Reaction Mechanism

Rate-Determining Step

Identifying the Rate-Determining Step

Identifying Intermediates

• THE PHYSICAL SIGNIFICANCE OF RATE EXPRESSIONS FOR ELEMENTARY STEPS RATE EXPRESSIONS FOR ELEMENTARY STEPS--the rate expression cannot be predicted from overall stoichiometry. The rate expression of an elementary step is given by the product of the rate constant and the concentrations of the reactants in the step. ELEMENTARY STEP MOLECULARITY RATE EXPRESSION A→ products unimolecular rate = k[A] A + B → products bimolecular rate =k[A][B] A + A → products rate =k[A]2 2 A + B → products* termolecular* rate = k[A]2[B] • THE PHYSICAL SIGNIFICANCE OF RATE EXPRESSIONS FOR ELEMENTARY STEPS - the more molecules the more collisions, the faster the rate - the faster the molecules are moving, the more likely they will collide, the faster the rate • MOLECULARITY AND ORDER - an elementary step is a reaction whose rate law can be written from its molecularity - NOT true of the overall reaction order!

Nitrogen oxide is reduced by hydrogen to give water and nitrogen, 2 H2(g) + 2 NO(g) → N2(g) + 2 H2O(g) and one possible mechanism to account for this reaction is 2 NO(g) → N2O2(g) (fast) N2O2(g) + H2(g) → N2O(g) + H2O(g) (slow) N2O(g) + H2(g) → N2(g) + H2O(g) (fast) What is the molecularity of each of the three steps? Show that the sum of these elementary steps is the net reaction. Step 1 bimolecularity Step 2 bimolecularity Step 3 bimolecularity Write the rate expression for the following mechanism and is this a valid expression? 2 NO(g) → N2O2(g) (fast) experimental rate law rate = k[NO]2[H2] N2O2(g) + H2(g) → N2O(g) + H2O(g) (slow) N2O(g) + H2(g) → N2(g) + H2O(g) (fast) Overall rxn stoichiometry: 2 H2(g) + 2 NO(g) → N2(g) + 2 H2O(g) Rate = k1[NO]2 Rate = k2[H2] Overall rate = k[NO]2[H2] This is a valid mechanism since it agrees with the stoichiometry and the experimental rate law

Collision Model

Collision Model

Factors Affecting Rate