7.3 Reaction Rates Text

7.3

Factors that Affect Rates of Reaction Have you noticed that some reactions occur quickly, while others occur slowly? A match burns almost immediately, but the epoxy glue used to repair a toy may take a day to completely harden (Figure 1). What factors affect the rate of reaction, and how can we explain these effects? There are four factors that affect the rate of reaction. You have seen the effects of temperature and concentration on the rate of reaction in Investigations 7.1 and 7.2. Other factors that affect rates 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 just finished a long day. You want to build a campfire to cook your meal and to provide warmth (Figure 2). What materials will you choose to burn? Why? How will you arrange those materials? How Figure 1 will you work with the fire to make it burn more quickly? How The two ingredients in epoxy glue combine and harden will you put the fire out before you go into your tent to sleep? in a chemical reaction that occurs at a definite rate. You must first decide how to set up the fire pit and what to burn. Rocks can be used to surround and contain the fire because they do not burn. All wood is flammable, but some kinds of wood burn faster than others. You will need to find dry wood because you need a high temperature to get the fire burning; wet wood uses much of this heat to evaporate the water. If you start your fire with paper or small pieces of wood, you can gradually add larger pieces as the fire gets going. To make your fire burn more quickly, you can blow on it or fan it. In building this fire, what factors are you changing to control the reaction? Before we answer this question, let’s review a model that might help our understanding of why reactions occur.

The Kinetic Molecular Theory: A Collision Model In previous grades, you used the particle theory of matter to Figure 2 explain changes of state. For example, as a solid is heated, the A number of factors control how fast a campfire burns. molecules move more quickly until they break apart from one another to form the liquid state. The kinetic molecular theory states that particles are constantly moving and that they move more D i d Yo u K n o w ? quickly at higher temperatures. Can the particle theory and the kinetic When you blow air on a fire, you molecular theory also be used to explain how various factors affect the rate are exhaling oxygen. The process of a reaction? of respiration produces carbon As particles, or molecules, move about, they hit or collide with each dioxide gas, but we also breathe other. If the molecules are moving slowly or if they are very stable, the out a considerable fraction of the colliding molecules just bounce off one another, and no reaction occurs. oxygen that we breathe in. But if the molecules move more quickly and hit harder, there is a chance 260 Chapter 7

that the molecules may come apart and the atoms may combine to form new molecules. Most collisions just result in the molecules bouncing off one another, with no change. But a small fraction of collisions are effective: they cause chemical bonds to break and re-form to make new molecules (Figure 3). The collision model states that the rate of reaction is affected by the number of collisions of reactant molecules. Molecules in a gas or a liquid are constantly colliding with each other. If all the collisions between molecules were effective, then reactions would occur instantaneously. According to the collision model, there are two ways to make a reaction go faster: • increase the number of collisions; • increase the fraction of collisions H2 that are effective. We can use the collision model to I2 help us understand how temperature, before concentration, surface area, and collision catalysts affect the rate of a reaction.

HI

HI collision

effective collision

Figure 3

Temperature

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

How does temperature affect the rate at which a reaction occurs? You know that sugar dissolves faster in hot water than in cold. Broiled burgers and French fries at your local hamburger restaurant are products of chemical reactions that occur faster at higher temperatures. Most reactions occur faster at higher temperatures. Of course, temperature can also be used to slow reactions down. The refrigerator and freezer at home are used to reduce the rate of natural decay reactions that occur in foods. At low temperatures, “cold-blooded” animals become less active (Figure 4). Temperature is the most important factor in making a reaction go faster. An increase in temperature of only 5°C or 10°C is enough to double the rate of many reactions. Why does this happen? As the temperature increases, the average speed of the molecules increases. As the molecules move faster around the container, they encounter and collide with more molecules. Although there are still some molecules that move slowly, there is a larger fraction of “quick” molecules. These molecules may hit each other hard enough for chemical bonds to break and new molecules to form. An increase in temperature makes the molecules collide more often and more effectively. Figure 4 No wonder it has such a dramatic effect on the rate The temperature of a cold-blooded animal depends on the temperature of of a reaction! its surroundings. As its environment cools, this toad’s body temperature drops, the reactions taking place in its body slow down, and the animal becomes sluggish.

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Concentration What happens to the rate of a reaction when the concentrations of the reactants increase? Concentrated hydrochloric acid reacts vigorously with metals (Figure 5) and other substances, but the same molecule is in your stomach right now! The hydrochloric acid that digests your food is much less concentrated because it is dissolved in water. As you learned in Investigation 7.2, increasing the concentration of a reactant increases the rate of the reaction. How can we use the collision model to explain the increase in the rate of reaction as the concentration of reactants increases? When more molecules are packed into a smaller space, they are more likely to collide with each other. As an analogy, imagine that you are measuring the number of times that you meet your friends by chance in the hallway during a typical 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 your friends were to move to a much smaller school, you would also be more likely to meet more friends in a given day. Increasing the concentration of the reactants in a container increases the number of collisions between the molecules.

Figure 5

Concentrated acids are very corrosive and must be treated with care.

Surface Area The rate of a reaction is affected by surface area. Surface area is the amount of area of a sample of matter that is visible and able to react. For example, the surface area of a whole orange is the outer peel; more surface area is exposed if we cut the orange in half. When reactants are present in different phases — for example, a solid and a gas — the area of contact, or surface area, determines the rate of reaction. The surface area can be increased by decreasing the sizes of the pieces of the reactant. For example, a pile of flour does not react quickly with oxygen from the air, even when a flame is directed onto it. But when a cloud of flour particles is puffed through a flame (Figure 6), the reaction happens much more quickly as the flour and oxygen combine. Such dust explosions have destroyed grain elevators and caused fatal accidents (Figure 7).

Figure 7

Figure 6

Increasing the surface area of a solid increases the rate of reaction. 262 Chapter 7

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

How can we use the collision model to explain the increase in rate of reaction as the surface area of the reactants increases? Reactions occur more quickly as the number of collisions between molecules increases. A reaction between a solid and a liquid or gas can occur only where the solid particles are in contact with the other phase. Increasing the number of solid particles that are available to react allows more molecules of the second phase to collide with them. Increasing the surface area increases the number of collisions between molecules and therefore the rate of reaction.

Catalysts What happens to the rate of a reaction when a catalyst is used? A catalyst is a substance that increases the rate of a chemical reaction without being consumed by the reaction. Thus, a catalyst is not a reactant. Catalysts are used in many industrial processes, including oil refining (Figure 8). As you may know, catalysts are also very important in controlling reactions in biological systems. To understand how a catalyst works, think of a chemical reaction as being similar to a car trip into the mountains. Imagine that you are travelling from

DESCRIBING THE RATE OF REACTION QUANTITATIVELY How can we calculate the rate of reaction? The rate of reaction may be described by a simple equation: quantity of product produced rate of reaction =

time required

Thus, a process has a greater rate of reaction if more product is produced in a given time. For example, imagine a car factory. Suppose that the workers produce 20 cars in a day. Thus, 20 cars rate of reaction =

1 day

= 20 cars/day

If the workers produce more cars in the same time (for example, 30 cars in a day) then the rate of reaction increases. 30 cars rate of reaction =

1 day

= 30 cars/day

The rate of reaction may also be higher if the same number of cars is produced in a shorter time (for example, 20 cars in half a day). 20 cars rate of reaction =

0.5 day

= 40 cars/day

Challenge 1 You have now learned how various factors affect the rates of reactions. During the manufacturing process of your product, can you determine what, if any, factors are used to increase the rate of reactions?

Work the Web Visit the Nelson web site at www.science.nelson.com. Follow the Science 10, 7.3 links to web sites that show reactions that occur at different rates. Figure 8

A catalytic cracking unit at an oil refinery. Catalysts are used to crack hydrocarbons into smaller fragments that can be re-formed into components of gasoline.

Controlling Chemical Reactions 263

Alphaville to Betaville (Figure 9). To get to Betaville, you could travel over a mountain through Gammaville. But if you take the road through Deltaville (representing a catalyst), you still arrive at the same place and you have an easier trip. A catalyst provides an easier way for a chemical reaction to occur. It does this by decreasing the amount of collision energy that molecules need to break bonds and form new molecules. Instead of one molecule in a thousand being able to Alphaville react, perhaps one molecule in ten can react. A catalyst increases the fraction of collisions that are effective. You will learn more about catalysts later in this chapter.

Gammaville

Betaville Deltaville

Figure 9

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

Understanding Concepts 1. 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 it makes a reaction go faster. 2. Explain how you would use your knowledge of factors that affect the rate of reaction to cook a steak as quickly as possible. 3. Consider a reaction in Container A in which 1000 collisions occur 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 are packed into the same space, so that 3000 collisions occur per second. How many effective collisions will now occur per second? (c) Suppose a catalyst is added to Container A so that twice the fraction of collisions are effective. How many effective collisions will now occur per second? 4. What effect on the rate of reaction of a metal with 20 mL of dilute 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 to the acid?

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Making Connections 5. Imagine that a factory is assembling cars. An efficiency expert is hired to try to analyze factors that affect how quickly cars are being assembled. On Monday, she divides the jobs so that different groups are working on different parts of the car at the same time. On Tuesday, she turns down the temperature in the factory. On Wednesday, she finds a worker who has invented a new-and-improved way to bolt together the vehicle’s parts. On Thursday, she hires more 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 day on 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 of reaction in a chemical system? 6. Look back at Investigation 6.9. What substance was used as a catalyst in that activity? 7. Why do photographers and technicians often store film and batteries in the refrigerator? 8. When you chew crackers, it is possible that three factors affect 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 involved in making a campfire? Explain.