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.
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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.
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4. 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.
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5. Activation Energy
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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

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.
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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.
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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.
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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.
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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.
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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.
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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. .
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15. Stresses 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.

16. Reactions 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.

17. Ch. 17/18
The End
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