260 Chapter 7

7.37.3

Have you noticed that some reactions occur quickly, while othersoccur slowly? A match burns almost immediately, but the epoxyglue used to repair a toy may take a day to completely harden(Figure 1). What factors affect the rate of reaction, and how can weexplain these effects?

There are four factors that affect the rate of reaction. You haveseen the effects of temperature and concentration on the rate ofreaction in Investigations 7.1 and 7.2. Other factors that affectrates of reaction are surface area and the presence of catalysts.How do these factors affect the rate of reaction?

Changing the Rate of a Reaction

Imagine that you are on a canoe camping trip and have justfinished a long day. You want to build a campfire to cook yourmeal and to provide warmth (Figure 2). What materials will youchoose to burn? Why? How will you arrange those materials? Howwill you work with the fire to make it burn more quickly? Howwill you put the fire out before you go into your tent to sleep?

You must first decide how to set up the fire pit and what toburn. Rocks can be used to surround and contain the fire becausethey do not burn. All wood is flammable, but some kinds of wood burnfaster than others. You will need to find dry wood because you need a hightemperature to get the fire burning; wet wood uses much of this heat toevaporate the water. If you start your fire with paper or small pieces ofwood, you can gradually add larger pieces as the fire gets going. Tomake your fire burn more quickly, you can blow on it or fan it. Inbuilding this fire, what factors are you changing to control thereaction? Before we answer this question, let’s review a model thatmight help our understanding of why reactions occur.

The Kinetic Molecular Theory:

A Collision Model

In previous grades, you used the particle theory of matter toexplain changes of state. For example, as a solid is heated, themolecules move more quickly until they break apart from oneanother to form the liquid state. The kinetic molecular theorystates that particles are constantly moving and that they move morequickly at higher temperatures. Can the particle theory and the kineticmolecular theory also be used to explain how various factors affect the rateof a reaction?

As particles, or molecules, move about, they hit or collide with eachother. If the molecules are moving slowly or if they are very stable, thecolliding molecules just bounce off one another, and no reaction occurs.But if the molecules move more quickly and hit harder, there is a chance

Factors that Affect Rates of Reaction

Figure 2

A number of factors control how fast a campfire burns.

When you blow air on a fire, youare exhaling oxygen. The processof respiration produces carbondioxide gas, but we also breatheout a considerable fraction of theoxygen that we breathe in.

Did You Know ?

Figure 1

The two ingredients in epoxy glue combine and hardenin a chemical reaction that occurs at a definite rate.

beforecollision

collision effectivecollision

H2

HI

HII2

Controlling Chemical Reactions 261

that the molecules may come apart and the atoms may combine to formnew molecules. Most collisions just result in the molecules bouncing offone another, with no change. But a small fraction of collisions areeffective: they cause chemical bonds to break and re-form to make newmolecules (Figure 3).

The collision model states that the rate of reaction is affected by thenumber of collisions of reactant molecules. Molecules in a gas or a liquid areconstantly colliding with each other. If all the collisionsbetween molecules were effective, then reactionswould occur instantaneously.According to the collision model, thereare two ways to make a reaction gofaster:• increase the number of collisions;• increase the fraction of collisions

that are effective.We can use the collision model to

help us understand how temperature,concentration, surface area, andcatalysts affect the rate of a reaction.

Temperature

How does temperature affect the rate atwhich a reaction occurs? You know that sugar dissolves faster in hot waterthan in cold. Broiled burgers and French fries at your local hamburgerrestaurant are products of chemical reactions that occur faster at highertemperatures. Most reactions occur faster at higher temperatures.

Of course, temperature can also be used to slow reactions down. Therefrigerator and freezer at home are used to reducethe rate of natural decay reactions that occur infoods. At low temperatures, “cold-blooded” animalsbecome less active (Figure 4).

Temperature is the most important factor inmaking a reaction go faster. An increase intemperature of only 5°C or 10°C is enough todouble the rate of many reactions. Why does thishappen? As the temperature increases, the averagespeed of the molecules increases. As the moleculesmove faster around the container, they encounterand collide with more molecules. Although there arestill some molecules that move slowly, there is alarger fraction of “quick” molecules. Thesemolecules may hit each other hard enough forchemical bonds to break and new molecules toform. An increase in temperature makes themolecules collide more often and more effectively.No wonder it has such a dramatic effect on the rateof a reaction!

Figure 4

The temperature of a cold-blooded animal depends on the temperature ofits surroundings. As its environment cools, this toad’s body temperaturedrops, the reactions taking place in its body slow down, and the animalbecomes sluggish.

Figure 3

Effective collisions between molecules can cause the formation of new molecules.

262 Chapter 7

Concentration

What happens to the rate of a reaction when the concentrations of thereactants increase? Concentrated hydrochloric acid reacts vigorously withmetals (Figure 5) and other substances, but the same molecule is in yourstomach right now! The hydrochloric acid that digests your food is muchless concentrated because it is dissolved in water. As you learned inInvestigation 7.2, increasing the concentration of a reactant increases therate of the reaction.

How can we use the collision model to explain the increase in the rateof reaction as the concentration of reactants increases? When moremolecules are packed into a smaller space, they are more likely to collidewith each other. As an analogy, imagine that you are measuring the numberof times that you meet your friends by chance in the hallway during atypical school day. If you increase the number of friends you have at school,you will likely encounter more friends during the day. If you and yourfriends were to move to a much smaller school, you would also be morelikely to meet more friends in a given day. Increasing the concentration ofthe reactants in a container increases the number of collisions between themolecules.

Surface Area

The rate of a reaction is affected by surface area. Surface area is theamount of area of a sample of matter that is visible and able to react. Forexample, the surface area of a whole orange is the outer peel; moresurface area is exposed if we cut the orange in half.

When reactants are present in different phases — for example, asolid and a gas — the area of contact, or surface area, determines therate of reaction. The surface area can be increased by decreasing thesizes of the pieces of the reactant. For example, a pile of flourdoes not react quickly with oxygen from the air, even when aflame is directed onto it. But when a cloud of flour particles ispuffed through a flame (Figure 6), the reaction happens muchmore quickly as the flour and oxygen combine. Such dustexplosions have destroyed grain elevators and caused fatalaccidents (Figure 7).

Figure 6

Increasing the surface area of a solid increases the rate of reaction.

Figure 7

Grain elevators are used to store wheat and other grains.Care must be taken not to have open flames where dustparticles might be suspended in the air and able to reactwith oxygen.

Figure 5

Concentrated acids are very corrosive andmust be treated with care.

Controlling Chemical Reactions 263

How can we use the collision model to explainthe increase in rate of reaction as the surface area ofthe reactants increases? Reactions occur more quicklyas the number of collisions between moleculesincreases. A reaction between a solid and a liquid orgas can occur only where the solid particles are incontact with the other phase. Increasing the numberof solid particles that are available to react allowsmore molecules of the second phase to collide withthem. Increasing the surface area increases thenumber of collisions between molecules andtherefore the rate of reaction.

Catalysts

What happens to the rate of a reaction when acatalyst is used? A catalyst is a substance thatincreases the rate of a chemical reaction withoutbeing consumed by the reaction. Thus, a catalyst isnot a reactant. Catalysts are used in many industrialprocesses, including oil refining (Figure 8). As youmay know, catalysts are also very important incontrolling reactions in biological systems.

To understand how a catalyst works, think of achemical reaction as being similar to a car trip intothe mountains. Imagine that you are travelling from

Figure 8

A catalytic cracking unit at an oil refinery. Catalystsare used to crack hydrocarbons into smaller fragmentsthat can be re-formed into components of gasoline.

DESCRIBING THE RATE OF REACTIONQUANTITATIVELY

How can we calculate the rate of reaction?The rate of reaction may be described by asimple equation:

rate of reaction =quantity of product produced

time required

Thus, a process has a greater rate of reaction ifmore product is produced in a given time. Forexample, imagine a car factory. Suppose thatthe workers produce 20 cars in a day. Thus,

rate of reaction =20 cars

= 20 cars/day1 day

If the workers produce more cars in the sametime (for example, 30 cars in a day) then therate of reaction increases.

rate of reaction =30 cars

= 30 cars/day1 day

The rate of reaction may also be higher if thesame number of cars is produced in a shortertime (for example, 20 cars in half a day).

rate of reaction =20 cars

= 40 cars/day0.5 day

1 You have now learned how various factors affect therates of reactions. During the manufacturing process ofyour product, can you determine what, if any, factors areused to increase the rate of reactions?

Challenge

Work the WebVisit the Nelson web site at www.science.nelson.com.Follow the Science 10, 7.3 links to web sites that showreactions that occur at different rates.

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Understanding Concepts1. Make a chart to summarize the four factors that affect the

rate of reaction. For each factor,

(a) describe how it affects rate;

(b) give an example; and

(c) draw a sketch to show, on a molecular level, how itmakes a reaction go faster.

2. Explain how you would use your knowledge of factors thataffect the rate of reaction to cook a steak as quickly aspossible.

3. Consider a reaction in Container A in which 1000 collisionsoccur per second, and 100 of these collisions are effective— that is, they break chemical bonds to make a product.

(a) What fraction of the collisions are effective?

(b) Consider Container B, in which more reactants arepacked into the same space, so that 3000 collisionsoccur per second. How many effective collisions willnow occur per second?

(c) Suppose a catalyst is added to Container A so thattwice the fraction of collisions are effective. How manyeffective collisions will now occur per second?

4. What effect on the rate of reaction of a metal with 20 mL ofdilute acid at room temperature would you expect if

(a) the acid was cooled to 10°C?

(b) 2 mL of concentrated acid was added?

(c) the metal was ground into powder before addition tothe acid?

Making Connections5. Imagine that a factory is assembling cars. An efficiency

expert is hired to try to analyze factors that affect howquickly cars are being assembled. On Monday, she dividesthe jobs so that different groups are working on differentparts of the car at the same time. On Tuesday, she turnsdown the temperature in the factory. On Wednesday, shefinds a worker who has invented a new-and-improved wayto bolt together the vehicle’s parts. On Thursday, she hiresmore workers to work in the factory.

(a) Which “rate factor” is being investigated each day?

(b) What effect do you think would be observed each dayon the rate of production of the cars?

(c) Which factor do you think would be most effective?

(d) How is this analogy not quite the same as the rate ofreaction in a chemical system?

6. Look back at Investigation 6.9. What substance was usedas a catalyst in that activity?

7. Why do photographers and technicians often store film andbatteries in the refrigerator?

8. When you chew crackers, it is possible that three factorsaffect the rate of breakdown of carbohydrates. Explain.

9. Would ground meat or a steak spoil more quickly? Explain,using the collision model.

10. Which factors that affect the rate of reaction are involvedin making a campfire? Explain.

AlphavilleDeltaville

Betaville

Gammaville

Alphaville to Betaville (Figure 9). To get to Betaville, you could travelover a mountain through Gammaville. But if you take the roadthrough Deltaville (representing acatalyst), you still arrive at the same placeand you have an easier trip.

A catalyst provides an easier way for achemical reaction to occur. It does this bydecreasing the amount of collision energythat molecules need to break bonds andform new molecules. Instead of onemolecule in a thousand being able toreact, perhaps one molecule in ten canreact. A catalyst increases the fraction ofcollisions that are effective. You will learnmore about catalysts later in this chapter.

Figure 9

A catalyst provides an easier path for a reaction to follow.