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DEMONSTRATION in Reaction Rate, Reaction Rate, Catalysts, Combustion, Activation Energy. Last updated December 12, 2017.

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Summary

In this series of demonstrations, students will be introduced to factors that affect the rates of chemical reactions. They will observe and record their observations, while also describing the rate-influencing factor for each demonstration as well as evidence supporting whether or not the reaction rate was increased or decreased by the factor.

Grade Level

High School

Objectives

By the end of this demonstration, students should be able to

Identify four factors that may affect rates of chemical reactions.
Determine if a factor will increase or decrease the rate of a reaction.
Create a data table and use it to record and analyze observations.

Chemistry Topics

This demonstration supports students’ understanding of

Kinetics
Chemical Reactions
Reaction rate
Activation energy
Catalyst
Collision theory of reaction rates

Time

Teacher Preparation: 15 minutes

Lesson: 45 minutes

Materials

Demonstration A (photo shown):

200 mL of 3% hydrogen peroxide
1 package (7 g) of activated dry yeast
2, 250 mL Erlenmeyer Flasks
100 mL Graduated Cylinder
10 mL Graduated Cylinder
Stirring Rod
250 mL Beaker
Matches or Lighter
Wooden Splint

Demonstration B:

Iron rod or nail
Tongs or Mitt
Bunsen Burner
Striker
Scoopula
Iron powder

Demonstration C:

2 chemical light sticks (suggest having extra in case any are defective)
beaker filled with hot water
beaker filled with ice water

Demonstration D:

10 mL of 1 M H₂SO₄
10 mL of 18 M H ₂SO₄
2, 50ml beakers
20g of granulated sugar, C₁₂H₂₂O₁₁ (10g per beaker)
2, 10ml graduated cylinders
2 stirring rods
Water bottle
Fume hood
Gloves
Tray or bin
1 liter beaker
Sodium bicarbonate, NaHCO₃
Spatula

Safety

Students and teacher should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
Always use caution around open flames. Keep flames away from flammable substances. Be sure that all flammable materials are removed from the area.
For combustion activities, be sure that students are at least three meters away and do not point the flame in the direction of students. Use a safety shield.
Always be aware of an open flame. Do not reach over it. Tie back hair, and secure loose clothing.
An operational fire extinguisher should be in the classroom.
Some of these demonstrations should be done in a fume hood. Irritating fumes may be given off.
Use extreme caution when handling sulfuric acid. Refer to SDS for 18M H₂SO₄ before using it.

Teacher Notes

Teachers should practice all of these demonstrations prior to using them with students in their classroom.
Demonstration A: Effect of concentration on reaction rate/ Effect of a catalyst:
- This demonstration shows the effect of using a catalyst to speed up a reaction. You could allow the hydrogen peroxide to decompose naturally but by adding the yeast, the rate of reaction is increased, observed by the formation of oxygen bubble as soon as the yeast contacts the hydrogen peroxide.
- Procedure:
  - 1. Prepare a 2% yeast suspension:
    - Mix 1 gram of yeast with 10 mL water OR mix the entire envelop (7 g) with 70 mL of water.
    - Stir well to create suspension.
  - 2. Write the equation for the decomposition of hydrogen peroxide on the board:
    - H₂O₂ (aq) → O₂ (g) + H₂O (l)
  - 3. Add 100 mL of 3% hydrogen peroxide to each of the 250 mL Erlenmeyer flasks.
  - 4. Ask students to describe what they see. (Nothing)
  - 5. Add 10 mL of the yeast suspension to one of the flasks.
  - 6. Ask students to describe what they see. (Gas bubbles)
  - 7. If you have not yet used a glowing ember to test for the production of oxygen, discuss the procedure with your students.
  - 8. Light a wooden splint using a match or lighter.
  - 9. After it has burned for a few seconds gently blow out the flame, leaving a glowing ember.
  - 10. Hold the glowing ember over the mouth of the flask containing the yeast suspension. The splint should reignite. See photos below.
  - 11. You can blow out the splint and reignite it several times.
  - 12. Ask students to describe what they see. (The flame reignites in the presence of oxygen gas.)
Demonstration B: The effect of surface area on reaction rate
Demonstration C: Effect of temperature on reaction rate
Demonstration D: Effect of concentration on reaction rate
- Procedure:
- *Teacher should wear gloves for this demonstration (as well as apron and goggles). Also note that this demonstration should be conducted in the fume hood!
  - 1. Place about 10 grams of sugar into each of two 50 mL beakers.
  - 2. Add a few drops of water to each and stir to mix well with the stirring rod.
  - 3. Leave the glass stirring rod in each of the two beakers.
  - 4. Place beakers on a tray or bin in the fume hood fume hood and turn on the fume hood.
  - 5. Add 10 mL of 1 M sulfuric acid to one beaker, stir briefly, and remove stirring rod.
  - 6. Add 10 mL of 18 M sulfuric acid to the other beaker, stir briefly, remove stirring rod, and close the fume hood door.
    - Important Notes:
      - Do not cover beakers. Creating a closed container will cause the beaker to explode.
      - This reaction is extremely exothermic and produces irritating fumes so must be done in the fume hood. It will begin by turning sulfuric acid yellow and then a dark orange. In about 30 seconds to a minute, a column of carbon will begin to appear in the beaker.
      - During the reaction, steam is generated which is very hot. It will smell like burned brown sugar and will produce fumes that are irritating. Make sure that the fume hood door is closed, and the ventilation is turned on in order reduce amount of contact with these vapors.
      - Do not allow students to touch carbon column as it will have sulfuric acid residue on it.
      - The beaker with lower concentration of sulfuric acid will not react. It should be the one you mix first so that the high concentration reaction does not begin before the fume hood door is closed.
      - Students should record observations of student handout.
      - Students should observe that the 1 M sulfuric acid did not induce a reaction but the higher concentrated 18 M sulfuric acid reacted at a high rate.
      - You can leave the lower concentration of acid in the fume hood for days and it will not react.
    - Clean Up:
      - Wearing disposable gloves lift the black carbon column from beaker and put it into a 1-liter beaker with some sodium bicarbonate.
      - With spatula, break the column up into smaller pieces.
      - Add a little water and set back on the tray.
      - Neutralize any acid spills with sodium bicarbonate and wipe clean.
      - After foaming stops, throw carbon in the trash.

Answers to Pre-Lab Questions:
- 1. Why is Hydrogen Peroxide (H₂O₂) stored in a brown plastic bottle?
- To protect it from ultraviolet light rays (UV) from the sun.
- 2. Is it possible to change how fast a reaction occurs?
- Yes, by adding a catalyst or by changing temperature, concentration, or surface area.

Answers to Analysis Questions:
- 1. How will rate of reaction be affected if concentration of reactants decreases?
- The lower the concentration of reactants, the lower the rate of reaction.
- 2. What was the yeast used as and what was the effect of adding it?
- It was used as a catalyst to speed up decomposition of hydrogen peroxide, H₂O₂.
- 3. Catalase is present in potatoes and liver. As an extension, you could try one or both of these items it see if the reaction proceeds at a different rate than with the yeast. What would a faster or slower rate say about the concentration of catalase present in the food?
- A faster rate would indicate a higher concentration of catalase.
- 4. What factor observed today explains why there is a risk of explosion in grain storage silos?
- Surface area - This is why there is a risk of explosion in grain silos due to grain dust in the air or in factories handling combustible powders.

For the Student

Background

When you hear the word rate, it usually refers to something happening in a specific amount of time, like how fast something grows (growth rate), or moves (rate of speed), heart rate (beats/min) or possibly an interest rate. In chemistry, the rate of a reaction describes how fast a reaction proceeds over time. In other words, a rate of reaction measures how quickly reactants are changed into products. If the volume remains constant, reaction rate can be defined in terms of changes in concentration, the change in concentration of reactants over time or the change in concentration of products over time.

The area of chemistry that involves the study of reaction rates and reaction mechanisms is called kinetics. It involves the study how molecules react, bond breaking and new bond formation. The reaction rate can be written for the disappearance of a reactant (such as a metal sample reacting with acid) or the appearance of a product (such as production of a gas). Reaction rate can also be described on a molecular level. For a reaction to occur, particles or molecules must contact each other with enough energy to break the chemical bonds and cause a chemical reaction. Molecules must also collide with a proper orientation in order to cause a chemical reaction. Any factor that affects the likelihood of an effective collision also affects the rate of reaction. There are several factors that affect the likelihood of an effective collision of molecules and therefore also affect the rate of reaction. These include:

The type of reactants involved
The concentration
The surface area
The temperature of the reactants
The addition of a catalyst

The Type of Reactants involved:

The nature of the reactants involved in a chemical reaction can affect reaction rate. Larger molecules may react slower than smaller molecules. The rate of reaction depends on the nature of the particular reactants and upon the reaction conditions. An example of this might be that under certain conditions, hydrogen and chlorine may combine rapidly but nitrogen and chlorine, under the same conditions may react slowly.

The Concentration:

Higher concentration can increase the reaction rate of a chemical reaction. More molecules equal more collisions and more collisions are necessary for reactions to occur. Imagine you're on a freeway in the middle of the night. Traffic is light, with few other cars on the road and you have little trouble getting from one point to another. However, that might not be the case at rush hour when the roadway is filled with cars. As more cars are added, the likelihood of a collision increases. If construction causes freeway lanes to be closed off, squeezing traffic into a smaller number of traffic lanes, it also increases the likelihood of a collision. The same is true of reactants. If more reactants are crammed into a space or if volume is decreased, concentration is increased and reactants collide more with each other, increasing the chance that there will be an effective collision. Any factor that affects the likelihood of an effective collision also affects the rate of reaction.

The Surface Area:

The rate of reaction can be increased by increasing the surface area of one of the reactants because it affects the ability to mix the reactants. A powdered solid will usually produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump. For example by cutting or grinding it into smaller pieces, the surface area of a reactant is increased. This means that more particles are exposed to the other reactant, causing more collisions and increasing the rate of reaction. This is why it is easier to start a campfire using small pieces of kindling than it is with larger logs.

The Temperature of the Reactants:

Changing the temperature of reactants impacts the reaction rate because higher temperature increases the kinetic energy of molecules and sufficient energy is needed in order to react (Activation Energy). Temperature is a measurement of the average kinetic energy or energy in motion within a system. As temperature increases, the kinetic energy of the system also increases. Molecules with more energy are likely to have more effective collisions with each other, resulting in an increased rate of reaction. An example of this might be grilling a burger. We add heat to increase the rate of reaction. Decreasing temperature decreases the energy of the molecules, causing the molecules to collide less, resulting in fewer reactions. Cooling food, such as burgers, helps to preserve it by slowing down reaction rates, which helps to keep microbes like bacteria from breaking down the food as fast, and helps to keep fresh fruits and vegetables from decomposing as fast.

The Addition of a Catalyst:

A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently changed. This does not mean that it is not involved in the reaction. Catalysts usually lower the activation energy of reactions causing the rate of the reaction to increase. The yeast in demonstration A is used only as a catalyst. An application involving hydrogen peroxide and a naturally occurring catalyst, called an enzyme, is the use of hydrogen peroxide on cuts or wounds. Catalase, the enzyme that catalyzes the decomposition of hydrogen peroxide, is in your blood. You may have seen the bubbling on a cut when the hydrogen peroxide is added. It does not bubble because the wound has germs but rather because the catalase in blood has catalyzed and speeded up the decomposition of the hydrogen peroxide into water and oxygen. The bubbling action helps to lift dirt and germs out of the wound.

Pre-Lab Questions

Why is Hydrogen Peroxide (H₂O₂) stored in a brown plastic bottle?
Is it possible to change how fast a reaction occurs? Explain.

Objective

You will observe each demonstration in order to identify factors that affect reaction rates. You will prepare a data table to record observations for each reaction. The table should reflect a rate-influencing factor for each reaction and should include a description of observations made that support whether the reaction rate was increased by the factor.

Safety

Students and teacher should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
Always use caution around open flames. Keep flames away from flammable substances. Remove all flammable materials from the area.
Students should be at least three meters away during combustion demonstrations.
Always be aware of an open flame. Do not reach over it and be sure to tie back hair, and secure loose clothing.
An operational fire extinguisher should be in the classroom.
Some of these demos need to be done in a fume hood, as irritating fumes may be released.

Data

Create a data table like the sample below. Identify the rate-influencing factor for each demo and include a description of observations made that support whether the reaction rate was increased by the factor.

Demo	Rate-Influencing Factor (s)	Observations
A

Analysis Questions

How will rate of reaction be affected if concentration of reactants decreases?
What was the yeast used as and what was the effect of adding it?
Catalase is present in potatoes and liver. As an extension, you could try one or both of these items it see if the reaction proceeds at a different rate than with the yeast. What would a faster or slower rate say about the concentration of catalase present in the food?
What factor observed today explains why there is a risk of explosion in grain storage bins such as grain silos?

Conclusion

Summarize what you learned through these demonstrations. Describe the meaning of the term, “rate of reaction,” identify four factors that can affect it, and how these factors could be used to speed up the rate of reaction.

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