Draw a Circle Draw a Star Draw a Smiley Face PRINT your full name (middle name too) Write 5 X’s Draw a pac-man Fold the paper in half and make a pile.

Reaction Mechanisms

Collision theory Collision between 2 particles is necessary for a reaction to occur One particle can decompose Three particles colliding with sufficient E and correct orientation is… HIGHLY unlikely

Reaction Mechanism Reactions occur by a series of steps called a Example: Overall reaction: NO2 + CO  NO + CO2 occurs by following steps Step 1: Step 2: N- blue, C- green O - Red

Intermediates Two molecules of NO2 collide in step 1 Oxygen is transferred, making NO3, the intermediate Intermediates are temporarily formed during a reaction They are neither a reactant nor a product Get used up in reaction Concept check! How does an intermediate differ from a catalyst?

Rules for Reaction Mechanisms Sum of elementary steps must equal overall balanced reaction. Mechanism must agree with experimental rate law

Elementary Step Steps in reaction from which a rate law can be directly written In step 1, 2 molecules of NO2 need to collide, therefore… Rate = k [NO2]2

Molecularity Rate law written based on molecularity Molecularity equals the number of things that have to collide Unimolecular – rxn depends on 1 molecule Bimolecular – rxn depends on 2 molecules Termolecular – rxn depends on 3 molecules Very rare!

Molecularity and rate law: Unimolecular (first order) rate=k[A] Bimolecular (second order) rate=k[A][B]

Rate Determining Step The slowest step in mechanism determines overall rate Rate cannot be faster than slowest step Overall rate law can be written from molecularity of slowest step Like an assembly line

How are mechanisms determined? Rate law is determined using experiment (method of initial rates, etc.) Chemist uses intuition to come up w/ various mechanisms Narrows down choices using rules for mechanisms No mechanism is ever absolutely proven

Practice problem 1 The rxn: 2NO2 (g) + F2 (g)  2NO2F (g) Experimental rate law: rate = k[NO2][F2] Suggested mechanism: NO2 + F2  NO2F + F Slow F + NO2  NO2F Fast Is this an acceptable mechanism? That is does it satisfy the two requirements?

Practice problem 2 The rxn: 2NO (g) + 2H2 (g)  N2 (g) + 2H2O (g) Experimental rate law: rate = k[NO]2[H2] Suggested mechanism: H2 +NO  ZZ Fast ZZ + NO  BB + H2O Slow BB + H2  N2 + H2O Fast Is this an acceptable mechanism? That is does it satisfy the two requirements?