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18.1-RATES OF REACTION
Standards: 8a, 8b, 8c, 8d
Vocabulary
rate, collision theory, activation energy, activated complex, transition state, inhibitor
Connecting to Your World
Metal objects can be damaged by corrosion, but this process can be useful to hungry soldiers, truck
drivers, and others who want a hot meal but do not have a place to cook it. Products now on the market use the
heat given off by the corrosion reaction of an iron-magnesium alloy with salt water to produce a hot meal.
Normally the reaction takes place at such a slow rate that the heat released from the process cannot be used. But
when the rate is increased by the addition of salt water, heat is produced rapidly. In this section, you will learn
some ways in which the rate of a reaction can be increased.
COLLISION THEORY
When you strike a match, it erupts into flame almost instantly and burns quickly. Some other reaction occurs
slowly. For example, millions of years were required for dead plants beneath Earth’s surface to be converted to
coal. These examples show that the speed of chemical reactions can vary from instantaneous to extremely slow.
The concept of speed is already familiar to you. The winning sprinter in figure 18.1 covers 100 meters in
11.5 second. She runs at a speed of 8.70 m/s. Another sprinter takes 15.0 seconds to cover the same distance.
She runs at a speed of 6.67 m/s. Both 8.70 m/s and 6.67 m/s express rates of travel.
A rate is a measure of speed of any change that occurs within an interval of time. The interval of time
may range from fractions of a second of centuries. Figures 18.2 show some familiar examples of rates of
change. If one half of a 1-molar piece of iron turns to rust in one year, the rate of which iron rusts might be
expressed as 0.5 mol/yr. In chemistry, the rate of chemical change or the reaction rate is usually expressed
as the amount of reactant changing per unit time. The progress of a typical reaction is illustrated in figure
18.3.
Rates of chemical reactions are related to the properties of atoms, ions, and molecules through a model
called collision theory. According to collision theory, atoms, ions, and molecules can react to form products
when they collide with one another, provided that the colliding particles have enough kinetic energy. Particles
lacking the necessary kinetic energy to react bounce apart unchanged when they collide. Figure 18.4 is a model
for the reaction of hydrogen and oxygen molecules to form water.
You can use two balls of soft modeling clay to illustrate collision theory. If you throw the balls of clay
together gently, they don’t stick to one another. This is analogue to colliding particles of low energy that fail to
react. The same balls of clay thrown together with great force, however, stick tightly to each other. This is
analogous to the collision of two high-energy particles that results in the formation of a product.
You can also use modeling clay to illustrate another point about chemical reactions. If you roll clay into
a rope and begin to shake one end more and more vigorously, you will come to a point where the clay rope will
break. Similarly, if enough energy is applied to a molecule, the bonds holding the molecule together can break
apart. Substances, when supplied with sufficient energy, decompose to simpler substances or reorganize
themselves into new substances. The minimum energy that colliding particles must have in order to react is
called the activation energy. In a sense, the activation energy for a chemical reaction is a barrier that reactants
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must cross to be converted to products. You can see in the energy diagram in figure 18.5 that when two reactant
particles with necessary activation energy collide, a new entity called the activated complex may form.
An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the
activation-energy barrier. An activating complex forms only if the colliding particles have sufficient energy and
if the atoms are oriented properly. The lifetime of an activated complex is typically about 10 -13s. Its brief
existence ends with the re-formation of products. The two outcomes are equally likely. Thus the activated
complex is sometimes called the transition state. Collision theory explains why some naturally occurring
reactions are immeasurably slow at room temperature. Carbon and oxygen react when charcoal burns, but this
reaction has a high activation energy. At room temperature, the collisions of oxygen and carbon molecules are
not energetic enough to break the O─O and C─C bonds. These both bonds must be broken to form the activated
complex. Thus the reaction rate of carbon with oxygen at room temperature is essentially zero.
√ Checkpoint: What are two events that might occur after the formation of an activate complex?
Factors Affecting Reaction Rates
Every chemical reaction proceeds at its own rate. Some reactions are naturally fast and some are naturally slow
under the same conditions. However, by varying the conditions of the reaction, the rate of almost any reaction
can be modified. The rate of a chemical reaction depends upon temperature, concentration, particle size,
and the use of a catalyst.
Temperature Usually, raising the temperature speeds up reaction, while lowering the temperature slows down
reactions. At higher temperatures, the motions of the reactant particles are faster and more chaotic than they are
at lower temperatures. Increasing the temperature increases both the frequency of collisions and the number of
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particles that have enough kinetic energy to slip over the activation-energy barrio to become products. An
increase in temperature, therefore, causes products to form faster. A familiar example of the effect of
temperature on reaction rate is the burning of charcoal. At room temperature, a bag of charcoal in contact with
air does not burn. But when a starter flame touches the charcoal, atoms of the reactants (carbon and oxygen)
collide with higher energy and grater frequency. Some collisions are at a high enough energy to form the
product (carbon dioxide). The heart released by the reaction then supplies enough energy to get more carbon
and oxygen over the activation-energy barrier. When the starter flame is removed, the reaction continues.
Concentration In a crowded room where people are moving about rapidly, you may find yourself bumping into
people frequently than if you were one of a few occupants. Similarly, the number of particles in a given volume
affects the rate at which reactions occur. Cramming more particles into a fixed volume increases the
concentration of reactants and consequently the frequency of collision. Increased collision frequency leads to a
higher reaction rate. The lighted splint in Figure 18.6 only glows in air, which is 20% oxygen. It may soon be
extinguished. But when the glowing splint is plunged into pure oxygen greatly speeds up the combustion
reaction. This is why flames are forbidden in areas where bottled oxygen is in use.
Particle Size If you put a bundle of sticks on a campfire, they bur quickly. A log, however, burns more slowly.
Small pieces of wood have more surface area than a log, and a surface area plays an important role in
determining the rate of reaction. The same is true for other chemical reactions. The total surface area of a solid
or liquid reactant affects the reaction rate. The smaller the particle size, the larger is the surface area for a given
mass of particle. An increase in surface areas increases the amount of the reactant exposed for reaction, which
further increases the collision frequency and the reaction rate.
One way to increase the surface area of solid reactants to dissolve them. In a solution, particles are
separated and more accessible to other reactants. You can also increase the surface area of a solid by grinding it
into a fine powder. Small dustlike particles, however, can be dangerous when suspended in air. As coal miners
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know , large chunks of coal pose little danger, but coal dust mixed with air is an explosive hazard because of its
large surface area. The same is true of dust in a flour mill or a grain elevator, as the dramatic photograph in
Figure 18.7 shows
Catalysts Increases the temperature is not always the best way to increase the rate of reaction. A catalyst
is often better. Recall that a catalyst is a substance that increases the rate of a reaction without being used up
during the reaction. Catalysts permit reactions to proceed along a lower energy path, as Figure 18.8 shows.
Notice that the activation energy barrier for the catalyzed reaction is lower than that of the uncatalyzed reaction.
With a lower activation-energy barrier, more reactants have the energy to form products within a given time.
For instance, the rate of the combination reaction of hydrogen and oxygen at room temperature in negligible,
but with a trace of finely divided platinum (Pt) as a catalyst, the reaction is rapid.
2H2(g) + O2(g)  2H2O(l)
Because a catalyst is not consumed during a reaction, it does not appear as a reactant or product in the
chemical equation. Instead, the catalyst is often written above the yield arrow, as in the equation above.
Catalysts are crucial for many life processes. Your body temperature is only 37C and cannot be raised
significantly without danger. Without catalysts, few reactions in the body would proceed fast enough. Enzymes
are biological reactions. When you eat a meal containing protein, enzymes in your digestive tract break down
the protein molecules in a few hours. Without enzymes, the digestion of protein at 370C would take many
years!
An inhibitor is a substance that interferes with the action of a catalyst. Some inhibitor molecules work
by reacting with, or “poisoning,” the catalyst itself. Thus the inhibitor reduces the amount of functional catalyst
available. Reaction slow or even stop when a catalyst is poisoned.
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18.1 Section Assessment
1. Key Concept How is the rate of a chemical reaction expressed?
2. Key Concept What are four factors the affect the rate of a chemical reaction?
3. Suppose a thin sheet of zinc containing 0.2 mol of the metal is completely converted to zinc oxide
(ZnO) in one month. How would you express the rate of conversion of the zinc?
4. Does every collision between reacting particles lead to products? Explain.
5. Refrigerated food stays fresh for long periods. The same food store at room temperature quickly
spoils. Why?
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