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Reaction Rates

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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Section 16-1

Section 16.1 A Model for Reaction Rates

• 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.

Section 16-1

Section 16.1 A Model for Reaction Rates (cont.)

reaction rate

collision theory

activated complex

activation energy

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

Section 16-1

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.

Section 16-1

Expressing Reaction Rates (cont.)

• Reaction rates are determined experimentally.

Section 16-1

Collision Theory

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

Section 16-1

Collision Theory (cont.)

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

Section 16-1

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.

Section 16-1

Collision Theory (cont.)

Section 16-1

Collision Theory (cont.)

Section 16-1

Collision Theory (cont.)

Section 16-1

Spontaneity and Reaction Rate

• 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.

A. A

B. B

C. C

D. D

Section 16-1

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Section 16.1 Assessment

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 an exothermic reaction.

C. Reacting substances must collide in the correct orientation.

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

A. A

B. B

C. C

D. D

Section 16-1

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Section 16.1 Assessment

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

End of Section 16-1

Section 16-2

Section 16.2 Factors Affecting Reaction Rates

• 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.

Section 16-2

The Nature of Reactants

• Some substances react more readily than others.

Section 16-2

Concentration

• Chemists change reaction rates by changing concentrations of reactants.

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

Section 16-2

Surface Area

• 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.

Section 16-2

Temperature

• Increasing temperature generally increases reaction rate.

• Increasing temperature increases the kinetic energy of the particles.

• Reacting particles collide more frequently at higher temperatures.

Section 16-2

Temperature (cont.)

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

• As temperature increases, reaction rate increases.

Section 16-2

Temperature (cont.)

Section 16-2

Catalysts and Inhibitors

• A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction.

• An inhibitor is a substance that slows or prevents a reaction.

Section 16-2

Catalysts and Inhibitors (cont.)

• Catalysts lower the activation energy.

• Low activation energy means more collisions between particles have sufficient energy to react.

Section 16-2

Catalysts and Inhibitors (cont.)

• 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.

A. A

B. B

C. C

D. D

Section 16-2

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Section 16.2 Assessment

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

A. A

B. B

C. C

D. D

Section 16-2

Section 16.2 Assessment

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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

End of Section 16-2

Section 16-3

Section 16.3 Reaction Rate Laws

• 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.

Section 16-3

Writing Reaction Rate Laws

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

• Rate = k[A] where [A] is the concentration and k is a constant.

Section 16-3

Writing Reaction Rate Laws (cont.)

• 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.

Section 16-3

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.

Section 16-3

Writing Reaction Rate Laws (cont.)

• The General Rate Law

– Rate = k[A]m[B]n

– 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

Section 16-3

Determining Reaction Order

• 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.

Section 16-3

Determining Reaction Order (cont.)

Section 16-3

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

A. A

B. B

C. C

D. D

Section 16-3

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Section 16.3 Assessment

What is the overall reaction order of the following reaction?

Rate = k[A]2[B]2

A. 1st

B. 2nd

C. 3rd

D. 4th

A. A

B. B

C. C

D. D

Section 16-3

Section 16.3 Assessment

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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

End of Section 16-3

Section 16-4

Section 16.4 Instantaneous Reaction Rates and Reaction Mechanisms

• 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.

Section 16-4

Section 16.4 Instantaneous Reaction Rates and Reaction Mechanisms (cont.)

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.

Section 16-4

Instantaneous Reaction Rates

• 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.

Section 16-4

Instantaneous Reaction Rates (cont.)

• 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)

Section 16-4

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.

Section 16-4

Reaction Mechanisms (cont.)

• 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.

Section 16-4

Reaction Mechanisms (cont.)

Section 16-4

Reaction Mechanisms (cont.)

• 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.

Section 16-4

Reaction Mechanisms (cont.)

A. A

B. B

C. C

D. D

Section 16-4

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Section 16.4 Assessment

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

A. compound reaction

B. complex reaction

C. multi-step reaction

D. combined reaction

A. A

B. B

C. C

D. D

Section 16-4

Section 16.4 Assessment

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What is the slowest step in a complex reaction called?

A. elementary step

B. reducing step

C. rate-determining step

D. intermediate step

End of Section 16-4

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Study Guide

Chapter Assessment

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Study Guide 1

Section 16.1 A Model for Reaction Rates

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.

Study Guide 2

Section 16.2 Factors Affecting Reaction Rates

Key Concepts

• 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.

Study Guide 3

Section 16.3 Reaction Rate Laws

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.

Study Guide 4

Section 16.4 Instantaneous Reaction Rates and Reaction Mechanisms

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.

A. A

B. B

C. C

D. D

Chapter Assessment 1

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The energy required to initiate a reaction is called ____.

A. initiation energy

B. activation energy

C. complex energy

D. catalyst energy

A. A

B. B

C. C

D. D

Chapter Assessment 2

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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

A. A

B. B

C. C

D. D

Chapter Assessment 3

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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

A. A

B. B

C. C

D. D

Chapter Assessment 4

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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

A. A

B. B

C. C

D. D

Chapter Assessment 5

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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

A. A

B. B

C. C

D. D

STP 1

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

A. mol/L ● s

B. L/s

C. M

D. mL/h

A. A

B. B

C. C

D. D

STP 2

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

A. 0.721 moles

B. 1.39 moles

C. 1.54 moles

D. 3.18 moles

A. A

B. B

C. C

D. D

STP 3

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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

A. A

B. B

C. C

D. D

STP 4

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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

A. A

B. B

C. C

D. D

STP 5

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

A. synthesis

B. double replacement

C. decomposition

D. single replacement

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CIM

Figure 16.4 Effect of Molecular Orientation on Collision Effectiveness

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