Reaction Rates and Equilibrium Chapter 18

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.

Collision Theory Collision Theory 18.1 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.

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 must collide to react. A reaction will occur if: The colliding particles have enough kinetic energy. The particles collide with the correct orientation Remember: energy is absorbed to break bonds and released when bonds form

Collision Theory 18.1 Effective Collision 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.

Collision Theory 18.1 Ineffective Collision 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.

Collision Theory When substances react, the following sequence of events occurs: Reactant(s) particles collide If with sufficient energy and orientation, existing bonds break Particles rearrange New bonds form product(s) Potential energy changes associated with these steps can be illustrated graphically:

Exothermic Potential Energy Diagram 18.1 Exothermic Potential Energy Diagram The activation-energy barrier must be crossed before reactants are converted to products. INTERPRETING GRAPHS a. Navigate Which are at a higher energy, the reactants or products? b. Read Is energy absorbed or released in progressing from the reactants to the activated complex? c. Interpret Once the activated complex is formed, will it always proceed to form products? Explain.

Potential Energy Diagrams 18.1 Potential Energy Diagrams The minimum energy needed by colliding reactant particles is called the activation energy. The activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. The difference in energy between the reactants and products is called the heat of reaction, ∆H ∆H = Hprod – Hreact Exothermic reactions have a -∆H Endothermic reactions have a +∆H

Endothermic Potential Energy Diagram

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Factors Affecting Reaction Rates What four factors influence the rate of a chemical reaction? The rate of a chemical reaction depends upon temperature, concentration, surface area, and the 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 provide an alternate pathway that requires less activation energy What is the effect on ∆H with a catalyst? 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?

Reversible Reactions and Equilibrium 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 A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. In a closed system, reversible reactions can attain equilibrium. At chemical equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. At chemical equilibrium, no net change occurs in the concentrations of reactants and products.

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. 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 SO3 decomposes to SO2 and O2 SO2 and O2 react to give SO3 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.

Reversible Reactions 18.2 What is EQUAL at chemical equilibrium? What is CONSTANT at chemical equilibrium?

18.2 Reversible Reactions These graphs show how the concentrations of O2, SO2, and SO3 vary with time. Left: Initially, SO2 and O2 are present. Right: Initially, only SO3 is present. INTERPRETING GRAPHS a. Navigate Where on the graphs can you find the initial concentrations of the reactants and products? The equilibrium concentrations? b. Read Which gas is most abundant at equilibrium? c. Interpret How do the equilibrium concentrations of O2, SO2, and SO3 compare?

Factors Affecting Equilibrium: 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. What three stresses can cause a change in the equilibrium position of a chemical system?

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, and changes in pressure.

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 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. 2NO2(g) ⇆ N2O4(g) + heat 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 Pressure – ONLY AFFECTS EQUILIBRIA WITH GASES Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium N2(g) + 3H2(g) ⇆ 2NH3(g) 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?

Spontaneous Reactions 18.4 Spontaneous Reactions Inside a pile of oily rags or a stack of hay that has not been thoroughly dried, decomposition causes heat to build up. When heat cannot escape, the temperature can become high enough to cause a fire. You will learn about the conditions that will produce a spontaneous chemical reaction.

18.4 Entropy Entropy (S) is a measure of the disorder (randomness) of a system.

18.4 Entropy For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid.Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Entropy 18.4 Entropy increases when a substance is divided into parts. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Enthalpy Enthalpy is a measure of the heat content (∆H) of a substance. Endothermic reactions make products with increased enthalpy. Exothermic reactions make products with decreased enthalpy.

Spontaneous Reactions A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions. Cadmium sulfide forms spontaneously when solutions of sodium sulfide and cadmium nitrate are mixed. Inferring Is free energy released in this reaction?

Spontaneous Reactions 18.4 Spontaneous Reactions Spontaneous reactions tend to create products with low enthalpy (energy), and create products with high entropy (disorder; randomness).

Enthalpy, Entropy, and Free Energy 18.4 Enthalpy, Entropy, and Free Energy