Kinetics Mechanism, Catalysts, and activation Energy

Reaction Mechanisms A believable mechanism must fulfill two criteria It will match the experimentally determined rate law. The elementary steps will add up to the overall equation.

Molecularity Elementary Step Reaction Elementary StepMolecularityRate expression [A]  productunimolecularrate = k[A] [A] + [B]  productsbimolecularrate = k[A] [B] [A] + [A] = productsbimolecularrate = k[A] 2 2[A] + [B]  productstermolecularrate = k[A] 2 [B]  An elementary step is a reaction whose rate law can be written from its molecularity  NOT true of the overall reaction order

2 H 2(g) + 2 NO (g)  N 2(g) + 2H 2 O (g) Possible mechanism 2 NO (g)  N 2 O 2(g) N 2 O 2(g) + H 2(g)  N 2 O (g) + H 2 O (g) N 2 O (g) + H 2(g)  N 2(g) + H 2 O (g )

Reaction Mechanism and Rate Expressions The rate of the overall reaction is limited by, and is exactly equal to, the combined rates of all elementary steps up to and including the slowest step in the mechanism. Intermediate-substance produced in one step but consumed in another. Catalyst-goes, in, comes out unchanged and DOES NOT show up in the final reaction. For prior example: If the experimental rate expression is: rate = k [NO] 2 Then the rate determining step is step one.

STOP!!!!!

Catalysts Alter mechanism to lower activation energy Do not change during the reaction Offer a different pathway Adsorption-collection of one substance on the surface of another Absorption-penetration of one substance into another

Heterogeneous catalyst -different phase than reactants How does it work? 1. Adsorption and activation of the reactants 2. Migration of the adsorbed reactants on the surface 3. Reaction of the adsorbed substances 4. Escape, or desorption, of the products

How a catalyst works…. Increases effective collisions by lowering activation energy required.

Activation energy- threshold energy that must be overcome to produce a chemical reaction Transition state = activated complex – Describe unstable collection of atoms at the top of the energy hump Taller the hump, larger the E a, smaller the rate constant

H 2 O 2 + Br -1  BrO -1 + H 2 O H 2 O + BrO -1  Br -1 + H 2 O + O 2 Overall reaction? Catalysts? Intermediates?

Relationship between kinetics and thermodynamics Reaction Coordinate (time) X 2 + Y 2 2 XY E activation -- a kinetic quantity  E  H --a thermodynamic quantity -endothermic—products end up higher in energy than reactants  heat is absorbed or taken into or added to the system and given a positive sign. -exothermic—pictured above, products are of lower energy than reactants  heat is lost to the surroundings and given a negative sign. -endothermic—products end up higher in energy than reactants  heat is absorbed or taken into or added to the system and given a positive sign. -exothermic—pictured above, products are of lower energy than reactants  heat is lost to the surroundings and given a negative sign.

Arrhenius Equation ln k = -E a 1 + ln A R T y = m x + b

Used to calculate -value of activation energy from temperature dependence of the rate constant -rate constant for a given temp - if the E* [also known as E a ] and A factor are known.

slope = -E a R E a = -R x slope E a = activation energy R = 8.31 J / K mol T = temperature in Kelvin 1 / T (1 / K) ln k

Points to remember!! 1. Ea is smaller; k is greater; the reaction is faster. 2. Ea is greater; k is smaller; the reaction is slower.