CHEMICAL KINETICSChemistry 11 - Kinetics

Source: Brown and LeMay’s Chemistry the Central Science

Objectives

Change

How does change affect us? What are some factors that we think about when

we discuss change? Today is Earth Day, what changes should be

discussed?

Chemical kinetics

Chemistry, by its very nature, is concerned with change.

What changes have we discussed / observed this semester in Chemistry?

Chemical kinetics

Chemical reactions convert substances into new substances with entirely different properties.

Aside from studying what changes occur, it is also important to study how these changes occur..

What are some things that might describe how a chemical reaction occurs?

Chemical kinetics

Chemical kinetics is concerned with how fast—or the rate at which—a reaction occurs.

There is an enormous range of rates of reaction.

Give an example of a quick reaction (fast rate). Give an example of a slow reaction (slow rate).

Chemical kinetics

Some examples of the application of kinetics include pharmaceuticals, environmental science, and industrial chemistry.

Our goals for this topic are: to understand how to determine the rates at which

reactions occur to consider the factors that control these rates

Factors that affect reaction rates

What factors determine how rapidly food spoils? What factors affect how quickly steel rusts?

Factors that affect reaction rates

How does a chemical reaction occur? What factors might affect how quickly a reaction

occurs?

Factors that affect reaction rates

On a molecular level, chemical reactions occur when molecules collide.

The more collisions, the faster the reaction. There are four factors that allow us to change

the rates at which particular reactions occur:1. the physical state of the reactants2. the concentrations of the reactants3. the temperature at which a reaction occurs4. the presence of a catalyst

Factors that affect reaction rates

Reactants must come together in order for a reaction to occur.

The more contact there is between molecules, the more rapidly they will react.

Of the three states of matter, which states are most ideal for molecules to interact with each other?

Physical state

Factors that affect reaction rates

Most of the reactions we will consider are either gases or liquid solutions because these states are most ideal.

Aside from the state (of matter), what other factors of physical state might affect the rate of reaction?

Physical state

Factors that affect reaction rates

How do you think concentration is related to reaction rates (e.g. burning of steel wool)?

Concentration

Factors that affect reaction rates

The higher the concentration of the reactants, the faster a reaction occurs.

Concentration

Factors that affect reaction rates

What effect does temperature have on molecules?

How does this affect a chemical reaction?

Temperature

Factors that affect reaction rates

Reactions occur more quickly at higher temperatures.

Recall that temperature is related to the average kinetic energy of molecules.

If molecules are moving around more, they will collide more frequently, which leads to increased reaction rates.

Think of spoiling milk.

Temperature

Factors that affect reaction rates

What does the word “catalyst” mean?

Catalysts

Factors that affect reaction rates

In chemistry, catalysts are agents / substances that increase reaction rate without being used up.

A very important example of a catalyst are enzymes in our bodies.

Catalysts

Reaction rates

What does the word “rate” imply?

Reaction rates

A rate describes how something changes over time.

In chemical reactions, what is changing over time?

Reaction rates

The rate of a reaction describes the in concentration of reactants or products over time.

Reaction rates

As time progresses, what happens to: the concentration of reactants? the concentration of products?

Reaction rates

Rate data for reaction of C4H9Cl with watert / s [C4H9Cl] / M Average rate

0.0 0.1000

50.0 0.0905

100.0 0.0820

150.0 0.0741

200.0 0.0671

300.0 0.0549

400.0 0.0448

500.0 0.0368

800.0 0.0200

1000.0 0

• Is C4H9Cl a reactant or product?

• What is happening to its concentration? Why?

• Calculate the rate of reaction at each time interval.

• What is the unit of measurement for reaction rate?

Reaction rates

Rate data for reaction of butyl chloride (C4H9Cl) with water

t / s [C4H9Cl] / M Average rate

0.0 0.1000

50.0 0.0905

100.0 0.0820

150.0 0.0741

200.0 0.0671

300.0 0.0549

400.0 0.0448

500.0 0.0368

800.0 0.0200

1000.0 0

• Construct a graph of the concentration of butyl chloride over time.

• How can we get the rate of reaction from the graph?

• Is the rate of reaction constant?

Reaction rates

Rate data for reaction of butyl chloride (C4H9Cl) with water

t / s [C4H9Cl] / M Average rate

0.0 0.1000

50.0 0.0905

100.0 0.0820

150.0 0.0741

200.0 0.0671

300.0 0.0549

400.0 0.0448

500.0 0.0368

800.0 0.0200

1000.0 0

• Let’s find the rate of reaction at t = 600s

• Now, find the rate of reaction at t = 50s.

• Determine the rate of reaction at t = 900s.

• Compare the rate at t = 50s and t = 900s.

Stoichiometry

The reaction in the previous example is represented as:C4H9Cl(aq) + H2O(aq) C4H9OH(aq) + HCl(aq)

How is the rate of disappearance of a reactant related to the rate of appearance of a product?

Stoichiometry

C4H9Cl(aq) + H2O(aq) C4H9OH(aq) + HCl(aq)

For this reaction,

However, notice that all the stoichiometric relationships are 1:1

Stoichiometry

What if the reaction were as follows?2HI(g) H2(g) + I2(g)

Would the relationship between disappearance of reactants and appearance of products be the same?

Stoichiometry

2HI(g) H2(g) + I2(g)

Stoichiometry

In general, for the reaction:aA + bB cC + dD

The rate is given by:

Stoichiometry

The reaction at which ozone (in the ozone layer of the atmosphere) disappears is expressed as:

2O3(g) 3O2(g)

(a) How is the rate of disappearance of ozone related to the rate of appearance of oxygen?

(b) If the rate at which O2 appears is 6.0 x 10-5 M/s, what is the rate at which ozone disappears?

Concentration

How does increasing the concentration of reactants affect rate?

Concentration

Consider the data for the following reaction:NH4

+(aq) + NO2-(aq) N2(g) + 2H2O(l)

Experiment number

Initial NH4+ conc.

(M)Initial NO2

- conc. (M)

Observed initial rate (M/s)

1 0.0100 0.200 5.4 x 10-7

2 0.0200 0.200 10.8 x 10-7

3 0.0400 0.200 21.5 x 10-7

4 0.0600 0.200 32.3 x 10-7

5 0.200 0.0202 10.8 x 10-7

6 0.200 0.0404 21.6 x 10-7

7 0.200 0.0606 32.4 x 10-7

8 0.200 0.0808 43.3 x 10-7

Concentration

The effect of concentration on rate of reaction can be expressed using a rate law:

Rate = k[A]m[B]n

where: k is the rate constant (depends on temperature)[A] and [B] are the concentrations of reactants A

and Bm and n are reaction orders

Concentration

For the rate law:Rate = k[A]m[B]n

the exponents m and n are called reaction orders. They describe how the concentration of the reactant affects rate.

1: first order2: second order3: third order, and so on…

Concentration

For the rate law:Rate = k[A]m[B]n

the exponents m and n are called reaction orders. They describe how the concentration of the reactant affects rate.

0: zeroth order

1: first order2: second order

3: third order, and so on… Reaction orders are determined experimentally.

Concentration

Rate = k[A]2[B] What will happen if [A] is doubled and [B] is

kept constant? If [B] is doubled and [A] is held constant? If [A] and [B] are doubled simultaneously?

Concentration

Consider the following data:Experiment

number[A] / M [B] / M Initial rate /

Ms-1

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

Concentration

Consider the following data:Experiment

number[A] / M [B] / M Initial rate /

Ms-1

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

Temperature

How does temperature affect reaction rate?

Temperature

If the temperature of a reaction is increased, reaction rate increases.

Why is that?

Temperature

In 1888, a Swedish scientist (Svante Arrhenius) suggested that molecules must possess a certain minimum amount of energy in order to react.

Golf analogy

Temperature

The minimum amount of energy required is called the activation energy.

Activation energy can also be determined experimentally and mathematically but we will not concern ourselves with that now.

Ener

gy

Reaction pathway

Catalysts

Catalysts increase reaction rate without being used up.

In terms of energy:

Ener

gy Uncatalyzed reaction

Reaction pathway

Catalyzed reaction

Catalysts

Examples of catalysts include platinum catalytic converters and enzymes.

Ener

gy Uncatalyzed reaction

Reaction pathway

Catalyzed reaction