Chapter 14Chemical Kinetics

Introduction to Kinetics

1. Chemistry is concerned with change. It is concerned with how chemicals can react to form new substances.

2. Equally important, is how rapidly these chemical reactions occur. They can take fractions of a second (an explosion) or millions of years (diamond formation).

Kinetics

1. Kinetics is the area of chemistry concerned with the speed (or rate) of reactions (chemical processes).

2. Why are rates important? Some examples:• Rate of food spoiling • Rate of steel rusting• Rate of paint drying • Rate of aging

Kinetic Outline You will learn:• 1. Collision theory What factors control rates. • 2. Reaction Rates How we measure rates.• 3.Rate Laws How the rate depends on amounts of reactants.• 4. Integrated Rate Laws How to calculate amount left or time to reach a given amount.• 5. Half-life How long it takes to react 50% of reactants.• 6. Arrhenius Equation How rate constant changes with temperature.• 7. Reaction Mechanisms How the reaction occurs at the molecular level.

Collision Theory

In order for a chemical reaction to occur:1.There must be collisions between the reactant

molecules. (More frequent collisions = faster rate)2. The collisions must be affective.3. To be affective, collisions must occur with: a) sufficient energy. b) suitable orientation.

Factors That Affect Reaction Rates

1. Physical State of the Reactants The likelihood of collisions is higher if the reactants are mixtures of homogeneous gases or liquids or solutions. Solids react faster if their exposed surface area is increased.

2. Concentration of ReactantsAs the concentration of reactants increases, so does the likelihood that

reactant molecules will collide.3. Temperature

At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.

4. CatalystsSpeed reactions without being used up by changing the reaction

mechanism. (Examples: enzymes)

Reaction Rates

• Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.

• Average Rate = [A] /t

D = change[ ] = concentration (Molarity)

A = the chemical t = time (usually seconds)

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

Reaction Rates In this reaction, the concentration of butyl chloride, C4H9Cl,

was measured at various times, t.

Notice: The concentration of the reactant, C4H9Cl, decreases as time goes on.

What happens to the concentration of the products?

The average rate of the reaction over each interval is the change in concentration divided by the change in time:

Average Rate

average rate = 0.00019 M/s

You do the next time interval.

Average Rate = 0.0905 – 0.0820 M 100.0 – 50.0 s

Average Rate = 0.00017 M/s

Notice that the average rate decreases as the reaction proceeds. Why? As the reaction goes forward, there are fewer collisions as the amount of reactant decreases.

Completed Chart

Reaction Rates

A plot of concentration vs. time for this reaction yields a curve like this.

The slope of a line tangent to the curve at any point is the instantaneous rate at that time.

Reaction Rates

The reaction slows down with time because the concentration of the reactants decreases.

Instantaneous Rate = slope

Instantaneous Rate = Rise/Run

Instantaneous Rate = [A] /t

Reaction Rates and Stoichiometry

• In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1.

• Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance of C4H9OH.

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

Rate = -[C4H9Cl]t

= [C4H9OH]t

Reaction Rates and Stoichiometry

• What if the ratio is not 1:1?H2(g) + I2(g) 2 HI(g)

• HI will appear twice as fast as H2 and I2 disappear. • To correct for this, you must equate the rates.

Negative sign for reactants! Why?

General Equation for Equating Rates

• For the reaction

aA + bB cC + dD

Reactants (decrease) Products (increase)

You try:

4 A (g) + 3 B (g) 7 C (g) + 2 D (g)

Which chemical appears or disappears the fastest?Why?

Sample Problem

Given: 4 A (g) + 3 B (g) 7 C (g) + 2 D (g)

• If the rate of disappearance of A = 0.48 mol/s, what is the rate of appearance of C?

• Easiest Way: set up proportion

4 moles A = 7 moles C 0.48 mol/s x

Cross-multiply 4 x = 7 (0.48) 4 x = 3.36 x = 0.84 answer = 0.84 mol/s