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Rate of Reaction
Understanding the factors that influence the rate of a chemical reaction is a fairly complex, but completely logical affair. Students readily accept the idea that molecules must collide if a reaction is to occur. However, many students have the misconception that every collision leads to the formation of product, when the truth of the matter is that many collisions do not go anywhere. A huge percentage of molecules within a sample may collide without ever turning into products. When creating a mental picture of a successful collision, students must envision molecules that possess both proper orientation and a sufficient amount of energy.
 
  1. The colliding molecules must collide with the correct orientation. Unless the alignment is favorable, the reaction will not be successful. Of course, as the overall number of collisions increases, it becomes more likely that a collision with a favorable orientation will occur.

  2. The molecules must collide with enough energy to overcome the activation energy barrier. The activation energy barrier can be lowered when a catalyst is present. Increasing the temperature gives the reactants molecules more kinetic energy, thus more high energy collisions can occur.
 
This topic of reaction rate is especially amendable to having students create their own experiment. 

Here is a possible list of variables to investigate:
 
    • changing the concentration of one of the reactants
    • changing temperature
    • use of a catalyst
    • changing the surface area (i.e. using a powdered vs. granular form of reactant).
 
Reactions in which a gaseous product forms are particularly easy for students to monitor. Students can measure the reaction rate by
 
    • monitoring the volume of gas produced in a determined amount of time.
    • monitoring  the change in mass of the system in a set amount of time.
 
Two possible reactions to investigate:
 
  1. Baking soda and vinegar ?
 
This uses cheap, easily obtained reactants that are very safe to use.
 
  1. Magnesium and hydrochloric acid  ?

Magnesium comes in powdered, granular, and ribbon form. This reaction can be catalyzed using a strip of copper metal. However, HCl must be used with care. The hydrogen gas produced is flammable,  no open flames can be allowed.


Experiment
 
Purpose: You and one partner will develop an experiment in which you investigate some factor that affects the rate of reaction between baking soda and acetic acid. You will have part of one period to experiment with the chemicals and devise a procedure. Another period will be devoted to fully conducting the experiment that you design.
 
Materials:
 
Cups and beakers
Graduated cylinder
Stopwatch
Baking soda
Vinegar
Soap
Thermometer
Syringes
 
Procedure:
 
1. Designing the Experiment
    1. Develop an experimental hypothesis.
    2. Clearly identify the independent variable. What factor did you choose to test?
    3. Clearly identify the dependent variable. How are you going to measure the rate of the reaction?
    4. Clearly identify the controls. Any factor that you are not testing must be maintained at a constant value. Explain how you will use controls in designing a reliable experiment.
 
2. Collecting the Data
    1. Reproducibility: How many trials are needed to collect sufficient data?
    2. What are the limits of your equipment in conducting this experiment?
    3. How many data points will you need to create a meaningful graph?
 
3. Processing the Data - Create a graph of your results.
    1. On which axis will the independent variable be placed?
    2. The dependent variable?
    3. Where is the line of best fit drawn? What is the equation for the line of best fit?
    4. How many data points are needed to create a good line?
    5. What does the slope of the line represent?
    6. What is the correlation coefficient for the line of best fit? How strongly does your data fit a linear pattern? Is an exponential curve a better fit than a linear curve?
 
    1. State your result.
    2. Was your hypothesis valid?
    3. What could be done to improve the experiment?



E Saylor

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