Investigating Chemical Reactions: Factors Which Influence the Rate of a Reaction.

Learning Goals:
1. This activity is designed for students to discover how temperature, surface area, and concentration of reactants affect the rate of a chemical reaction.
2. This activity is designed for students to design good experimental tests of hypotheses.
3. This activity is designed for students to synthesize ideas & to develop hypotheses and create tests to find solutions.

Key Concepts to be discovered:
1. Increasing the concentration of a reactant in a chemical reaction increases the amount of reactant particles in a given volume, which increases the frequency of successful collisions between reactant particles, thereby increasing the rate of the chemical reaction.
2. Increasing the temperature of a reactant in a chemical reaction increases the average speed of the reactant particles, as well as the average energy of the collisions between reactant particles, which increases the frequency of successful collisions between reactant particles, thereby increasing the rate of the chemical reaction.
3. Increasing the surface area of a reactant in a chemical reaction increases the contact between reactant particles, thereby increasing the rate of the chemical reaction.

Key Concepts to be reviewed:
1. In designing a simple experiment to test a hypothesis, only one independent variable should be present, with all other variables being held constant.

Vocabulary Words to be reviewed:
1. Chemical reaction
2. Collision theory
3. Reactant
4. Product

This activity is a lab meant to introduce students to the factors which affect the rate of a chemical reaction. It is intended for use in a 9th grade Physical Science class of roughly 28 students. Roughly 90 minutes of class time is required to have students working in pairs complete the lab. To complete the lab it is necessary to have access to hydrochloric acid, bromothymol blue, mortar and pestles, and a classroom set of one-hole stoppers outfitted with a glass bend attached to a small length of tubing in the hole. All other materials should be standard chemistry classroom materials or are easily obtained from the grocery store. This activity is one of the last that will be completed in a unit on chemical reactions. Students will have explored what a chemical reaction is, the collision theory of chemical reactions, possible observations that lead one to believe a chemical reaction has occurred, types of chemical reactions, balancing of chemical equations to show conservation of mass, and determining the products of a chemical reaction given the reactants.

In order to carry out this activity, the following materials will be needed:
-2M hydrochloric acid(HCl) -alka seltzer -bromothymol blue
-test tube -one-hole stopper sized to test tube
-test tube holder -stopwatch -water
-glass bend with tubing -goggles -beaker
-mortar & pestle -hot plate -ice -lycopodium powder
-flame source

To introduce the lab the class will first review the collision theory of chemical reactions. From here students will observe a demonstration involving lycopodium powder to illustrate that the rate of a chemical reaction can vary. When a flame source is brought into contact with a pile of lycopodium powder, no noticeable effect is observed. The lycopodium powder will then be allowed to fall down in dust format onto the flame, and a ball of flame will be produced. The students will be told that no new reactants were introduced...so why the difference in the observations? Students will explore this in the inquiry lab.

A handout explaining the lab will be distributed. Students will be shown a chemical reaction involving 2M hydrochloric acid (HCl) and an alka-seltzer tablet. To begin with, a beaker will be filled with 100 ml of water and 10 drops of bromothymol blue and stirred. 5 ml of the 2M HCl will be placed in a test tube. A tablet of alka-seltzer will then be placed into the test tube. The test tube will immediately have a one-hole stopper placed onto it, with a glass bend and tubing coming out of the stopper. The other end of the tubing will be in the beaker of water and bromothymol blue. Students will observe that over an amount of time the water and bromothymol blue beaker changes to the color yellow when the carbon dioxide gas produced by the alka-seltzer and HCl bubbles through it. It will be explained that the faster the carbon dioxide gas is produced, the faster the bromothymol blue and water mixture will turn to the color yellow.

The students will now be directed that they are to find three different things that could be done to change the rate at which the carbon dioxide gas is produced in the chemical reaction between HCl and alka-seltzer. Various materials, including those listed in the materials section will be provided for the students, or be on hand if asked for by the students. Before entering the lab area, students should first write up some hypotheses as to what they think may affect the rate of the reaction in their lab books. Students will be reminded that they should use sound design in their experiments. They will be asked to write down what they will hold constant for each hypothesis that they test, as well as what they will change, and what they will measure. They will then construct a data table to collect data in for their given hypotheses.

After preparing the above in their lab book, students will then enter the lab area to test out their hypotheses. They should collect data to show if their hypotheses are correct or not. If necessary, students should come up with additional hypotheses to test as needed. I will monitor their progress, seeing what kind of conclusions the students obtain. Students will need to write up their conclusions in their lab book. Lastly, students will need to answer some extension questions related to these three factors affecting the rate of a chemical reaction. Assignment Handout (Microsoft Word 30kB May27 11)

For the lycopodium demonstration, make sure that proper safety precautions are taken. Safety glasses should be worn. If the demonstration is performed in a small room, make sure to use a safety shield between the class and the demonstration area. Make sure that MSDS sheets are referenced before using hydrochloric acid and bromothymol blue.

I have never actually carried out this activity using this inquiry based method. My guess is that students will struggle to identify some possible factors influencing the reaction rate. Equipment available for their use may have to be shown first, and from this then I suspect that they will be able to come up with some hypotheses.

This activity differs from how rates of chemical reactions have been previously covered in our curriculum. In the past, students were told what exactly they should use as their independent variable in the reaction. For example, in one part they were told to use a whole alka-seltzer and then a crushed up alka-seltzer, from which they were to see that the one with greater surface area caused the reaction to proceed at a quicker pace. Now students need to discover that surface area is a factor influencing the reaction rate on their own.

In order to determine whether or not students are achieving the learning goals for the activity:
1. I will monitor progress in the lab to see what kind of results students are obtaining.
2. Students will turn in their collected data in a lab write-up, where they will summarize their findings.

Minnesota Academic Science Standards for High School Chemistry:
9-12.II.B.2 - reaction rate factors
The student will explain the influence of temperature, surface area, agitation and catalysts on the rate of a reaction.