chapter 16: reaction rates chemistry matter and change

Chapter 16: Reaction RatesChapter 16: Reaction Rates

CHEMISTRY Matter and Change

Section 16.1 A Model for Reaction Rates

Section 16.2 Factors Affecting Reaction Rates

Section 16.3 Reaction Rate Laws

Section 16.4 Instantaneous Reaction Rates and Reaction Mechanisms

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CHAPTER

16 Table Of Contents

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• Calculate average rates of chemical reactions from experimental data.

energy: the ability to do work or produce heat; it exists in two basic forms: potential energy and kinetic energy

• Relate rates of chemical reactions to collisions between reacting particles.

SECTION16.1

A Model for Reaction Rates

reaction rate

collision theory

activated complex

activation energy

Collision theory is the key to understanding why some reactions are faster than others.

SECTION16.1

A Model for Reaction Rates

Expressing Reaction Rates• The reaction rate of a chemical reaction is stated

as the change in concentration of a reactant or product per unit of time.

SECTION16.1

A Model for Reaction Rates

• Reaction rates are determined experimentally (and always expressed as a positive value).

• Average rate of reaction: the change in concentration of a reactant or product that occurs during a specific time interval.

Expressing Reaction Rates (cont.)

SECTION16.1

A Model for Reaction Rates

• Collision theory states that atoms, ions, and molecules must collide in order to react.

SECTION16.1

A Model for Reaction Rates

Collision Theory

• Correct orientation and

• Sufficient energy• (Lego example)

SECTION16.1

A Model for Reaction Rates

Collision Theory (cont.)

• An activated complex is a temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.

SECTION16.1

A Model for Reaction Rates

Collision Theory (cont.)

• The minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction is called the activation energy.

• High activation energy means that few collisions have the required energy and the reaction rate is slow.

Collision Theory (cont.)

SECTION16.1

A Model for Reaction Rates

• What does an exothermic reaction look like on a plot of energy vs. time?

• Draw a plot in your notes.

Collision Theory (cont.)

SECTION16.1

A Model for Reaction Rates

SECTION16.1

A Model for Reaction Rates

Collision Theory (cont.)

• Are more spontaneous reactions faster than less spontaneous reactions?

• ΔG indicates only the natural tendency for a reaction to proceed—it does not affect the rate of a chemical reaction.

SECTION16.1

A Model for Reaction Rates

Spontaneity and Reaction Rate

Which of the following is NOT a requirement for a reaction to occur, according to the collision theory?

A. Reacting substances must collide.

B. Reacting substances must be in anexothermic reaction.

C. Reacting substances must collide in the correct orientation.

D. Reacting substances must collide with sufficient energy to form an activated complex.

SECTION16.1

Section Check

A temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming is called ____.

A. reaction complex

B. reaction substrate

C. activated complex

D. activated molecule

SECTION16.1

Section Check

• Identify factors that affect the rates of chemical reactions.

concentration: a quantitative measure of the amount of solute in a given amount of solvent or solution

catalyst

inhibitor

heterogeneous catalyst

homogeneous catalyst

• Explain the role of a catalyst.

Factors such as reactivity, concentration, temperature, surface area, and catalysts affect the rate of a chemical reaction.

SECTION16.2

Factors Affecting Reaction Rates

• Some substances react more readily than others. (Alkali metals, halogens)

• The phase of the reactants matters. Some substances react faster in aqueous solutions than if combined as solids.

SECTION16.2

Factors Affecting Reaction Rates

The Nature of Reactants

• Chemists change reaction rates by changing concentrations of reactants.

• When concentrations are increased, more molecules are available to collide per unit volume, and therefore collisions occur more frequently.

SECTION16.2

Factors Affecting Reaction Rates

Concentration

• Greater surface area allows particles to collide with many more particles per unit of time.

• For the same mass, many small particles have more surface area than one large particle.

• Reaction rate increases with increasing surface area.

• Steel wool v. solid steel example

SECTION16.2

Factors Affecting Reaction Rates

Surface Area

• Increasing temperature generally increases reaction rate.

• Increasing temperature increases the kinetic energy of the particles.

• Reacting particles collide more frequently at higher temperatures.

SECTION16.2

Factors Affecting Reaction Rates

Temperature

• High-energy collisions are more frequent at a higher temperature.

• As temperature increases, reaction rate increases.

SECTION16.2

Factors Affecting Reaction Rates

Temperature (cont.)

SECTION16.2

Factors Affecting Reaction Rates

Temperature (cont.)

• A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction. Note: a catalyst does not increase the amount of product .

• An inhibitor is a substance that slows or prevents a reaction.(Refrigeration, preservatives)

SECTION16.2

Factors Affecting Reaction Rates

Catalysts and Inhibitors

• Catalysts lower the activation energy.

• Low activation energy means that more of the collisions between particles will have sufficient energy to overcome the activation energy barrier and bring about a reaction.

Catalysts and Inhibitors (cont.)

SECTION16.2

Factors Affecting Reaction Rates

• A heterogeneous catalyst exists in a physical state different than that of the reaction it catalyzes.

• A homogeneous catalyst exists in the same physical state as the reaction it catalyzes.

SECTION16.2

Factors Affecting Reaction Rates

Catalysts and Inhibitors (cont.)

Which of the following generally does not increase the rate of a chemical reaction?

A. increasing concentration

B. adding a catalyst

C. adding an inhibitor

D. increasing temperature

Section CheckSECTION16.2

High-energy particle collisions are more frequent:

A. when an inhibitor is present

B. when temperature is decreased

C. when activation energy is higher

D. when temperature is increased

Section CheckSECTION16.2

• Express the relationship between reaction rate and concentration.

reactant: the starting substance in a chemical reaction

rate law

specific rate constant

reaction order

method of initial rates

• Determine reaction orders using the method of initial rates.

The reaction rate law is an experimentally determined mathematical relationship that relates the speed of a reaction to the concentrations of the reactants.

SECTION16.3

Reaction Rate Laws

• A rate law expresses the relationship between the rate of a chemical reaction and the concentration of the reactants.

SECTION16.3

Reaction Rate Laws

Writing Reaction Rate Laws

• The symbol k is the specific rate constant, a numerical value that relates the reaction rate and the concentrations of reactants at a given temperature.

• The specific rate constant is unique for every reaction.

SECTION16.3

Reaction Rate Laws

Writing Reaction Rate Laws (cont.)

• The reaction order for a reactant defines how the rate is affected by the concentration of that reactant.

• Rate = k[H2O2]

• The reaction is first order, so the rate changes in the same proportion the concentration of H2O2 changes.

SECTION16.3

Reaction Rate Laws

Writing Reaction Rate Laws (cont.)

• 2NO(g) + 2H2(g) → N2(g) + 2H2(g)

–Rate = k[NO]2[H2]

–If H2 is doubled, the rate doubles.

–If NO is doubled, the rate quadruples because 22 = 4.

–First-order H2, second-order NO, third-order overall

SECTION16.3

Reaction Rate Laws

Writing Reaction Rate Laws (cont.)

• The method of initial rates determines reaction order by comparing the initial rates of a reaction carried out with varying reactant concentrations.

• Initial rate measures how fast the reaction proceeds at the moment when reactants are mixed.

SECTION16.3

Reaction Rate Laws

Determining Reaction Order

SECTION16.3

Reaction Rate Laws

Determining Reaction Order (cont.)

• Doubling [A] doubles the reaction rate, so [A] is first order.

• Doubling [B] quadruples the reaction rate, so [B] is second order.

• Rate = k[A][B]2

SECTION16.3

Reaction Rate Laws

Determining Reaction Order (cont.)

What is the overall reaction order of the following reaction?

Rate = k[A]2[B]2

A. 1st

B. 2nd

C. 3rd

D. 4th

SECTION16.3

Section Check

In the following reaction, what is the overall reaction order if doubling [A] results in quadrupling the reaction rate and doubling [B] results in a reaction rate eight times faster?

Rate = k[A]m[B]n

A. 12

B. 5

C. 6

D. 10

SECTION16.3

Section Check

• Calculate instantaneous rates of chemical reactions.

decomposition reaction: a chemical reaction that occurs when a single compound breaks down into two or more elements or new compounds

• Understand that many chemical reactions occur in steps.

• Relate the instantaneous rate of a complex reaction to its reaction mechanism.

SECTION16.4

Instantaneous Reaction Rates and Reaction Mechanisms

instantaneous rate

complex reaction

reaction mechanism

intermediate

rate-determining step

The slowest step in a sequence of steps determines the rate of the overall chemical reaction.

SECTION16.4

Instantaneous Reaction Rates and Reaction Mechanisms

• This figure shows the concentration of H2O2 over time during the decomposition reaction 2H2O2(aq) → 2H2O(l) + O2(g).

• The instantaneous rate is the slope of the straight line tangent to the curve at the specific time.

SECTION16.4

Instantaneous Reaction Rates

Instantaneous Reaction Rates and Reaction Mechanisms

• Instantaneous rate can be calculated if the concentrations are known, the temperature is known, and the experimentally determined rate law and specific rate constant at that temperature are known.

• 2N2O5(g) → 4NO2(g) + O2(g)

• Rate = k[N2O5]

• k = 1.0 × 10–5 s–1 and [N2O5] = 0.350 mol/L

• Rate = (1.0 × 10–5 s–1)(0.350 mol/L) = 3.5 × 10–6 mol/(L•s)

Instantaneous Reaction Rates (cont.)

SECTION16.4

Instantaneous Reaction Rates and Reaction Mechanisms

• Most chemical reactions consist of sequences of two or more simpler reactions.

• Each step is called an elementary step.

• A complex reaction contains two or more elementary steps.

Reaction Mechanisms

SECTION16.4

Instantaneous Reaction Rates and Reaction Mechanisms

• A reaction mechanism is the complete sequence of elementary steps that makes up a complex reaction.

• An intermediate is a substance produced in one of the elementary steps and consumed in a subsequent elementary step.

• Intermediates do not appear in the net chemical equation.

SECTION16.4

Reaction Mechanisms (cont.)

Instantaneous Reaction Rates and Reaction Mechanisms

Chlorine-catalyzed decomposition of ozone

SECTION16.4

Reaction Mechanisms (cont.)

Instantaneous Reaction Rates and Reaction Mechanisms

• Every complex reaction has one elementary step that is slower than the others.

• The slowest elementary step in a complex reaction is called the rate-determining step.

SECTION16.4

Reaction Mechanisms (cont.)

Instantaneous Reaction Rates and Reaction Mechanisms

SECTION16.4

Reaction Mechanisms (cont.)

Instantaneous Reaction Rates and Reaction Mechanisms

What is a reaction with two or more elementary steps called?

A. compound reaction

B. complex reaction

C. multi-step reaction

D. combined reaction

SECTION16.4

Section Check

What is the slowest step in a complex reaction called?

A. elementary step

B. reducing step

C. rate-determining step

D. intermediate step

SECTION16.4

Section Check

Chemistry Online

Study Guide

Chapter Assessment

Standardized Test Practice

Reaction Rates

Resources

CHAPTER

16

Key Concepts• The rate of a chemical reaction is expressed as the rate

at which a reactant is consumed or the rate at which a product is formed.

• Reaction rates are generally calculated and expressed in moles per liter per second (mol/(L ● s)).

• In order to react, the particles in a chemical reaction must collide.

• The rate of a chemical reaction is unrelated to the spontaneity of the reaction.

SECTION16.1

A Model for Reaction Rates

Study Guide

• Key factors that influence the rate of chemical reactions include reactivity, concentration, surface area, temperature, and catalysts.

• Raising the temperature of a reaction generally increases the rate of the reaction by increasing the collision frequency and the number of collisions that form an activated complex.

• Catalysts increase the rates of chemical reactions by lowering activation energies.

SECTION16.2

Factors Affecting Reaction Rates

Study Guide

Key Concepts

• The mathematical relationship between the rate of a chemical reaction at a given temperature and the concentrations of reactants is called the rate law.

rate = k[A]rate = k[A]m[B]n

• The rate law for a chemical reaction is determined experimentally using the method of initial rates.

SECTION16.3

Reaction Rate Laws

Study Guide

Key Concepts

• The reaction mechanism of a chemical reaction must be determined experimentally.

• For a complex reaction, the rate-determining step limits the instantaneous rate of the overall reaction.

SECTION16.4

Study Guide

Key Concepts

Instantaneous Reaction Rates and Reaction Mechanisms

The energy required to initiate a reaction is called ____.

A. initiation energy

B. activation energy

C. complex energy

D. catalyst energy

Chapter Assessment

Reaction RatesCHAPTER

16

In general, which of the following does not cause a reaction rate to increase?

A. increasing surface area

B. increasing temperature

C. increasing volume

D. adding a catalyst

Chapter Assessment

Reaction RatesCHAPTER

16

What is the overall reaction order of the following reaction?

Rate = k[A][B]2[C]

A. 1st order

B. 2nd order

C. 3rd order

D. 4th order

Chapter Assessment

Reaction RatesCHAPTER

16

A substance produced by an elementary step in a complex reaction that is consumed later and does not show up in the net reaction is called a(n) ____.

A. activated complex

B. catalyst

C. enzyme

D. intermediate

Chapter Assessment

Reaction RatesCHAPTER

16

Increasing the temperature of a reaction increases the rate of reaction by:

A. increasing the collision frequency

B. increasing the number of high-energy collisions

C. both a and b

D. none of the above

Chapter Assessment

Reaction RatesCHAPTER

16

Which of the following is an acceptable unit for expressing a rate?

A. mol/L ● s

B. L/s

C. M

D. mL/h

Reaction RatesCHAPTER

16

Standardized Test Practice

How many moles are in 4.03 × 102 g of calcium phosphate (Ca3(PO4)2)?

A. 0.721 moles

B. 1.30 moles

C. 1.54 moles

D. 3.18 moles

Reaction RatesCHAPTER

16

Standardized Test Practice

Doubling the concentration of one reactant in a reaction causes the reaction rate to double. What is the order of that reactant?

A. 1st

B. 2nd

C. unable to determine

D. none of the above

Reaction RatesCHAPTER

16

Standardized Test Practice

The rate law for the reaction A + B + C → Product is rate = k[A]2[B][C]. If [A] = 0.350M, [B] = .500M, [C] = .125M, and k = 6.50 × 10–5 L3/(mol3 ● s), what is the instantaneous rate of reaction?

A. 2.84 × 10–6 mol/L ● s

B. 4.98 × 10–7 mol/L ● s

C. 5.84 × 10–6 mol/L ● s

D. 2.84 × 10–7 mol/L ● s

Reaction RatesCHAPTER

16

Standardized Test Practice

H2O2 breaks down to form hydrogen and oxygen gas in what type of reaction?

A. synthesis

B. double replacement

C. decomposition

D. single replacement

Reaction RatesCHAPTER

16

Standardized Test Practice

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