Summary

In this demonstration, students observe the effect of temperature, concentration, and particle size on the rate of a chemical reaction.

Grade Level

High or middle school

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence
• Crosscutting Concepts:
  • Patterns: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

AP Chemistry Curriculum Framework

This lesson supports the following learning objectives:

• Big Idea 4: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.
  • 4.1 The student is able to design and/or interpret the results of an experiment regarding the factors (i.e., temperature, concentration, surface area) that may influence the rate of a reaction.

Objectives

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

• Describe how temperature affects the rate of a chemical reaction.
• Describe how concentration of reactants affects the rate of a chemical reaction.
• Describe how particle size or surface area affects the rate of a chemical reaction.

Chemistry Topics

This demonstration supports students’ understanding of

• Kinetics
• Chemical Reactions
• Reaction Rates
• Indicators of Chemical Change

Time

Teacher Preparation: 10 minutes

Lesson: 15 - 20 minutes

Materials

• 250 ml beakers, 7
• Alka-Seltzer tablets, 8
• Water, 900 ml
• Optional: food coloring

Safety

• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• Alka-Seltzer is composed of three solid ingredients – acetylsalicylic acid (aspirin), citric acid, and sodium bicarbonate.
• When added to water, the citric acid reacts with the sodium bicarbonate, which is a base. Carbon dioxide is produced, which causes the bubbles.
• Students should know that in order for a reaction to occur, reactant particles must collide with enough energy to overcome the activation energy of that specific reaction.
• The following factors affect the rate of a reaction:
  • Temperature - increasing the temperature also increases the velocity (kinetic energy) of the particles. Higher energy reactants will collide more often and produce higher energy collisions. This increases the rate of the reaction.
  • Concentration – increasing the amount of reactant present will also increase reaction rate. Since more particles are present the number of collisions will also increase.
  • Surface Area – Solid reactants react on their surface. Crushing a solid increases the surface area available for a reaction to occur and increases reaction rate.
• As an alternative, the demonstration can be also be set up as an inquiry lab. Provide the materials and ask students to design a lab that will show the effect of changes in temperature, concentration and particle size on the rate of a reaction.
• Before class, set up the following stations:
  • Control: Add 150 mL of room temperature water to a beaker. Have one Alka-Seltzer Tablet ready to use.
  • Note: You can add a drop or two of food coloring to each beaker to make the reactions easier to see.
  • Temperature: Add 150 ml of hot water to a 250 ml beaker and 150 ml of cold water to a second 250 ml beaker. Have two Alka-Seltzer tablets ready to use.
  • Concentration: Add 150 ml of room temperature water to each of two 250 ml beakers. Have two Alka-Seltzer tablets ready to use.
  • Particle size/Surface area: Add 150 ml of room temperature water to each of two 250 ml beakers. Have two Alka-Seltzer tablets ready to use. Crush one of them and put it in a small beaker or cup.

Demonstration Procedure:

1. Introduction: The purpose of the demonstration is to show the effect of temperature, concentration, and surface area on the rate of a chemical reaction. Have students brainstorm about ways you could increase the energy or number of molecular collisions during a reaction.
2. Control: Ask students to predict what happens when you add an Alka-Seltzer tablet to water. Add the tablet to the water and ask for observations and evidence of a chemical reaction. Show them the chemical equation. C₆H₈O₇ + 3NaHCO₃ → ₃CO₂ + 3H₂O + 3Na₂C₆H₅O₇
3. Temperature: Ask students to predict the effect of increasing the temperature on the rate of reaction. At the same time, add a tablet to the hot and cold water. Ask for observations. The rate of reaction in the hot water is faster than that in the cold. Ask students to use what is happening at the particle level to explain why rate increases at higher temperatures.
4. Concentration: Ask students to predict the effect of increasing the amount of Alka-Seltzer on the rate of reaction. At the same time, add one tablet to one beaker and two tablets to the others. The rate of reaction in the beaker with two tablets is faster than that in the beaker with one tablet. Ask students to use what is happening at the particle level to explain why rate increases at higher concentrations.
5. Surface Area: Ask students to predict the effect of crushing the Alka-Seltzer on the rate of reaction. At the same time, add one tablet to one beaker and a crushed tablet to the others. The rate of reaction in the beaker with the crushed tablet is faster than that in the beaker with the whole tablet. Ask students to use what is happening at the particle level to explain why rate increases with increased surface area/smaller particle size.