A Model for Reaction RatesCopyright © McGraw-Hill Education

Unit 8: Rates of Reaction Chapter 16Outline

• Collision Theory

• Factors affecting the rate of reaction

• Average rate

• Rate Laws and Reaction Orders

• Instantaneous Rates

• Reaction Mechanisms*

A Model for Reaction RatesCopyright © McGraw-Hill Education

Collision Theory

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

A Model for Reaction RatesCopyright © McGraw-Hill Education

Collision Theory

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

A Model for Reaction RatesCopyright © McGraw-Hill Education

Collision Theory

• 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.

A Model for Reaction RatesCopyright © McGraw-Hill Education

Collision Theory

A Model for Reaction RatesCopyright © McGraw-Hill Education

Collision Theory

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

The Nature of Reactants

• Some substances react more readily than others.

Concentration

• 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.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

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.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

Temperature

• Increasing temperature generally increases reaction rate.

• Increasing temperature increases the kinetic energy of the particles.

• Reacting particles collide more frequently at higher temperatures.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

Temperature

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

• As temperature increases, reaction rate increases.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

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.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

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.

Factors Affecting Reaction RatesCopyright © McGraw-Hill Education

Catalysts and Inhibitors

• 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 Model for Reaction RatesCopyright © McGraw-Hill Education

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.

• Reaction rates are determined experimentally.

A Model for Reaction RatesCopyright © McGraw-Hill Education

CALCULATE AVERAGE REACTION RATES

Use with Example Problem 1.

Problem In a reaction between butyl chloride (C4H9Cl) and water, the concentration of C4H9Cl is 0.220M at the beginning of the reaction. At 4.00 s, the concentration of C4H9Cl is 0.100M. Calculate the average reaction rate over the given time period expressed as moles of C4H9Cl consumed per liter per second.

Response

ANALYZE THE PROBLEMYou are given the initial and final concentrations of the reactant C4H9Cl and the initial and final times. You can calculate the average reaction rate of the chemical reaction using the change in concentration of butyl chloride in four seconds.

KNOWN UNKNOWN

t1 = 0.00 s Average reaction rate = ? mol/(L • s)t2 = 4.00 s

[C4H9Cl] at t1 = 0.220M

[C4H9Cl] at t2 = 0.100M

SOLVE FOR THE UNKNOWN

• State the average reaction rate equation.

= [C4H9Cl] at t2 – [C4H9Cl]at t1

t2 – t1

• Substitute t2 = 4.00 s, t1 = 0.00 s, [C4H9Cl] at t2 = 0.100M, and [C4H9Cl] at t1 = 0.220M.

= 0.100M – 0.220M

4.00 s –0.00

A Model for Reaction RatesCopyright © McGraw-Hill Education

CALCULATE AVERAGE REACTION RATES

SOLVE FOR THE UNKNOWN (continued)

• Substitute mol/L for M and perform the calculations. Note that the minus sign cancels out.

=0.100 mol/L – 0.2 20 mol/L

4.00 s – 0.00 s

Average reaction rate =

0.120 mol/L4.00 s

= 0.0300 mol/(L•s)

EVALUATE THE ANSWERThe average reaction rate of 0.0300 moles C4H9Cl consumed per liter per second is reasonable based on the starting and ending amounts. The answer is correctly expressed in three significant figures.

A Model for Reaction RatesCopyright © McGraw-Hill Education

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.

Molecular Orientation and Collision Effectiveness

Animation https://www.youtube.com/watch?v=IkqoBbFZV4Q

FPO

Add link to animation from page 564 here.

A Model for Reaction RatesCopyright © McGraw-Hill Education

Reaction Rate LawsCopyright © McGraw-Hill Education

Writing Reaction Rate Laws

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

Reaction Rate LawsCopyright © McGraw-Hill Education

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.

Reaction Rate LawsCopyright © McGraw-Hill Education

Writing Reaction Rate Laws

• 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.

Reaction Rate LawsCopyright © McGraw-Hill Education

Determining Reaction Order

• 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

Reaction Rate LawsCopyright © McGraw-Hill Education

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.

Reaction Rate LawsCopyright © McGraw-Hill Education

Writing Reaction Rate Laws

• 2NO(g) + 2H2(g) → N2(g) + 2H2(g)

Reaction Rate LawsCopyright © McGraw-Hill Education

Writing Reaction Rate Laws

• 2NO(g) + 2H2(g) → N2(g) + 2H2(g)

–Write the General Rate = k[NO]m[H2]n

–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

Reaction Rate LawsCopyright © McGraw-Hill Education

Writing Reaction Rate Laws

• 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 overallp. 577 #19-22

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

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.

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

Instantaneous Reaction Rates

• 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 and Reaction MechanismsCopyright © McGraw-Hill Education

CALCULATE INSTANTANEOUS REACTION RATES

Use with Example Problem 2.

Problem

The following reaction is first order in H2

and second order in NO with a rate constant of 2.90 × 102(L2/(mol2・s)).

2NO(g) + H2(g) → N2O(g) + H2O(g)

Calculate the instantaneous rate when the reactant concentrations are

[NO] = 0.00200M and [H2] = 0.00400M.

Response

ANALYZE THE PROBLEM

The rate law can be expressed by rate = k[NO]2[H2]. Therefore, the instantaneousreaction rate can be determined by inserting reactant concentrations and the specific rate constant into the rate law equation.

KNOWN UNKNOWN

[NO] = 0.00200M rate = ? mol/(L · s)

[ H2 ] = 0.00400M

k = 2.90 × 102(L2 /(mol2・s))

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

CALCULATE INSTANTANEOUS REACTION RATES

EVALUATE THE ANSWER

Units in the calculation cancel to give mol/(L・s), which is a common unit for reaction rates. A magnitude of approximately 10-6 mol/(L・s) fits with the quantities given and the rate law equation. The answer is correctly expressed with three significant figures.

SOLVE FOR THE UNKNOWN

• State the rate law.

rate = k[NO]2[H2]

• Substitute k = 2.90 × 102(L2/(mol2 · s)), [NO] = 0.00200M, and [H2] = 0.00400M.

rate = (2.90 × 102 L2/(mol2 · s))(0.00200 mol/L)2(0.00400 mol/L)

• Multiply the numbers and units.

rate = 4.64 × 10-6 mol/(L · s)

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

*Reaction Mechanisms

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

• 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.

• 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.

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

*Reaction Mechanisms

Chlorine-catalyzed decomposition of ozone

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

*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.

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

*Reaction Mechanisms

Instantaneous Reaction Rates and Reaction MechanismsCopyright © McGraw-Hill Education

Review

Essential Questions• How are instantaneous rates of chemical reactions calculated?

• What substances and steps are involved in a reaction mechanism?

• How is the instantaneous rate of a complex reaction related to its reaction mechanism?

Vocabulary

• instantaneous rate• complex reaction• reaction mechanism

• intermediate• rate-determining step