Lesson

Problem

How do temperature and concentration affect the rate at which a reaction occurs?

Background

OxiClean contains sodium percarbonate (Na₂CO₃·1.5 H₂O₂) and sodium carbonate (Na₂CO_3, also known as washing soda). When dissolved in water, sodium percarbonate breaks down into sodium carbonate, hydrogen peroxide, and oxygen gas. Hydrogen peroxide can act as a bleaching agent by oxidizing and breaking down sections of chemical structures that can cause color. Sodium percarbonate is generally safer to use and more environmentally friendly than household bleach (sodium hypochlorite, NaOCl).

Materials

• OxiClean Powder Versatile Stain Remover (no dyes or perfumes)
• Water
• Digital balance
• Spoon or scoopula (for measuring out OxiClean)
• 250 mL beakers
• 100 mL graduated cylinder
• 50 mL graduated cylinder
• Hot plate (for heating water)
• Blue food coloring
• Ice cubes
• Stirring rod
• Stopwatch or timer

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash your hands thoroughly before leaving the lab.
• Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.

Pre-Lab

1. What do you think of when you hear the word “rate”?
2. If a reaction is occurring at a faster rate, will the time it takes for the reaction to occur increase or decrease?
3. Predict how you think temperature and concentration will affect reaction rate.

Procedure: Part 1 (Creating Mixtures)

1. You will make two different OxiClean mixtures.
2. Using a graduated cylinder measure the water necessary for OxiClean Mixture A. Place the water in a beaker. *Note that the water should be warm—if warm water isn’t available from the sink, use a hot plate to carefully heat the water.
3. Using a spoon or scoopula, measure the mass of OxiClean necessary for Mixture A onto the scale. Add it to the water in the beaker.
4. Stir well until the OxiClean powder is completely dissolved.
5. Repeat these steps to make OxiClean Mixture B.
6. Set the mixtures aside and move on to Part 2 of the Procedure.

Procedure: Part 2 (Determining the Effect of Temperature on Reaction Rate)

1. You will create two blue food dye solutions.
   1. In beaker 1 combine 100 mL hot water and 1 drop blue food coloring. *Note that the water should be hot—if hot water isn’t available from the sink, use a hot plate to carefully heat the water.
   2. In beaker 2 combine 100 mL cold water and 1 drop blue food coloring. *Note that the water should be cold—if cold water isn’t available from the sink, use ice cubes to cool the water.
2. Gently stir each solution to ensure the blue dye is evenly mixed. 
3. Obtain two graduated cylinders.
4. In each graduated cylinder measure 50 mL of “OxiClean Mixture A”:
5. Next, you will perform step a and b at the same time:
   1. Pour the 50 mL contents of one graduated cylinder (containing “OxiClean Mixture A”) to the cold water blue dye beaker.
   2. Pour the 50 mL contents of the other graduated cylinder (also containing “OxiClean Mixture A”) to the hot water blue dye beaker.
6. Start a timer.
7. Gently stir each solution.
8. Record qualitative observations and the amount of time it takes for the blue color to completely fade in each sample.

Observations & Data

Class Data: Part 2 (Determining the Effect of Temperature on Reaction Rate)

Collect data from the other groups in the class, and find the average of the results.

Results

Use the average to write a statement that quantitatively describes the results of Part 2.

Procedure: Part 3 (Determining the Effect of Concentration on Reaction Rate)

1. You will create two blue food dye solutions.
   1. In beaker 1 combine 100 mL room temperature water and 1 drop blue food coloring.
   2. In beaker 2 combine 100 mL room temperature water and 1 drop blue food coloring.
2. Gently stir each solution to ensure the blue dye is evenly mixed. 
3. Obtain two graduated cylinders.
4. In each graduated cylinder measure premade OxiClean mixtures:
   1. Graduated Cylinder 1: 50 mL “OxiClean Mixture A”
   2. Graduated Cylinder 2: 50 mL “OxiClean Mixture B”
5. At the same time, add each of the OxiClean to a beaker and start a timer.
6. Gently stir each solution.
7. Record qualitative observations and the time it takes for the blue color to completely fade in each sample.

Observations & Data

Class Data: Part 3 (Determining the Effect of Concentration on Reaction Rate)

Collect data from the other groups in the class, and find the average of the results.

Results

Use the average to write a statement that quantitatively describes the results of Part 3.

Summary

1. Write a one-sentence summary of how changing temperature affects reaction rate.
2. Write a one-sentence summary of how changing concentration affects reaction rate.

3. Explain, in terms of collision theory, how increasing temperature affects the rate of a reaction. Use results from the lab in your response. (recall: temperature measures average kinetic energy)
4. Explain, in terms of collision theory, how increasing the concentration of a reactant affects the rate of a reaction. Use results from the lab in your response.
5. Why might it be important to slow down or speed up a chemical reaction? Give one example of each.

Application Questions

Answer the following questions using what you have learned about reaction rates.

1. Given the balanced equation representing a reaction

2 HCl(aq) + Na₂S₂O₃(aq) → S(s) + H₂SO₃(aq) + 2 NaCl(aq)

Increasing the concentration of Na₂S₂O₃(aq) increases the rate of reaction because the:

a. Activation energy decreases.
b. Activation energy increases.
c. Frequency of effective collisions decreases.
d. Frequency of effective collisions increases.

2. Why can an increase in temperature lead to more effective collisions between reactant particles and an increase in the rate of a chemical reaction?
   1. The activation energy of the reaction increases.
   2. The activation energy of the reaction decreases.
   3. The number of molecules with sufficient energy to react increases.
   4. The number of molecules with sufficient energy to react decreases.

3. Given the balanced equation representing a reaction:

Zn(s) + 2 HCl(aq) → H₂(g) + ZnCl₂(aq)

Which set of reaction conditions produces H₂(g) at the fastest rate?

a. 1.0-g lump of Zn(s) in 50 mL of 0.5 M HCl(aq) at 20 degrees Celsius.
b. 1.0-g lump of Zn(s) in 50 mL of 0.5 M HCl(aq) at 30 degrees Celsius.
c. 1.0 g of powdered Zn(s) in 50 mL of 1.0 M HCl(aq) at 20 degrees Celsius.
d. 1.0 g of powdered Zn(s) in 50 mL of 1.0 M HCl(aq) at 30 degrees Celsius.

4. At 20°C, a 1.2-gram sample of Mg ribbon reacts rapidly with 10.0 milliliters of 1.0 M HCl(aq). Which change in conditions would have caused the reaction to proceed more slowly?
   1. Increasing the initial temperature to 25°C.
   2. Decreasing the concentration of HCl to 0.1 M.
   3. Using 1.2 g of powdered Mg.
   4. Using 2.4 g of Mg ribbon.