Ch 1718 Reaction Kinetics Chemical Equilibrium Reaction Mechanisms

Ch. 17/18 Reaction Kinetics & Chemical Equilibrium

Reaction Mechanisms • Reaction Mechanism: the step-by-step sequence of reactions by which the overall chemical change occurs. • Intermediates: species that appear in some steps but not in the net equation. • Homogeneous Rxn: a rxn whose reactants and products exist in a single phase. 17 -2

Collision Theory • Collision Theory: the set of assumptions regarding collisions and reactions. * Molecules must collide with sufficient force and correct orientation to produce a rxn. 17 -3

Activation Energy • Activation Energy (Ea): the minimum energy required to transform the reactants into an activated complex. • Activated Complex: a transitional structure that results from an effective collision and that persists while old bonds are breaking and new bonds are forming. • ΔE (delta E) or change in energy: for the forward reaction, energy change is E of products – E of reactants. 17 -4

Activation Energy 17 -4 a) Ea: energy difference between activated complex and reactants. b) Ea’: energy difference between activated complex and products. c) ΔE: energy difference between reactants and products ΔE=Ep-Er

Reaction Rate • Reaction Rate: the change in concentration of reactants per unit time as a reaction proceeds. • Chemical Kinetics: the area of chemistry that is concerned with reaction rates and reaction mechanisms. 17 -6

Rate-Influencing Factors • The Nature of the Reactants. • Surface Area. • Temperature (temp increase favors endothermic rxns) • Concentration. • Presence of Catalysts. * # of collisions, which is affected by the above. 17 -7

Reaction Rates • Heterogeneous Reactions: involve reactants in two different phases. • Catalyst: a substance that changes the rate of a chemical rxn without itself being permanently consumed. • Homogeneous Catalyst: a catalyst in the same phase as the reactants and products in a rxn system. • Heterogeneous Catalyst: a catalyst in a different phase from the reactants. 17 -8

Rate Laws • Rate Law: an equation that relates reaction rate and concentrations of reactants. R = k A n B m R = rate (lower case) k = specific rate constant A = molar concentration of reactant A n = the power to which the concentration of A is raised. 17 -9

Reaction Rates • Rate Determining Step: the slowest rate step. • Exothermic Rxn: Releases energy. (feels hot) • Endothermic Rxn: Absorbs energy. (feels cold)

Equilibrium • Reversible Rxn: a chemical rxn in which the products can react to re-form the reactants. • Chemical Equilibrium: when the rate of the forward rxn equals the rate of the reverse rxn and the concentrations of the products and reactants remain unchanged. 17 -11

Equilibrium A + B →← C + D Initially, concentrations of C and D are zero, A and B are maximum. As the rxns progress, A and B are consumed, C and D are formed until eq. is established. 17 -12

Equilibrium n. A + m. B →← x. C + y. D Eq. constant (capital K) is equal to: K = C x D y A n B m With C being the concentration in mol/L. Equilibrium Constant: the ratio of the mathematical product of the concentration of substances formed at eq. to the mathematical product of the concentrations of reacting substances. Each concentration is raised to a power equal to the coefficient of that substance. (Pure solids and liquids can not change concentration and are not a factor in K)

Shifting Equilibrium • Le Chatelier’s Principle: if a system at eq. is subjected to a stress, the eq. is shifted in the direction that tends to relieve that stress. . 17 -14

Stresses to an Equilibrium 1. Changes in Pressure: the rxn will shift to the side that has less number of particles. Ex. N 2(g) + 3 H 2(g) ↔ 2 NH 3(g) 2. Changes in Concentration: A + B ↔ C + D an increase in the concentration of A or B will push the rxn towards product, increase C or D and the reactants will form faster. 3. Changes in Temperature: an increase in temp. will shift eq. toward the endothermic rxn, a decrease in temp. will shift toward the exothermic rxn.

Reactions that go to Completion • Reactions run to completion when reacting ions are almost completely removed from solution. 1. Formation of a gas: rxns that form a gas as a product go to completion, gases are frequently released during a rxn. 2. Formation of a percipitate (PPT): product is usually an insoluble solid that can not reform the reactants. 3. Formation of a slightly ionized product: when molecules are the product, ions are taken out of solution. • Spectator Ions: don’t take part in a chemical rxn, are present as a reactant and a product.

Ch. 17/18 The End 17 -18