Chemical Kinetics The area of chemistry that is

concerned with the speeds, or rates, of reactions is called chemical kinetics.

Our goal in this chapter is to understand how to determine the rates at which reactions occur and to consider the factors that control these rates.

Factors that Affect Reaction Rates

1. The physical state of the reactants: The easier reactants collide, the more

rapidly they react When reactants are in different phases, the

reaction is limited to their area of contact Need solids with increased surface area

Factors that Affect Reaction Rates

2. The concentrations of the reactants: Increase the concentration of one or more

of the reactants Increase in concentration leads to an

increase in the frequency with which the reactant molecules collide

Factors that Affect Reaction Rates

3. The temperature at which the reaction occurs:

Increasing temperature increases kinetic energy of molecules

As molecules move more rapidly, they collide more frequently with higher energy

Factors that Affect Reaction Rates

4. The presence of a catalyst: Catalysts are agents that

increase reaction rates without being used up

They affect the kinds of collisions that occur (the reaction mechanism)

Reaction Rates The speed of an event is defined as the

change that occurs in a given interval of time.

Reaction rate: the change in the concentration of reactants or products per unit of time

Progress of a hypothetical reaction A  B, starting with 1.00 mol A. Each red sphere represents 0.01 mol A, and each blue sphere represents 0.01 mol B. (a) At time zero, the vessel contains 1.00 mol A (100 red spheres) and 0 mol B (0 blue spheres). (b) After 20s, the vessel contains 0.54 mol A and 0.46 mol B. (c) After 40s, the vessel contains 0.30 mol A and 0.70 mol B. 

Reaction Rates

Rate of this reaction can be expressed either as the rate of disappearance of reactant A or as the rate of appearance of product B.

Average rate of appearance of B =

change in concentration of B change in time

Reaction Rates

Average rate of disappearance of A =

- change in concentration of A change in time

Rates are always expressed as positive quantities

How do we know that A and B are in a one to one mole ratio?

Change of Rate with Time

Instantaneous Rate

Graphs showing how the concentration of a reactant or product changes with time allow us to evaluate the instantaneous rate

○ Rate at a particular moment in the reaction

Instantaneous Rates

From now on “rate” will mean “instantaneous rate” unless indicated otherwise

Rate = - Δ[reactant] Δt Rate = Δ[product] Δt

Reaction Rates and Stoichiometry

In general for a reaction:

aA + bB cC + dD

The rate is given by:

Relating Rates at Which Products Appear and Reactants Disappear (a) How is the rate at which ozone

disappears related to the rate at which oxyen appears in the reaction 2O3(g) 3O2(g) ?

(b) If the rate at which O2 appears, is 6.0x10-5 M/s, at what rate is O3 disappearing at this same time?

Relating Rates at Which Products Appear and Reactants Disappear The decomposition of N2O5 proceeds

according to the following equation:

2N2O5(g) 4NO2(g) + O2(g)

If the rate of decomposition of N2O5 at a particular instant in a reaction vessel is 4.2x10-7 M/s, what is the rate of appearance of (a) NO2, (b) O2?

The Rate Law

One way of studying the effect of concentration on reaction rate is to determine the way in which the rate at the beginning of a reaction (the initial rate) depends on the starting concentrations.Measure concentration of reactants and a

function of time

NH4+(aq) + NO2

- (aq) N2 (g) + 2H20(l)

Experiment # Initial NH4+

Concentration (M)Initial NO2

Concentration (M)Observed Initial

Rate (M/s)

1 0.0100 0.200 5.4 x10-7

2 0.0200 0.200 10.8 x10-7

3 0.200 0.0202 10.8 x10-7

4 0.200 0.0404 21.6 x10-7

5 0.200 0.0808 43.3 x10-7

The Rate Law

An equation which shows how the rate depends on the concentrations of reactants, is called a rate law.

For a general reaction, aA + bB cC + dD

The rate law takes the form:

Reaction Orders: The Exponents in the Rate Law Rate laws for most reactions have the

general form:

Rate = k[reactant 1]m [reactant 2]n … The exponents m and n in a rate law are

called reaction orders The overall reaction order is the sum

of the orders with respect to each reactant in the rate law.

Reaction Orders: The Exponents in the Rate Law The exponents in a rate law indicate

how the rate is affected by the concentration of each reactant.

Although the exponents in a rate law are sometimes the same as the coefficients in the balanced equation, this is not necessarily the case.

Units of Rate Constants

Units of the rate constant depend on the overall reaction order of the rate law.

units of rate = (units of rate constant)*

(units of concentration)

Units of rate constant = (units of rate)

(units of concentration)

Determining Reaction Orders and Units for Rate Constants What is the unit for the rate constant for

the following rate law? 2N2O5 4NO2 + O2

(a) What is the reaction order of the reactant H2 in the following equation: H2 + I2 2HI? (b) What are the units of the rate constant for this equation?

Using Initial Rates to Determine Rate Laws Must be determined experimentally Observe the effect of changing the initial

concentrations of the reactants on the initial rate of the reaction

The rate of reaction depends on concentration, the rate constant does not depend on concentration

Using Initial Rates to Determine Rate Laws Reaction orders: 0 = changes in concentration of the reactant

will have no effect on the rate because any concentration raised to the zero power = 1

1= changes in concentration of reactant will produce proportional changes in the rate

2= changes in concentration of reactant will result in changes by the factor of x2

Determining Rate Law from Initial Rates The initial rate of a reaction A + B C

Using this data, determine (a) the rate law for the reaction, (b) the rate constant, (c) the rate of the reaction when [A] = 0.050M and [B] = 0.100M.

Experiment # [A] (M) [B] (M) Initial Rate (M/s)

1 0.100 0.100 4.0 x 10-5

2 0.100 0.200 4.0 x 10-5

3 0.200 0.100 16.0 x 10-5

Determining Rate Law from Initial Rates 2 NO(g) + 2H2(g) N2(g) + 2H20(g)

Using this data, determine (a) the rate law for the reaction, (b) the rate constant, (c) the rate of the reaction when [NO] = 0.050M and [H2] = 0.150M.

Experiment # [A] (M) [B] (M) Initial Rate (M/s)

1 0.10 0.10 1.23 x 10-3

2 0.10 0.20 2.46 x 10-3

3 0.20 0.10 4.92 x 10-3

The Change of Concentration with Time Rate laws can be converted into

equations that show the relationship between the concentrations of the reactants or products and time. First-order reactionsSecond-order reactions

First-Order Reactions

A first-order reaction is one whose rate depends on the concentration of a single reactant raised to the first power.

Integrated rate laws can be used to determine (1) the concentration of a reactant remaining at an time after the reaction has started, (2) the time required for a given fraction of a sample to react, or (3) the time required for a reactant concentration to fall to a certain level.

Using the Integrated First-Order Rate Law The decomposition of a certain insecticide

in water follows first-order kinetics with a rate constant of 1.45yr-1 at 12°C. A quantity of this insecticide is washed into a lake on June 1, leading to a concentration of 5.0x10-7g/cm3. (a) What is the concentration of the insecticide on June 1 of the following year? (b) How long will it take for the concentration of the insecticide to decrease to 3.0x10-7g/cm3?

Using the Integrated First-Order Rate Law The decomposition of (CH3)2O at 510°C

is a first-order process with a rate constant of 6.8x10-4s-1. If the initial pressure of (CH3)2O is 135torr, what is its pressure after 1420s?

Second-Order Reactions

A second-order reaction is one whose rate depends on the reactant concentration raised to the second power or on the concentrations of two different reactants, each raised to the first power.

Examples

Half-Life

The half-life of a reaction, t1/2, is the time required for the concentration of the reactant to reach one-half of its initial value, [A]t1/2 = ½ [A]0.

Describes how fast a reaction occurs A fast reaction will have a short half-life

Determining the Half-Life of a First-Order Reaction At 600K, the half-life for a particular

process is 3.3x105s. (a) What is the rate constant at this temperature? (b) At 300K, the rate constant is 2.1x10-5s-1. What is the half-life at this temperature?

Temperature and Rate The rates of most chemical reactions

increase as the temperature rises. Why? The faster rate at

higher temperature is due to an increase in the rate constant with increasing temperature

The Collision Model

We know reaction rates depend on concentration and temperature.The collision model explains why on a

molecular level

Based on the kinetic molecular theory

The Collision Model

Molecules must collide to react The greater the number of collisions, the

greater the reaction rate

1. How does this explain rate increasing with increasing concentration?

2. How does this explain rate increasing with increasing temperature?

The Collision Model For a reaction to occur, more is required

than just a collision

The Orientation Factor

The Orientation Factor

Molecules must be oriented a certain way during collisions for a reaction to occur.

The relative orientationsof molecules during their collisions determine whether the atoms are suitably positioned to form new bonds.

Activation Energy

Molecules must possess a certain minimum amount of energy to react

This energy comes from the kinetic energies of the colliding molecules

Upon collision, the kinetic energy of the molecules can be used to stretch, bend, and ultimately break bonds, leading to chemical reactions

Activation Energy

If molecules move too slowly, with too little kinetic energy, they merely bounce off one another without changing

The minimum amount of energy required to initiate a chemical reaction is called the activation energy, Ea.

Activation Energy The particular arrangement of atoms at

the top of the barrier is called the activated complex or transition state.

The rate depends on the magnitude of Ea

The lower Ea, the faster the reaction

Determining Rate Law Lab

-Add 2mL of HCl-Time until no longer see +

-Add 2mL of Na2S2O3

-Time until no longer see +

Determining Rate Law Lab