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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Performance 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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Performance 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

Sample Test Questions

1. In chemical reactions, what do temperature, concentration, surface area, and agitation affect?
a. pH of a solution
b. catalysts
c. reaction rate
d. equilibrium

2. What change will make a chemical reaction occur more quickly?
a. add an inhibitor
b. add a preservative
c. increase temperature
d. increase reactant size

3. What must be true for a chemical reaction to occur?
a. reactants must collide effectively
b. a catalyst must be present
c. solids must be removed
d. reactants must be chilled

4. What factors affect the rate of a chemical reaction?
a. color, concentration, and clarity
b. temperature, agitation, and concentration
c. surface area, color, and pressure
d. preservatives, clarity, and pressure

5. What must bond to form molecules?
a. atoms
b. ions
c. neutrons
d. catalysts

6. How can the reaction rate of zinc with hydrochloric acid be slowed down?
a. increase the temperature
b. dilute the hydrochloric acid
c. cut the zinc into smaller pieces
d. stir the reaction mixture

7. As the temperature of a reaction increases, what must also increase?
a. solution pH
b. color intensity
c. activation energy
d. reaction rate

8. What change will slow down a reaction between aluminum and sulfuric acid?
a. add a catalyst
b. increase the surface area
c. increase the temperature
d. agitate the mixture

9. What factor explains why steel wool rusts more quickly than steel nails when they stored in the same container?
a. catalysis
b. temperature
c. agitation
d. surface area

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. (c)
2. (c)
3. (a)
4. (b)
5. (a)
6. (b)
7. (d)
8. (b)
9. (d)

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PerformanceBenchmark 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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