CHEM 109A CLAS & Kinetics

Thermodynamics describes the equilibrium position and relative amounts of Rs and Ps. Answers the question ‘How much?’.

Equilibrium constant (K) Numerical expression for the relative concentrations (or pressures) of Rs and Ps at equilibrium.

K = [products]their coefficients for and Acid-Base rxn K = 10±∆pKa [reactants]their coefficients

if K > 1, Ps favored at equilibrium ([P] > [R]) K < 1, Rs favored at equilibrium ([P] < [R]) *The more stable the compound, the greater its concentration at equilibrium

Enthalpy (∆H, ∆Ho) ~energy at constant P

∆Ho = Σ∆Hoproducts - Σ∆Horeactants ~ ∆B.D.E. = ΣB.D.E. bonds broken - ΣB.D.E. bonds formed (Listed in Tbl 3.2, pg 146)

Exothermic reaction: -∆Ho, releases heat Endothermic reaction: +∆Ho, consumes heat

Entropy (∆S, ∆So) ~ freedom/disorder

When two reactants come together to form a single , there is a decrease in the disorder/freedom of the system and the is negative → at 25oC the T∆So term is -7 kcal/mol

If the disorder/freedom of the system increases, the sign on entropy is +

Gibbs Free Energy (∆G, ∆Go) ∆Go = Σ∆Goproducts - Σ∆Goreactants = -RTln(K) = ∆Ho - T∆So o ∆G 298K = -1.4log(K)

Exergonic reaction: -∆Go, releases energy Endergonic reaction: -∆Go, releases energy

*Formation of Ps w/ stronger bonds and greater freedom of motion causes -∆Go.

Ps thermodynamically stable if -∆Go unstable if +∆Go

Page 1 of 4 CHEM 109A CLAS Thermochemistry & Kinetics

Kinetics describes how quickly reactants become products (and vice versa). Answers the question ‘How fast?’.

Energy of Activation (∆G‡) barrier.

Smaller ∆G‡ means faster reaction, so destabilizing Rs (inc. G of Rs) or stabilizing transition state (T.S.) (dec. G of T.S.) will make reaction occur more quickly.

Rs thermodynamically stable if large ∆G‡ unstable if small ∆G‡

Rate of Reaction Speed at which Rs are used up (or Ps formed) Depends on 1. # of collisions between Rs per time 2. # of collisions w/ sufficient energy to overcome ∆G‡ 3. # of collisions that occur w/ proper orientation

Can increase rate of reaction by… Inc. [R] (inc #1) Inc. temperature (T) (inc. #1 and 2)

Units of the rate depend on the order of the reaction 1st Order: Rate depends only on concentration of one R. Rate = k[A] *smaller k, slower reaction

2nd Order: Rate depends on concentration of two Rs or one R2. Rate = k[A][B] or = k[A]2

Rate-Determining or Rate-Limiting Step Slowest step/rxn in the mechanism, determines the overall rate of reaction. In the reaction diagram (see below) it is the step that has a T.S. as the highest point (free energy) in the diagram.

Arrhenius Equation Rate constants (k) depend on temperature (T) and Ea (experimental energy of activation) k = Ae-Ea/RT (A is a frequency factor that represents #3) or lnk =lnA – Ea RT *An inc. in T of 10oC will double k & rate.

(If rate constants are measured at various temperatures, the Ea and ∆H‡, ∆G‡ and ∆S‡ as shown in box on pg 157 and Ch 3 #54)

Page 2 of 4 CHEM 109A CLAS Thermochemistry & Kinetics Use knowledge of thermodynamics (and kinetics) to draw… Energy Diagram (Free Energy Diagram or Rxn Coordinate Diagram) to pictorially show energy change taking place at each step of the reaction.

For single step endergonic (1) and exergonic (2) reactions: T.S

T.S P1 ‡ o o ‡ ∆G1 ∆G1 G ∆G2 [kcal/ mol] R o ∆G2

P2

Rxn progress

For multi-step reactions, write a diagram for each step and add them to get the overall reaction diagram

EX. Ch 3 #31. Draw a reaction coordinate diagram for a two-step reaction in which the first step is endergonic, the second step is exregonic and the overall reaction is endergonic. Label R, P, intermediates and T.S.

Page 3 of 4 CHEM 109A CLAS Thermochemistry & Kinetics T.S T.S o Go G [kcal [kcal P R /mol] /mol] Step 1 Step 2 P Endergonic Exergonic R

Rxn progress Rxn progress

T.S. T.S. Go [kcal/ Intermediate mol] P + ∆Go R

Rxn progress [time]

Try Ch3 #32…

Also try Ch3 #20-30

Aue also makes use of the Hammond Postulate and the Evans-Polanyi Principle to answer “why?” questions. The Hammond Postulate is discussed in your text and my explanation of both models can be found in ‘What is the EP Principle?’ handout.

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