Ch. 18—Reaction Rates and Equilibrium

18.1—Rates of Reactioncollision theory, activation energy, reaction rates

18.2—Reversible Reactions and Equilibriumreversible 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--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

18.1

Collision TheoryIn 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.

Collision Theory

Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.

18.1

Collision TheoryAccording 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.

18.1

Effective Collision

Collision Theory18.1

Ineffective Collision

The minimum energy that colliding particles must have in order to react is called the activation energy.

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.

18.1

Factors Affecting Reaction RatesThe rate of a chemical reaction depends upon

a) temperature

b) concentration

c) particle size

d) use of a catalyst.

18.1

Factors Affecting Reaction RatesTemperature

• Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action.

18.1

Factors Affecting Reaction RatesConcentration

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.

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.

18.1

Factors Affecting Reaction Rates

• Catalysts

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.

18.1

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 ReactionsIf 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.

18.2

A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

Reversible Reactions18.2

At equilibrium, all three types of molecules are present.

SO2 and O2 react to give

SO3

SO3 decomposes to SO2 and O2

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.

18.2

Factors Affecting Equilibrium: Le Châtelier’s Principle

Stresses that upset the equilibrium of a chemical system include

a) changes in the concentration of reactants or products

b) changes in temperature

c) changes in pressure

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.

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.

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.

18.2

Factors Affecting Equilibrium: Le Châtelier’s Principle

Pressure• Pressure affects a mixture of nitrogen, hydrogen,

and ammonia at equilibrium

18.2

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. leftc. right

d. right

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.

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.

18.2

18.1 Section Quiz.1. The units below that would be appropriate to

measure the rate of a chemical reaction isa) 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