Summary

In this lab students will observe an endothermic chemical reaction involving baking soda (sodium bicarbonate) and vinegar (acetic acid). Students will investigate the signs a chemical reaction has occurred (gas production, change in temperature). Students will perform the lab in an open system so they can see the change of mass due to gas production. This lab is a lead into the topic of conservation of mass. After the lab is completed, the teacher should do a demonstration of the exothermic reaction Hydrogen peroxide and potassium iodide.

Grade Level

High or middle school

Objectives

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

• Observe, identify, and describe signs of a chemical reaction
• Calculate the mass before and after a reaction
• Use the Law of the Conservation of Mass to determine why the mass before the reaction was different than the mass after the reaction
• Differentiate between endothermic and exothermic reactions

Chemistry Topics

This lab supports students’ understanding of

• Chemical Reactions
• Conservation of Mass
• Endothermic Reactions
  
• Exothermic Reactions

Time

Teacher Preparation: 30 minutes

Lesson: One-two 90 minute class periods or two-three 50 minutes class periods

Materials

• Acetic Acid (vinegar, 50 ml for each group)
• Sodium bicarbonate (baking soda, 1 tablespoon for each group)
• 50 ml to 100 ml graduated cylinders (1 for each group plus one for demo)
• 400 ml beaker (1 for each group plus one for demo)
• balance (1 beam balance or 1 electronic balance for each group)
• thermometer ( 1 for each group plus demo)
• hydrogen peroxide (50 mL) teacher use only
• potassium iodide ( 1 teaspoon) for teacher use only
• weighing paper

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• When baking soda and vinegar are mixed together, an endothermic chemical reaction occurs. Gas is produced and observed as bubbles. The temperature decreases. If the experiment is performed in an open system, the final mass will be less than the initial mass. This is because a gas is produced during the reaction. The discrepancy in mass is due to the gas escaping the system. If the experiment were to be performed in a closed system then the final mass would the same as the starting mass (ignoring human error). When hydrogen peroxide and potassium iodide are mixed together an exothermic chemical reaction occurs. Gas is produced and energy is released in the form of heat.
• Teacher should set up the lab stations for the lab groups.
• Students should be in groups of 3 to 4 students.
• The teacher will have the potassium iodide and hydrogen peroxide demo set up in a central location for the students to observe.
• Students will need to be introduced to the concepts of chemical changes, signs of chemical changes, and the Law of Conservation of Mass.
• Prior to starting the lab, the teacher should go over the “problem” questions with the students.
• In the pre-lab questions students are asked to find the chemical formulas for acetic acid and sodium bicarbonate. At this point in the class students are not yet able to write a balanced equation. The purpose of this is to introduce them to chemical formulas.
• Students should notice bubble formation which indicates gas formation in a chemical reaction.
• Because the experiment is performed in an open system, teacher should explain how the mass lost after the reaction was due to the gas escaping the system.
• For a more advanced lab students could perform the exothermic reaction themselves
  • The demo should be performed before students perform their own experiments
  • The teacher should measure 50 mL of hydrogen peroxide in a graduated cylinder then pour it into a 400 mL beaker and record the mass.
  • Once the peroxide is in the beaker, the teacher should measure the temperature with a thermometer.
  • The teacher should then mass 1 teaspoon of potassium iodide and add it to the hydrogen peroxide beaker.
  • Students should be observing the chemical reaction taking place: bubble formation, change of color, heat production

• For a longer lab, students can perform the same endothermic reaction in a closed system to observe how the mass of the product is the same as the starting mass of the reaction.