Presentation on theme: "Ch. 17/18 Reaction Kinetics & Chemical Equilibrium"— Presentation transcript:
1Ch. 17/18 Reaction Kinetics & Chemical Equilibrium
2Reaction MechanismsReaction 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
3Collision TheoryCollision Theory: the set of assumptions regarding collisions and reactions.* Molecules must collide with sufficient force and correct orientation to produce a rxn.17-3
4Activation EnergyActivation 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
5Activation Energy17-4a) 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
6Reaction RateReaction 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
7Rate-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
8Reaction RatesHeterogeneous 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
9Rate LawsRate Law: an equation that relates reaction rate and concentrations of reactants.R = kAnBmR = rate(lower case) k = specific rate constantA = molar concentration of reactant An = the power to which the concentration of A is raised.17-9
11EquilibriumReversible 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
12EquilibriumA + B →← C + DInitially, 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
13Equilibrium nA + mB →← xC + yD Eq. constant (capital K) is equal to:K = Cx DyAn BmWith 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)
14Shifting EquilibriumLe 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
15Stresses to an Equilibrium Changes in Pressure: the rxn will shift to the side that has less number of particles.Ex. N2(g) + 3H2(g) ↔ 2NH3(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.
16Reactions that go to Completion Reactions run to completion when reacting ions are almost completely removed from solution.Formation of a gas: rxns that form a gas as a product go to completion, gases are frequently released during a rxn.Formation of a percipitate (PPT): product is usually an insoluble solid that can not reform the reactants.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.