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Bond Strength of Ionic Salts (1 Favorite)

LESSON PLAN in Polarity, Ionic Bonding, Electronegativity, Bond Energy, Intramolecular Forces, Lewis Structures. Last updated November 18, 2019.


Summary

In this lesson, students will discover that dissolving salts changes the temperature of a solution even though it is a physical change. Students will first collect data during an investigation to compare the temperature change when dissolving three different salts (NaCl, KCl, and CaCl2). Then students will use magnets to construct an explanation of the temperature change based on collision of particles and properties of the metals.

Grade Level

High School

NGSS Alignment

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

  • HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
  • HS-PS1-4: Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
  • HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects).
  • Scientific and Engineering Practices:
    • Developing and Using Models
    • Analyzing and Interpreting Data

Objectives

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

  • Recognize patterns in temperature change within experimental data.
  • Describe how bond strength varies in different salts.
  • Describe how a salt dissolves in water.
  • Model the interaction of particle collisions at a microscopic level.

Chemistry Topics

This lesson supports students’ understanding of

  • Ionic Bonding
  • Bond Energy
  • Electronegativity
  • Intramolecular Forces
  • Lewis Dot Structures
  • Polarity

Time

Teacher Preparation: 30 minutes

Lesson: 120 minutes (2 days)

Materials (Per group of four students)

  • 3.0 g Sodium Chloride (solid)
  • 3.0 g Potassium Chloride (solid)
  • 3.0 g Calcium Chloride (solid)
  • 50.0 mL Water
  • 250 mL beaker
  • 100 mL graduated cylinders
  • Stirring rod
  • Thermometer
  • Balance
  • Weigh boat
  • 1 Metallic marble
  • 1 Glass marble
  • Books / blocks of wood (to create a ramp for collisions on magnetic ruler)
  • Stopwatch
  • 3 Rulers with different strength magnets connected to center. (See picture and explanation below as a reference.)
  • I suggest using three different colored rulers to represent the strength of magnet.
    • Red (weakest) has two ceramic magnets connected.
    • Yellow (medium) has three ceramic magnets connected.
    • Blue (strongest) has one neodymium magnet connected.
    • *Choose rulers that can have a “track” for the marbles to roll down the center.

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.

Teacher Notes

  • This lesson can lead into discussing bond strength for covalent bonds. In my class I do this lesson prior to discussing bond enthalpy for chemical reactions. Video references can be found at:
  • Investigation: Virtual Lab, Dissolving Salts in Water
    • You can reinforce the first investigation by running the tests on this simulation. It provides several salt options and graphs temperature change.
  • Salty Roads Simulation
    • I use this as a pre-lab activity to introduce how a salt dissociates with water.
  • PhET Simulation: Sugar and Salt Solutions
    • If students are having a difficult time understanding how salt is dissolved in water at the molecular level, this simulation is also helpful to show.
  • Part Two Suggestions:
    • When using the magnetic rulers, you will want the students to suggest ideas and guide them by reminding them that they need to collect evidence to explain what happened to the energy when a salt dissolves.
    • Remind them about the temperature drop that occurred when NaCl dissolved from the first investigation. Try asking them, what does a temperature drop indicate about the kinetic energy?
    • Suggested ideas for testing include:
      • Timing the glass marble rolling down the entire ruler without colliding and breaking the bond, then timing with the glass marble breaking the bond.
      • Timing the glass marble before collision, and then after collision.
      • Comparing the collision when the glass marble collides for different strength magnets.
      • Using cell phone to take a slow motion video during the collision.
    • This investigation works best when students have had practice engaging in inquiry and designing investigations. Students that are not used to inquiry may need a little more guidance.
    • More information on using magnets to show energy transfer during physical and chemical reactions can be found at Next Generation Science Storylines.
  • An answer Key has been provided for teacher reference.

For the Student

Lesson

Background

When an ionic salt dissolves in water, the energy of the system can change. Measuring the temperature of the substances before and after dissolving can unlock clues to the behavior of the particles. The change in temperature can be calculated by the following equation:

Change in temperature = final temperature – initial temperature
or
ΔT = Tf – Ti

Where ΔT represents the overall change in the solution temperature, Tf represents the lowest/highest temperature reached during dissolving, and Ti represents the water and salt at room temperature.

Prelab Questions

Use the Salty Roads Simulation to help you to complete the following questions:

  1. Can dissolving a salt be defined as a physical or chemical change? Why?
  2. Sketch how the salt is dissolved below.
  3. What is happening to the substance while it is dissolving? What is the water doing to dissolve the substance?
  4. Use evidence from the simulation to explain why the salt melts the ice in terms of particle motion.
  5. How is temperature related to kinetic energy?

Part 1: Dissolving Salts

Objective

What happens to the temperature of a solution when a salt is dissolved?

Materials

  • 3.0 g Sodium Chloride (solid)
  • 3.0 g Potassium Chloride (solid)
  • 3.0 g Calcium Chloride (solid)
  • 50.0 mL Water
  • 250 mL beaker
  • 100 mL graduated cylinder
  • Stirring rod
  • Thermometer
  • Balance
  • Weigh boat

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.

Procedure

  1. Measure and record the temperature of 50.0 mL of room temperature water in the Data Collection Section below.
  2. Measure 3.0 g of salt (potassium chloride, KCl).
  3. Add the salt to the water and stir it until the temperature stops changing.
  4. Record the lowest/highest temperature of the mixture.
  5. Repeat steps 1-4 for sodium chloride and calcium chloride.
  6. Calculate the change in temperature for each salt in the Calculations Section, and add your group results to the class shared results.
  7. Create an average for the class temperature data. Create a bar graph for the average class data on the space provided in the Analysis Section.

Data Collection

Salt Water Initial Temperature (Ti) in °C Solution Final Temperature (Tf) in °C Observations
Potassium Chloride (KCl)
Sodium Chloride (NaCl)
Calcium Chloride (CaCl2)

Calculations

Change in Temperature: Group Results

Salt Tf – Ti = ΔT
KCl
NaCl
CaCl2

Change in Temperature: Class Results

ΔT for KCl ΔT for NaCl ΔT for CaCl2
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Average ΔT

Analysis

Graph the average temperature change calculated for each salt.Be sure to label the x-axis and y-axis, and create a title for your graph.

  1. State any patterns or trends in data:

Conclusion

  1. When a salt dissolves in water, what happens to the temperature?
  2. Does the temperature change the same when different salts are dissolved? Explain why this may occur.
  3. Locate the metals for each salt on the periodic table. Using evidence from the investigation, create an explanation of the temperature change differences based on the metal in the salt.
  4. What happened to the thermal energy of the water when a salt is dissolved?

Part 2: Modeling Particle Interactions

Prelab Questions

  1. How can the magnets and marbles be used to show bonding between particles? List your ideas below.
  2. Create a sketch that illustrates how atoms in a salt are bonded together. Refer to your ideas you generated about using magnets for connections. Does your sketch show why a substance requires energy when breaking bonds?

Objective

What happens at a microscopic level to the energy of particles in salt when they are dissolved in water?

Materials

  • 3 Rulers with different strength magnets connected to center
  • Metallic marbles
  • Glass marbles
  • Books / block of wood (to create a ramp for collisions on magnetic ruler)
  • Stopwatch

Procedure

Using the magnets provided, with your group design a model that explains why the temperature changes when a salt is dissolved in water and why the temperature change is different for different types of salts. Design an investigation using the materials to collect evidence of how the temperature changes when a salt dissolves in water.

Before beginning, think about…

  • How could you use the magnet and metallic marble to represent an ionic bond?
  • What marble will represent the water?
  • How can the marbles have kinetic energy represented?
  • What data could be collected to see if the kinetic energy of the particles is changing when bonds are broken?
  • How can we model different types of salts?

Drawing of your experimental design:

Write your procedure here: (remember to make clear directions – could this experiment be repeated by another group with the same results?)

Data

Create a data table to collect evidence for your experiment below. (Remember the data collected should be able to provide evidence about how the kinetic energy of the particles is changing when bonds are broken.)

Analysis

Present an overview of your investigation and your results to your classmates.
Take notes about what other groups did and what they found.

After presentations, answer the following questions.

  1. Provide evidence that the kinetic energy of particles decreased when a bond was broken.
  2. What happened there wasn’t enough energy transferred during the collision to break the salt bond?
  3. Provide evidence from your investigation that the kinetic energy needed to break the bond of different salts changes. Why does this happen?
  4. Describe how conservation of energy occurs when the bonds of the salt are broken.

Conclusion

Sketch a drawing to explain why the temperature changes when an ionic salt dissolves in water.