« Return to AACT homepage

AACT Member-Only Content

You have to be an AACT member to access this content, but good news: anyone can join!


Need Help?

Investigating Enthalpy and Entropy (4 Favorites)

LAB in Entropy, Exothermic & Endothermic, Gibb's Free Energy , Enthalpy, Spontaneous vs. Non-spontaneous Reactions. Last updated April 26, 2019.


Summary

In this lab, students will be able to observe and measure energy changes during the formation of solutions. The students should be able to explain and describe these changes in terms of entropy, enthalpy and free energy.

Grade Level

High School

NGSS Alignment

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

  • 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-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data

AP Chemistry Curriculum Framework

  • Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
    • 5.12 The student is able to use representations and models to predict the sign and relative magnitude of the entropy change associated with chemical or physical processes.
    • 5.13 The student is able to predict whether or not a physical or chemical process is thermodynamically favored by determination of (either quantitatively or qualitatively) the signs of both ΔH° and ΔS°, and calculation or estimation of ΔG° when needed.
    • 5.14 The student is able to determine whether a chemical or physical process is thermodynamically favorable by calculating the change in standard Gibbs free energy.

Objectives

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

  • Define enthalpy, entropy and free energy.
  • Explain the difference between an endothermic and exothermic process in terms of temperature of system and surroundings.
  • Measure the changes in enthalpy of a reaction.
  • Indicate the sign (positive or negative) of free energy if the enthalpy, entropy and temperature signs are known.
  • Write thermochemical equations for the formation of solutions.

Chemistry Topics

This lab supports students’ understanding of

  • Energy & Thermodynamics
  • Enthalpy
  • Entropy
  • Formation of solutions
  • Spontaneous & Non-Spontaneous Reactions
  • Free energy
  • Exothermic
  • Endothermic

Time

Teacher Preparation: 30 minutes

Lesson: 45 minutes

Materials (per group)

  • Thermometer (digital or alcohol)
  • 3 plastic cups or 100 mL beakers
  • 3 plastic spoons
  • Tap Water (13-15 grams per trial)
  • Balance (lab groups may share)
  • 5-7 grams of NaCl
  • 5-7 grams NH4Cl
  • 5-7 grams CaCl2 (not anhydrous)

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

  • Enthalpy, entropy and free energy are challenging topics for students to understand. This lab is more conceptual in that is focuses on using the signs and not as much on the numerical values to determine the relationships between the variables. If you want the students to use the numerical values to calculate enthalpy and free energy, it is possible. Entropy can be found in reference books, textbooks or online if necessary.
  • All substances can be safely rinsed down the drains with excess water.
  • The students do not need to measure any of the values except the temperature to determine the exothermic/endothermic information.
  • If the students don’t measure the temperature to at least one decimal place, they may not see a change in the temperature for the NaCl dissolving in water. Digital thermometers work the best, but if you have alcohol thermometers, make sure students are using Celsius scale and estimating the last digit.
  • See downloadable answer key for expected student lab results.

For the Student

Lesson

Background

In every chemical reaction, heat (enthalpy) is either released or absorbed and entropy (randomness) either increases or decreases. How do these two energy factors determine the course of a reaction? The size and direction of enthalpy and entropy changes together to determine whether a reaction is spontaneous (thermodynamically favored); meaning that the reaction favors products and releases free energy. An exothermic reaction accompanied by an increase in entropy is definitely spontaneous because both factors are favored. The reverse reaction is nonspontaneous (non-thermodynamically favored) because neither the enthalpy nor the entropy change is favorable. A reaction is also spontaneous if a decrease in entropy is offset by a large release of heat. Similarly, an endothermic reaction is spontaneous if an entropy increase offsets the heat absorption. For example, enthalpy change and entropy change work in opposition when ice melts. The melting of ice is an endothermic reaction, but the process is still spontaneous above 0°C. At these temperatures, the absorption of heat is more than offset by a favorable entropy change.

Prelab Questions

  • Define enthalpy.
  • Define entropy.
  • Define exothermic process.
  • Define endothermic process.

Objective

To observe and measure energy changes during the formation of solutions. To describe and explain these changes in terms of entropy and enthalpy.

Materials

  • Thermometer
  • 3 plastic cups or 100 mL beakers
  • 3 plastic spoons
  • Tap Water (13-15 grams per trial)
  • Balance
  • 5-7 grams of NaCl
  • 5-7 grams NH4Cl
  • 5-7 grams CaCl2 (not anhydrous)

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. Place 13-15 grams of water in a plastic cup.
  2. Record the actual mass in the data table.
  3. Measure the initial temperature (T1) of the water.
  4. Add 5-7 grams of solid NaCl to the cup.
  5. Record the actual mass in the data table.
  6. Stir gently with the thermometer.
  7. Stir gently and measure the highest or lowest temperature (T2).
  8. Record all temperature values in the data table.
  9. Record any qualitative observations made during the formation of the solution.
  10. Repeat the procedures in new cups with the other 2 ionic solid compounds (NH4Cl and CaCl2).
  11. Dispose of all mixtures down the drain with lots of water and throw the cups in the trash can when finished.
  12. Rinse and dry off the thermometer.

Data

Qualitative Observations of Solids and Solutions

Substance

Before Addition to Water

During and After Addition of Water

NaCl

NH4Cl

CaCl2

Quantitative Data

Mixture

Mass of H2O (g)

Mass of Solid (g)

T1
(°C)

T2
(°C)

∆T
(°C)

(1)
NaCl(s) +H2O(l)

(2)
NH4Cl(s) +H2O(l)

(3)
CaCl2(s) +H2O(l)

Analysis and Conclusion

  1. Calculate the ∆T for each mixture. Show work.

Mixture 1:
Formula Used:
Substitution of Quantities:

Final answer with sf & units:

Mixture 2:
Formula Used:
Substitution of Quantities:

Final answer with sf & units:

Mixture 3:
Formula Used:
Substitution of Quantities:

Final answer with sf & units:

  1. An exothermic process gives off heat to its surroundings. An endothermic process absorbs heat from its surroundings. Identify and explain which solutions are exothermic and endothermic. What is the sign of ∆H of each mixture?
Mixture 1 (NaCl) Mixture 2 (NH4Cl) Mixture 3 (CaCl2)

Exothermic or Endothermic

Explanation:

Exothermic or Endothermic

Explanation:

Exothermic or Endothermic

Explanation:

∆H

∆H

∆H

  1. Which solution(s) had little or no change in the temperature?
  2. When sodium chloride dissolves in water, the ions dissociate.

Write thermochemical equation, similar to the one above, that describe how NH4Cl and CaCl2 each dissociate as they dissolve in water. Include heat as a reactant or product in each equation.

NH4Cl:

CaCl2:

  1. Which solids in this experiment rapidly dissolve in water? Does the dissolving process usually occur with an increase or decrease in entropy? What is the sign of ∆S in each case?
Mixture 1 (NaCl) Mixture 2 (NH4Cl) Mixture 3 (CaCl2)

Increase or Decrease

Increase or Decrease

Increase or Decrease

∆S

∆S

∆S

  1. Consider the equation ∆G=∆H-T∆S. For each dissolving process, substitute the sign of T, ∆S and ∆H into the equation and determine the sign of ∆G. For which process might ∆G be either positive or negative?

Mixture 1 (NaCl) Mixture 2 (NH4Cl) Mixture 3 (CaCl2)

∆G

∆H

∆S

T

Conclusion

Summarize the purpose of the lab and whether the purpose was met. Research what “ice melt” is. What are the different types of ice melt and why does it work to melt snow. Are there different types of ice melt and why? Which would you use?