Kinetics and Equilibrium review ( Items 114-132 of 200 ways ..)

1. 18.1 • Kinetics and Equilibrium review (Items 114-132 of 200 ways ..) • Kinetics deals with the rates of chemical reactions. In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time. Equilibrium refers to the condition where forward and reverse reactions are occurring at the same rates.

2. 18.1 Collision Theory • Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.

3. 18.1 Collision Theory • Effective Collision • According to collision theory, atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy and have the proper orientation.

4. 18.1 Collision Theory • Ineffective Collision

5. 18.1 • The minimum energy that colliding particles must have in order to react is called the activation energy. • An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. • The activated complex is sometimes called the transition state.

6. 18.1 • The rate of a chemical reaction depends upon • temperature, • concentration, • particle size, • the use of a catalyst. • Factors affecting Reaction Rates • What four factors influence the rate of a chemical reaction?

7. 18.1 Factors Affecting Reaction Rates • Temperature • Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action.

8. 18.1 Factors Affecting Reaction Rates • Concentration • a. In air, a lighted splint glows and soon goes out. • b. When placed in pure oxygen (higher oxygen concentration), the splint bursts into flame.

9. 18.1 Factors Affecting Reaction Rates • Particle Size • The minute size of the reactant particles (grain dust), and the mixture of the grain dust with oxygen in the air caused the reaction to be explosive, destroying the grain elevator.

10. 18.1 Factors Affecting Reaction Rates • Catalysts

11. A reversible reactionis one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. • When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium. The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction. At chemical equilibrium, no net change occurs in the actual concentration of the reactants and products – i.e. concentrations are constant. • (Note this does not mean that the concentration of reactants and products are equal – avoid this common mistake).

12. Reversible Reactions • How do the amounts of reactants and products change in a chemical system at equilibrium? If the rate of the shoppers going up the escalator is equal to the rate of the shoppers going down, then the number of shoppers on each floor remains constant, and there is an equilibrium. At chemical equilibrium, no net change occurs in the actual amounts or concentration of the reactants and products.

13. Reversible Reactions SO3 decomposes to SO2 and O2 SO2 and O2 react to give SO3 At equilibrium, all three types of molecules are present.

14. Reversible Reactions

15. 18.2 The French chemist Le Châtelier proposed what has come to be called 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. • Three stresses can cause a change in the equilibrium position of a chemical system? • changes in the concentration of reactants or products, • changes in temperature, • changes in pressure (for equilibria involving gases).

16. for Conceptual Problem 18.1

17. Equilibrium Constants What does the value of the equilbrium constant, Keq , indicate about the equilibrium position of a reaction? The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation. aA + bB cC + dD ←

18. Equilibrium Constants • A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.

19. Kinetics and Equilibrium

20. Physical Equilibria – Phases Equilibria • H2O (s) H2O (l) • At 0 oC (273 K) there exists a phase equilibrium between solid and liquid H2O. • At 100 oC (373 K) there exists a phase equilibrium between liquid and gaseous H2O. • H2O (l) H2O (g)

22. for Sample Problem 17.4 How many joules of heat are required to melt a 10.0 grams popsicle at 0 oC. Assume that the popsicle has the same heat of fusion as water.

23. Heats of Vaporization and Condensation The quantity of heat absorbed by a melting a solid is exactly the same as the quantity of heat released when the liquid freezes; that is, ∆Hfusion = –∆Hsolidification The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, ∆Hvaporization = –∆Hcondensation

24. 17.3 Heats of Vaporization and Condensation • Enthalpy changes accompany changes in state.

25. 17.3

26. Heating (Endothermic) and Cooling (Exothermic) Curves • Heating versus Cooling Curves

27. Physical Equilibria – Saturated Solutions • Saturated solutions are another example involving physical equilibrium. The term “saturated solutions” refers to a solution containing the maximum amount of solute that will dissolve at a given temperature. The rate at which a substance crystallizes out of solution is equal to the rate at which it dissolves. • pHET simulation for saturated solutions • http://phet.colorado.edu/en/simulation/soluble-salts

28. Saturated Solution: = contains the maximum amount of solute for a given quantity of solvent at a constant temperature & pressure  if additional solute is added, it will not dissolve; rather it will precipitate out Unsaturated Solution: = a solution that contains less solute than a saturated solution at a given temperature & pressure  if additional solute is added, it will dissolve Supersaturated Solution: = contains more solute than it can theoretically hold at a given temperature  crystallization will be initiate if a very small “seed crystal” of solute is added **very unstable**

29. Sample Individual Solubility Curve- Potassium nitrite Saturated Solution: = contains the maximum amount of solute for a given quantity of solvent at a constant temperature & pressure  if additional solute is added, it will not dissolve; rather it will precipitate out Unsaturated Solution: = a solution that contains less solute than a saturated solution at a given temperature & pressure  if additional solute is added, it will dissolve Supersaturated Solution: = contains more solute than it can theoretically hold at a given temperature  crystallization will be initiate if a very small “seed crystal” of solute is added **very unstable** http://www.stevespanglerscience.com/experiment/00000078