Ph chem chap 18 - rates of reaction b


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Reaction Rates, things that affect reaction rates, gibbs energy, le chatlier principal

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Ph chem chap 18 - rates of reaction b

  1. 1. Reaction Rates and Equilibrium
  2. 2. Essential Question: How is the rate of a chemical change expressed, and what four factors influence the rate of a chemical reaction? Rates of Reaction
  3. 3. Reaction Rates • The speed of chemical reactions can vary from instantaneous to extremely slow. • A rate is a measure of the speed of any change per interval time. • Reaction rate = amount of reactant changing per unit time.
  4. 4. Collision Theory • Particles react when they collide with one another. • The collision must have sufficient kinetic energy to break existing bonds. • This minimum energy is called activation energy.
  5. 5. Energy Diagrams
  6. 6. Energy Diagrams • Activation Energy – minimum energy that colliding particles must have in order to react. • Activated Complex – unstable arrangement of atoms that forms momentarily • Transition State – synonym for the activated complex
  7. 7. Factors Affecting Reaction Rates • Temperature • Concentration • Particle Size • Presence of a catalyst (or inhibitor)
  8. 8. Effect of a Catalyst
  9. 9. Essential Question • How do the amounts of reactants and products change at equilibrium, and what three stresses can cause a change in the equilibrium position? Reversible Reactions and Equilibrium
  10. 10. At Chemical Equilibrium • Products are being formed… • …reactants are being formed… • …but no net change occurs in the actual amounts of either the reactants or products.
  11. 11. Reversible Reactions
  12. 12. Changes in Concentrations
  13. 13. Equilibrium Position • Expresses the relative concentrations of reactants and products at equilibrium. A B A B 1% 99% 99% 1% Certain factors can affect this position
  14. 14. Le Châtelier’s Principle • “If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.”
  15. 15. Equilibrium Constants • The ratio of product concentrations to reactant concentrations at equilibrium • Each concentration is raised to a power equal to their coefficient in a balance chemical equation.
  16. 16. Equilibrium Constants
  17. 17. What Keq Tells Us • Keq greater than 1 means that products are favored • Keq less than 1 means that reactants are favored • Keq equal to 1 means that products and reactants are equally favored
  18. 18. Factors Affecting Equilibrium • Changes in concentration of reactants or products • Changes in temperature • Changes in pressure (of gases)
  19. 19. Changes in Concentration
  20. 20. Changes in Temperature
  21. 21. Changes in Pressure
  22. 22. Effect of Pressure on Equilibrium
  23. 23. Essential Question • What does the solubility product constant tell you about the solubility of a compound, and how can you predict whether a precipitate will form when salt solutions are mixed? Solubility Equilibrium
  24. 24. Solubility Product Constant • Takes the same form as equilibrium constants • AgCl(s) Ag→ + (aq) + Cl− (aq)
  25. 25. Common Ion Effect • Lowering the solubility of an ionic compound as a result of the addition of a common ion • If the product of the concentrations of two ions in the mixture is greater than Ksp of the compound formed from the ions, a precipitate will form.
  26. 26. Essential Question • What two factors determine the spontaneity of a reaction, and what are the characteristics of spontaneous reactions? Entropy and Free Energy
  27. 27. Spontaneous Reactions • Occur naturally and favor the formation of products • Produce substantial amounts of products at equilibrium
  28. 28. Nonspontaneous Reactions • Does not favor the formation of products • Do not produce substantial amounts of products at equilibrium • Note: Speed is NOT a factor
  29. 29. Entropy Changes • A measure of the disorder of a system • Law of Disorder: the natural tendency is for systems to move in the direction of maximum disorder or randomness • An increase in entropy favors spontaneous chemical reactions
  30. 30. Enthalpy Changes • The decrease in enthalpy (exothermic reactions) indicates a spontaneous reaction. • An increase in enthalpy (endothermic reactions) indicates a non-spontaneous reaction.
  31. 31. Gibbs Free-Energy • The maximum amount of energy that can be use to do work. • ΔG is negative in spontaneous reactions because the system loses free energy.
  32. 32. Essential Question • What does the specific rate constant, k, communicate and how does one interpret a reaction progress curve? The Progress of Reactions
  33. 33. Specific Rate Constant • The value of the specific rate constant, k, is large if the products form quickly • The value is small if the products form slowly.