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

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 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. 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.

Ineffective Collision 18.1 Collision Theory 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.

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. 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.

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

A reversible reaction is 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).

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.

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

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?

changes in the concentration of reactants or products, 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).

for Conceptual Problem 18.1

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 ←

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.

Kinetics and Equilibrium Types of Equilibria Equilibria Physical Phase Changes Saturated Solutions Chemical Equilibria Reversible Chemical Reactions

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)

A heating curve graphically describes the enthalpy changes that take place during phase changes. INTERPRETING GRAPHS a. Identify In which region(s) of the graph is temperature constant? b. Describe How does the amount of energy required to melt a given mass of ice compare to the energy required to vaporize the same mass of water? Explain. c. Apply Concepts Which region of the graph represents the coexistence of solid and liquid? Liquid and vapor?

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.

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

Heats of Vaporization and Condensation 17.3 Heats of Vaporization and Condensation Enthalpy changes accompany changes in state. Enthalpy changes accompany changes in state. Fusion and vaporization are endothermic processes. Solidification and condensation are exothermic processes. Interpreting Diagrams Which arrows represent processes that release heat to the surroundings?

17.3 A heating curve graphically describes the enthalpy changes that take place during phase changes. INTERPRETING GRAPHS a. Identify In which region(s) of the graph is temperature constant? b. Describe How does the amount of energy required to melt a given mass of ice compare to the energy required to vaporize the same mass of water? Explain. c. Apply Concepts Which region of the graph represents the coexistence of solid and liquid? Liquid and vapor?

Heating (Endothermic) and Cooling (Exothermic) Curves Heating versus Cooling Curves

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

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**

Unsaturated 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** Sample Individual Solubility Curve- Potassium nitrite http://www.stevespanglerscience.com/experiment/00000078