Content Benchmark P.12.A.5
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Students know chemical reactions can take place at different rates, depending on a variety of factors (i.e. temperature, concentration, surface area, and agitation). E/S

Chemical reactions vary widely in how fast they occur. In order for chemical reactions to occur, the reacting particles must collide. These collisions must occur with enough energy and the correct orientation in order to have a chemical reaction occur. The energy that these colliding particles must possess in order to react is activation energy. If a collision occurs without these considerations being met, then no reaction will occur. If a reaction occurs, reactants change into products. The rate at which this change occurs is called the reaction rate.

There are many factors which affect the rate of chemical reactions, including (1) surface area, (2) temperature, (3) agitation, and (4) concentration. The following is a common example to see how these factors affect the rate of reaction.

Figure 1: A campfire represents some factors that affect reaction rate. (from http://coop.jsc.nasa.gov/photo/2004/fallsocial/index.htm)

Suppose you roast hot dogs over a fire as a special treat after taking a nature hike. How do you get the fire started? If you just take some logs and pile them up in a spot safe to burn them, it is virtually impossible to light the logs on fire. Instead, you must build a fire, using small twigs on the bottom, then bigger twigs, and eventually some logs. The small items have a lot of surface area where the oxygen from the air can get into contact with the wood. When you light the fire, you put the flame to the base of where you have placed the kindling. You have used some chemistry to get this fire to light. You have taken the surface area of your reactants into consideration. The larger logs are virtually impossible to light with a match or fire starter because they don’t have enough surface area for oxygen to get into contact with them to start to burn. By using smaller pieces of wood, you have increased the amount of surface area at which the fire can begin to burn.

To learn more about how surface area affects reaction rates, go to http://www.chemguide.co.uk/physical/basicrates/surfacearea.html.

Once the kindling starts to burn, the temperature increases. When the temperature is high enough, the larger twigs will start to burn and eventually the logs. You have used another factor to get your larger logs to burn. You have increased the temperature of the reactants. The average kinetic energy of the wood and oxygen molecules has increased. Now there are more particles that have sufficient kinetic energy (activation energy) to make effective collisions so that they can burn. The temperature of the flame that you used to start the fire wasn’t high enough to get the larger logs to burn directly. By raising the temperature of the larger logs and the air (with the oxygen in it) around the fire, those logs began to burn.

More detailed information about the temperature dependence of reaction rates can be found at http://www.chem.ufl.edu/~itl/2045/lectures/lec_m.html.

Sometimes, as the fire is burning, you may use agitation to increase the reaction rate. By stoking the fire, you are exposing more unburned wood to oxygen in the air. You have allowed more air to get in contact with the wood. You might even fan the fire or force fresh air into the fire. This increases the concentration of oxygen in contact with the wood. Now more particles are colliding effectively, allowing the fire to burn more quickly. Thus you have used concentration of reactants to increase the rate of reaction.

To learn more about factors affecting reaction rate, go to http://www.webchem.net/notes/how_far/kinetics/rate_factors.htm#3.%20Physical%20state

Once you have finished using the fire to roast hot dogs, you must either put the flames out or simply let the fire burn out. If you put the fire out, you can douse it with plenty of water or you can shovel dirt on top of it. In both cases, you are again using chemistry. By either of these actions, you are lowering the concentration of oxygen that can get to the burning logs to keep them burning. You are putting up a physical barrier of either water or dirt to cover the burning logs and put the fire out. You are also lowering the temperature of the reaction. Both the water and dirt are at a lower temperature than the logs and oxygen that are reacting chemically (burning). This lowers the kinetic energy of the particles that are reacting until they are at a low enough temperature to cease burning. The molecules no longer have sufficient energy to make effective collisions, and the rate decreases.

If you decide simply to let the fire burn out, you will see and sense evidence that the fire has stopped. There will no longer be flames visible around the reaction. The remnants of the fire will have a much lower temperature. Also, there will be a pile of ash where the fire used to be. The materials that are left from the chemical reaction of burning are very different from the original wood and oxygen. In fact, throughout the burning process, there have been many evidences that the properties of the materials produced are different from the properties of the wood and oxygen. There might be some soot that is produced from the burning wood. Also carbon dioxide and water vapor will have been produced during the burning. If you were to hold a cool piece of aluminum foil high over the fire, you might be able to collect some of the soot and condense water vapor produced in the burning. The black color of the soot and the liquid phase of the condensed water vapor will show you that the properties of the materials produced are very different from the color of the wood you burned and the gas phase of the oxygen used. Since these materials have different properties, they are different substances than the reactants. This is evidence that a chemical reaction has occurred. New molecules of carbon dioxide and water have been produced from the wood and oxygen that were burned.

To learn more about combustion and fire, go to http://science.howstuffworks.com/fire1.htm.

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Content Benchmark P.12.A.5

Students know chemical reactions can take place at different rates, depending on a variety of factors (i.e. temperature, concentration, surface area, and agitation). E/S

Common misconceptions associated with this benchmark:

1. Some students incorrectly think that increasing the concentration of reactants always increases the reaction rate.

Increasing the concentration of reactants will increase the reaction rate on many liquids or gases. But, there are a few cases in which this is not true.

For more information about this misconception, go to
http://www.chemguide.co.uk/physical/basicrates/concentration.html.

2. Some students incorrectly believe that activation energy must always be supplied to reactants in order to bring about a reaction.

Although, this is true in many cases, in some instances, the reactants already possess enough energy when mixed for the reaction to occur.

For some illustrations of activation energy, go to
http://www.chem.vt.edu/RVGS/ACT/notes/act_energy.html.

3. Some students incorrectly think that adding a solid or a liquid reactant to a reaction mixture increases the concentration of that reactant.

Only gases or solutes in a solution have concentration. Thus the only reactions in which concentration is a factor are those in which the reactants are in solution, either aqueous or gaseous. The rate factors for reactions in which there are liquid or solid reactants are temperature, agitation and particle size.

For more information on the effect of increasing concentration of reactants, go to http://www.science.uwaterloo.ca/~cchieh/cact/c123/coneffec.html

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Content Benchmark P.12.A.5

Sample Test Questions

1^st Item Specification: Describe factors affecting the rate at which a reaction proceeds.

Depth of Knowledge Level 1

1. Which of the following could increase the rate at which a chemical reaction proceeds?
1. Grinding a solid reactant into powder.
2. Lowering the temperature of one reactant.
3. Decreasing the concentration of the reactants.
4. Removing a catalyst from the reaction.

2. Identify the combination of factors that will most likely increase the rate of a chemical reaction.
1. Decreasing temperature, using a large block of the solid, and stirring.
2. Decreasing temperature, chopping up the solid block, and stirring.
3. Increasing temperature, using a large block of the solid, and stirring.
4. Increasing temperature, chopping up the solid block, and stirring.

3. When two chemicals react their molecules must collide with each other with enough energy to make the reaction occur. Which of the following will increase the rate of collision between the molecules?
1. Increasing the size of the reactants.
2. Increasing the temperature of the reactants.
3. Decreasing the surface area of the reactants.
4. Decreasing the volume of the reactants.

Depth of Knowledge Level 2

4. Yeast is a single-celled organism used to make bread dough rise. When yeast and sugar are dissolved in water and added to bread dough the yeast cells use the sugar as food and in the process release carbon dioxide gas which creates gas pockets in the bread dough. Which of the following could a baker do to make bread dough rise faster?
1. Dissolve the yeast in cold water.
2. Dissolve the yeast in warm water.
3. Put the yeast in the dough without water.
4. Add milk instead of water to the dough.

5. Increasing the temperature, concentration, and surface area of the reactants in a chemical reaction will all cause an increase in the rate of a reaction because the molecules of the reactants will
1. be brought closer together.
2. be spread farther apart.
3. increase in amount.
4. increase in volume.

6. In an automobile engine gasoline and air are compressed by a piston in a cylinder. This forces the gasoline and air mixture into a smaller space bringing their molecules close enough together to react. A spark from a spark plug then ignites the gasoline and air mixture causing combustion. Which of the following factors causes the rate of combustion to increase?
1. A decrease in pressure.
2. A decrease in air.
3. An increase in temperature.
4. An increase in gasoline.

2^nd Item Specification: Predict the result of a given factor on reaction rate.

Depth of Knowledge Level 1

7. The metal iron reacts with oxygen in the air to produce the compound iron oxide or rust. What effect will painting an iron gate have on the rate of the rusting reaction?
1. Paint will increase the reaction rate.
2. Paint will decrease the reaction rate.
3. Paint will destroy the reaction.
4. Paint will not affect the reaction.

8. The rate of a certain reaction doubles with every 10° C increase in temperature. Which of the following would be true of this reaction?
1. The particles of the reactants are increasing in size.
2. The particles of the reactants are losing energy.
3. The particles of the reactants are getting farther apart.
4. The particles of the reactants are colliding more frequently.

Depth of Knowledge Level 2

9. 100 g of the same solid are reacting with 200 ml of the same liquid in each of the following containers. The molecules of the solid are represented by the symbol.

The container in which the reaction rate will be the fastest in

1. container I.
2. container II.
3. container III.
4. container IV.

3^rd Item Specification: Identify the effect of catalysts on reaction rate.

Depth of Knowledge Level 1

10. Which of the following is true about catalysts in chemical reactions?
1. Catalysts are used up in chemical reactions.
2. Catalysts slow down or stop chemical reactions.
3. Catalysts are changed in chemical reactions.
4. Catalysts increase chemical reaction rates.

11. Which of the following is an example of a catalyst?
1. An enzyme helping to break down starch in your intestine.
2. Salt being added to meat to keep it from rotting or spoiling.
3. Combining baking soda and vinegar to cause a reaction.
4. Adding heat energy to increase the rate of a chemical reaction.

12. Iron in cars will combine with oxygen in the presence of water to form rust. In northern climates salt is put on roads to lower the temperature at which ice will form. Some of the salt is transferred to the cars that drive on the road. This salt when combined with water and oxygen increases the rate at which rusting occurs. In the rusting reaction salt acts as a(n)
1. reactant.
2. product.
3. catalyst.
4. enzyme.

Depth of Knowledge Level 2

13. Sugar in our body cells reacts very quickly with oxygen. The same sugar will react very slowly with the oxygen in air. The reason for this difference is because
1. our bodies are warmer than the air.
2. cells contain catalysts to speed the reaction.
3. the sugar exposed to air is in solid form.
4. the sugar in our cells contains more energy.

14. In industry speeding up a chemical reaction can mean making a new product faster. Heating a reaction can increase the rate at which it occurs but this method is expensive. Which of the following may be a less expensive way to increase chemical reaction rate?
1. Use a larger amount of chemicals in the reaction.
2. Use a smaller amount of chemicals in the reaction.
3. Use larger containers for the chemical reaction.
4. Use a catalyst to increase the rate of the reaction.

15. A catalyst is a substance that changes the rate of a chemical reaction without itself being changed by the reaction. Enzymes are biological catalysts. Which of the following predictions is most likely true of an enzyme in our digestive system?
1. The enzyme will increase the rate of the digestive process.
2. The enzyme will decrease the rate of the digestive process.
3. The enzyme will increase the amount of food that is digested.
4. The enzyme will decrease the amount of food that is digested.

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Content Benchmark P.12.A.5

Students know chemical reactions can take place at different rates, depending on a variety of factors (i.e. temperature, concentration, surface area, and agitation). E/S

Answers to Sample Test Questions

1. A, DOK Level 1
2. D, DOK Level 1
3. B, DOK Level 1
4. B, DOK Level 2
5. A, DOK Level 2
6. C, DOK Level 2
7. B, DOK Level 1
8. D, DOK Level 1
9. C, DOK Level 2
10. D, DOK Level 1
11. A, DOK Level 1
12. C, DOK Level 1
13. B, DOK Level 1
14. D, DOK Level 2
15. A, DOK Level 2

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Content Benchmark P.12.A.5

Students know chemical reactions can take place at different rates, depending on a variety of factors (i.e. temperature, concentration, surface area, and agitation). E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Laboratory Activities
There are many excellent websites that have hands-on or simulated laboratory activities that can help students understand how reaction rates are affected by temperature, concentration, surface area, and agitation.

• The Bayer Company’s Alka Seltzer site has two lessons that cover reaction rate. The lesson called “Experiment 2” shows the effect of particle size and is found at http://www.alkaseltzer.com/as/experiment/student_experiment2.htm. The lesson called “Experiment 3” shows the effect of concentration on reaction rate and is found at http://www.alkaseltzer.com/as/experiment/student_experiment3.htm

• Iowa State has a virtual lab of reaction rates at
http://www.chem.iastate.edu/group/Greenbowe/ sections/projectfolder/flashfiles/kinetics2/iodine_clock.html

• A lesson from a chemistry teacher in Kent, Washington at
http://www.kent.k12.wa.us/curriculum/science/Mong_Kirkland/Chemistry/Rate.dot

• Virtual chemistry labs by Davidson University at
http://www.chm.davidson.edu/ChemistryApplets/kinetics/ReactionRates.html

2. Activation Energy Notes
This site hosted by the University of Vermont has some excellent notes on kinetic and activation energy. This would be a great site for students to do research.

To learn more, go to http://www.chem.vt.edu/RVGS/ACT/notes/act_energy.html

3. Theory of Reaction Rates
There are several websites that do a good job of simplifying the theory of reaction rates, especially in their introductory text. Some of these are below:
• http://www.chem4kids.com/files/react_rates.html
• http://www.newi.ac.uk/buckleyc/react.htm
• http://www.purchon.com/chemistry/rates.htm
• http://www.chemguide.co.uk/physical/basicratesmenu.html
• http://www.wpbschoolhouse.btinternet.co.uk/page03/3_31rates.htm

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