Rates of Reaction

1. Rates of Reaction 18.1 • The heat given off by the corrosion reaction of an iron-magnesium alloy with salt water can produce a hot meal. The rate of reaction is increased by adding salt water, so heat is produced rapidly. You will learn some ways in which the rate of a reaction can be increased.

2. 18.1 Collision Theory • Collision Theory • How is the rate of a chemical change expressed? • In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time.

3. 18.1 Collision Theory • A rate is a measure of the speed of any change that occurs within an interval of time.

4. 18.1 Collision Theory • Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.

5. 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.

6. 18.1 Collision Theory • Ineffective Collision

7. 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.

8. 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?

9. Factors Affecting Reaction Rates • Explore animations to better understand the factors that control the speed of a reaction. • Reversible reactions PhET: http://phet.colorado.edu/en/simulation/reversible-reactions • Video regarding Haber process and effect of catalyst: http://glencoe.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::600::450::/sites/dl/free/0023654666/339141/catalysisASSEMBLED.swf::Catalysis • Virtual lab on kinetics of iodine clock: • http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/kinetics2/iodine_clock.html

10. 18.1 Factors Affecting Reaction Rates • Temperature • Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action.

11. 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.

12. 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.

13. 18.1 Factors Affecting Reaction Rates • Catalysts

14. 18.1 Factors Affecting Reaction Rates • An inhibitor is a substance that interferes with the action of a catalyst. Antioxidants and antimicrobials used in drying fruits and preserving fruit juices slow the action of microbes and limit contact with air.

15. 18.1 Section Quiz. • 18.1.

16. 18.1 Section Quiz. • 1. The units below that would be appropriate to measure the rate of a chemical reaction is • mol/s. • mol/L. • kJ/mol. • h/mol.

17. 18.1 Section Quiz. • 2. In a chemical reaction, the energy of reactants is always • greater than the energy of the products. • more than the activation energy. • less than the activation energy. • less than the energy of the products.

18. 18.1 Section Quiz. • 4. A catalyst works because it • lowers the activation energy. • increases the temperature. • is permanently changed in a reaction. • supplies energy to a reaction.

19. Reversible Reactions and Equilibrium 18.2 • In the early 1900s, German chemists refined the process of making ammonia from elemental nitrogen and hydrogen.This process allows the manufacture of nitrogen fertilizers. You will learn how reaction conditions can influence the yield of a chemical reaction. • Activities: • POGIL and Lab on Equilibrium and Le Chatelier’s principle • Animations on collision theory and equilibria

20. 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).

21. 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.

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

23. Reversible Reactions

24. 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).

25. 18.2 Factors Affecting Equilibrium: Le Châtelier’s Principle • Concentration Rapid breathing during and after vigorous exercise helps reestablish the body’s correct CO2:H2CO3 equilibrium, keeping the acid concentration in the blood within a safe range.

26. Factors Affecting Equilibrium: Le Châtelier’s Principle • Temperature Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in ice.

27. Factors Affecting Equilibrium: Le Châtelier’s Principle Pressure Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium

28. Factors Affecting Reaction Rates - Explore several factors that control the speed of a reaction. • Reversible reactions PhET: http://phet.colorado.edu/en/simulation/reversible-reactions • Video regarding Haber process and effect of catalyst: http://glencoe.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::600::450::/sites/dl/free/0023654666/339141/catalysisASSEMBLED.swf::Catalysis • Virtual lab on kinetics of iodine clock: • http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/kinetics2/iodine_clock.html

29. Conceptual Problem 18.1

30. Conceptual Problem 18.1

31. for Conceptual Problem 18.1

32. 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 (Missing equilibrium arrows for this generalized example ; TO Draw) ←

33. 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.

34. 18.1

35. 18.1

36. for Sample Problem 18.2

37. 18.2 Section Quiz. • 1. In a reaction at equilibrium, reactants and products • decrease in concentration. • form at equal rates. • have equal concentrations. • have stopped reacting.

38. 18.2 Section Quiz. • 2. In the reaction 2NO2(g)  2NO(g) + O2(g), increasing the pressure on the reaction would cause • the amount of NO to increase. • the amount of NO2 to increase. • nothing to happen. • the amount of O2 to increase.

39. 18.2 Section Quiz. • 4. For the following reaction, Keq = 1. • A(g) + B(g)  C(g) + D(g) Therefore, at equilibrium • [C] = [A]. • [A][B] = 0. • [AB] = [CD] = 1. • [A][B] = [C][D].