Unit 14

Chemical Kinetics

In chemical kinetics, we study the rate at which a process occurs. The rates of reactions span an enormous range, from those that are complete within fractions of seconds, such as explosions, to those that take thousands or even millions of years, such as the formation of diamonds or other minerals in Earth’s crust.

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. What factors determine how fast food spoils? What determines the rate at which steel rusts? What controls the rate at which fuel burns in an automobile? We will not answer these specific questions, but we will see that the rates of all chemical reactions are subject to the same basic principles.

Four Factors That Affect Reaction Rates

1. The physical state of the reactants2. The concentrations of the reactants3. The temperature at which the reaction

occurs4. The presence of a catalyst

1.Physical State of the Reactants

Reactants must come together to react. The more readily molecules collide with each other, the more rapidly they react. Most of the reactions we consider are homogenous, which means the reactants are in the same phase. When reactants are in different phases, the reaction is limited to their area of contact. Reactions that involve solids tend to proceed faster if the surface area of the solid is increases. For instance, a medicine in the form of a fine powder will dissolve in the stomach and enter the bloodstream quicker than the same medicine in the form of a tablet.

2. Concentrations of the Reactants

Most chemical reactions proceed faster if the concentration of one or more reactants is increased.

Steel wool burns with difficulty in air, which contains 20% oxygen, but bursts into a brilliant white flame in pure oxygen.

3. The Temperature at Which the Reaction Occurs

The rates of chemical reactions increase as temperature is increased. We refrigerate perishable foods such as milk for this reason. Why does increasing the temperature lead to an increase in reaction rates? Increasing the temperature increases the kinetic energy of the molecules. As molecules move more rapidly, they collide more frequently and with higher energy, leading to increased 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 lead to a reaction occurring. Catalysts play very important roles in our lives. The physiology of most living species depends on enzymes, protein molecules that act as catalysts, increasing the rates of selected biochemical reactions.

Reaction Rates

The speed of an event is defined as the change that occurs in a given interval of time. For example, when we talk about the speed of a car, we usually talk in terms of miles per hour. The speed of a chemical reaction – its reaction rate– is the change in the concentration of reactants or products per unit of time. The units for a reaction rate are usually molarity per second(M/s).

Progress of a Reaction

A B

The 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. The average rate of appearance B is given by the change in concentration of B divided by the change in time:avg. rate of appearance of B = change in concentration of B change in time = [B] at t2- [B]at t1 = /\ [B]

t2-t1 /\ t

The average rate of disappearance of A would be expressed as [A] at t2- [A]at t1 = - /\ [A]

t2-t1 /\ t

Note the minus sign in this equation. By convention, rates are always expressed as positive quantities. Because [A]is decreasing with time, /\ A is a negative number. We use the negative sign to convert negative /\ A to a positive rate.

Study Check

From the data given in the captions below, calculate the average rate at which A disappears over the time interval from 20 s to 40s.

Change of Rate With Time

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

• A plot of [C4H9Cl] 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.

The instantaneous rate is the rate at that particular moment in the reaction. This rate is determined from the slope of the curve at this point of interest.To help you better understand the difference between average rate and instantaneous rate, imagine that you have just driven 98 mi in 2.0 hours. Your average speed is 49 mi/hr, but your instantaneous speed at any moment is the speedometer reading at that time.

It is typical for rates to decrease as a reaction proceeds, because the concentration of the reactants decreases.

Study Check

Using the following graph, determine the slope of the line at t= 0 to determine the instantaneous rate of the reaction at t=0.