Ch. 18—Reaction Rates and Equilibrium 18.1—Rates of Reaction collision theory, activation energy, reaction rates 18.2—Reversible Reactions and Equilibrium reversible reactions, chemical equilibrium, Le Chatelier’s principle (skip equilibrium constants) 18.3—Solubility Equilibrium (skip) solubility product constant, common ion effect 18.4—Entropy and Free Energy (skip) Gibbs free energy, spontaneous reactions, entropy

18.1 18.1--Rates of Reaction 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.

18.1 Collision Theory In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time. A rate is a measure of the speed of any change that occurs within an interval of time.

18.1 Collision Theory Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. As time passes, the amount of reactant (red squares) decreases and the amount of product (blue spheres) increases. Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. Interpreting Diagrams Assuming equal time intervals between the boxes, how can you tell that the rate of conversion of reactant to product is not constant throughout this reaction?

18.1 Collision Theory 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. Effective Collision

Collision Theory 18.1 Ineffective Collision The minimum energy that colliding particles must have in order to react is called the activation energy. If colliding particles have enough kinetic energy and collide at the right orientation, they can react to form a new product. a) An effective collision of reactant molecules produces product molecules. b) An ineffective collision of reactant molecules produces no reaction, and the reactants bounce apart unchanged.

18.1 Collision Theory 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.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates The rate of a chemical reaction depends upon a) temperature b) concentration c) particle size d) use of a catalyst.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Temperature Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Refrigeration Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Meat kept at freezing temperatures has a lifetime of months.

Factors Affecting Reaction Rates 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. The rate of a reaction depends upon the concentrations of the reactants. a) In air, a lighted splint glows and soon goes out. b) When placed in pure oxygen, the splint bursts into flame. Inferring What accounts for the difference in reactivity?

Factors Affecting Reaction Rates 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. An explosion destroyed this grain elevator. 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.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Catalysts A catalyst increases the rate of a reaction by lowering the activation-energy barrier. INTERPRETING GRAPHS a. Navigate How does the catalyst affect the magnitude of the activation energy? b. Read Does the catalyst change the amount of energy released in the reaction? c. Interpret Along which of the two reaction paths are reactants converted more rapidly to products?

Factors Affecting Reaction Rates 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.

18.2--Reversible Reactions and Equilibrium 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.

18.2 Reversible Reactions How do the amounts of reactants and products change in a chemical system at equilibrium? At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.

18.2 Reversible Reactions 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. A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. If the rate at which shoppers move from the first floor to the second is equal to the rate at which shoppers move from the second floor to the first, then the number of shoppers on each floor remains constant. Applying Concepts Is it necessary that an equal number of shoppers be on each floor? Explain.

SO3 decomposes to SO2 and O2 18.2 Reversible Reactions SO2 and O2 react to give SO3 SO3 decomposes to SO2 and O2 Molecules of SO2 and O2 react to give SO3 . Molecules of SO3 decompose to give SO2 and O2 . At equilibrium, all three types of molecules are present in the mixture. At equilibrium, all three types of molecules are present.

18.2 Reversible Reactions 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.

Factors Affecting Equilibrium: Le Châtelier’s Principle 18.2 Factors Affecting Equilibrium: Le Châtelier’s Principle Stresses that upset the equilibrium of a chemical system include changes in the concentration of reactants or products changes in temperature changes in pressure

Le Châtelier’s Principle 18.2 Le Châtelier’s Principle 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.

Factors Affecting Equilibrium: Le Châtelier’s Principle 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. The rapid exhalation of CO2 during and after vigorous exercise helps reestablish the body’s correct CO2:H2CO3 equilibrium. This keeps the acid concentration in the blood within a safe range.

Factors Affecting Equilibrium: Le Châtelier’s Principle 18.2 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. 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. Interpreting Illustrations How does an increase in temperature affect the equilibrium of a mixture of these gases?

Factors Affecting Equilibrium: Le Châtelier’s Principle 18.2 Factors Affecting Equilibrium: Le Châtelier’s Principle Pressure Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium. a) The system is at equilibrium. b) Equilibrium is disturbed by an increase in pressure. c) A new equilibrium position is established with fewer gas molecules. Interpreting Diagrams What effect does a decrease in volume have on the number of gas molecules?

a. Addition of product, shifts the equilibrium to the left, forming more PCl5 b. The equation shows 2 mol of gaseous product and 1 mol of gaseous reactant. The increase in pressure is relieved if the equilibrium shifts to the left, because a decrease in the number of moles of gaseous substances produces a decrease in pressure. c. The removal of heat causes the equilibrium to shift to the left, because the reverse reaction is heat-producing. d. The removal of PCl3 causes the equilibrium to shift to the right to produce more PCl3

a. left b. left c. right d. right

Equilibrium Constants 18.2 Equilibrium Constants Equilibrium Constants What does the value of 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.

Equilibrium Constants 18.2 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.

18.1 Section Quiz. 1. The units below that would be appropriate to measure the rate of a chemical reaction is a) mmol/s. b) mol/L. c) kJ/mol. d) h/mol.

18.1 Section Quiz. 2. In a chemical reaction, the energy of reactants is always a) greater than the energy of the products. b) more than the activation energy. c) less than the activation energy. d) less than the energy of the products.

18.1 Section Quiz. 3. An increase in which one of the following will NOT increase the reaction rate? a) temperature b) concentration of reactants c) total mass of reactants d) surface area of reactants

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

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

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

18.2 Section Quiz. 3. Changing which of the following would NOT affect the equilibrium position of a chemical reaction? a) concentration of a reactant only b) concentration of a product only c) temperature only d) volume only