
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. O 3* O 2 + O bimolecular: = 2 (these are the most common) i. e. HI + HI activated complex H 2 + I 2 termolecular: = 3 (rare, due to probability of orientation and energy both being correct. ) i. e. O(g) + O 2(g) + N 2(g) O 3(g) + “energetic” N 2(g)1
The Raschig process for the preparation of hydrazine (N 2 H 4) Overall RXN: 2 NH 3(g) + Na. OCl(aq) N 2 H 4(aq) + Na. Cl(aq) + H 2 O(l) Proposed Mechanism: (Only from experiment) Step 1: NH 3(aq) + OCl-(aq) NH 2 Cl(aq) + OH-(aq) Step 2: NH Cl (aq) + NH (aq) N H + + Cl-(aq) 2 3 2 5 Step 3: N 2 H 5+(aq) + OH-(aq) N 2 H 4(aq) + H 2 O(l) “Cancel intermediates and “add steps” to give overall RXN. ” 2 NH 3(g) + OCl-(aq) N 2 H 4(aq) + Cl-(aq) + H 2 O(l) The overall rate law, mechanism, and the total order can’t be predicted from the stoichiometry, only by experiment. 2
The following is only true for individual steps: The rate law of an elementary step is given by the product of a rate constant and the conc. of the reactants in the step. Step A Product(s) Molecularity uni rate law rate = k[A] A + B Product(s) bi rate = k[A][B] A + A Product(s) bi rate = k[A]2 ter rate = k[A]2[B] 2 A + B Product(s) The overall mechanism must match the observed rate law. Usually one STEP is assumed to be the rate determining step. 3
Example: Overall RXN: 2 NO 2(g) + F 2(g) 2 NO 2 F(g) Observed Experimental rate law: rate = k[NO 2][F 2] Question: Why does this rule out a single step RXN? Answer: rate law for single step process would be: rate = k[NO 2]2[F 2] “Let’s try to work out a Mechanism that matches the observed rate law. ” 4
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