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Ch. 17/18 Reaction Kinetics & Chemical Equilibrium.

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Presentation on theme: "Ch. 17/18 Reaction Kinetics & Chemical Equilibrium."— Presentation transcript:

1 Ch. 17/18 Reaction Kinetics & Chemical Equilibrium

2 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

3 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

4 Activation Energy Activation Energy (E a ): 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

5 Activation Energy 17-4 a) E a : energy difference between activated complex and reactants. b) E a ’: energy difference between activated complex and products. c) ΔE: energy difference between reactants and products ΔE=E p -E r

6 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

7 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

8 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

9 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

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

11 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

12 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

13 Equilibrium nA + mB → ← xC + yD 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)

14 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

15 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) + 3H 2 (g) ↔ 2NH 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.

16 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.

17 Ch. 17/18 The End 17-18


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