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Utilizing Hess's Law (5 Favorites)

LAB in Calorimetry, Heat, Hess's Law, Enthalpy, Error Analysis. Last updated December 18, 2020.


Summary

In this lab, students will use a coffee cup calorimeter to collect data that will allow them to calculate ∆H for two reactions. The first reaction, between sodium bicarbonate and hydrochloric acid, is endothermic. The second, between sodium carbonate and hydrochloric pressure, is exothermic. They will then use their experimental values and Hess’s Law to determine ∆H for the decomposition of sodium bicarbonate, compare their calculated value to the theoretical value, and calculate the percent error. This resource includes a prelab presentation and sample calculations.

Grade Level

High School

AP Chemistry Curriculum Framework

This lab supports the following unit, topic, and learning objectives:

  • Unit 6: Thermodynamics
    • Topic 6.4: Heat Capacity and Calorimetry
      • ENE-2.D: Calculate the heat q absorbed or released by a system undergoing heating/ cooling based on the amount of the substance, the heat capacity, and the change in temperature.
    • Topic 6.9: Hess’s Law
      • ENE-3.C: Represent a chemical or physical process as a sequence of steps.
      • ENE-3.D: Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of the individual steps.

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.
  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data
    • Planning and Carrying Out Investigations

Objectives

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

  • Perform a calorimetry experiment to successfully calculate ΔHrxn.
  • Understand the concepts behind Hess’s Law.
  • Be able to solve problems using Hess’s Law.
  • Apply Hess’s Law to successfully calculate ΔHrxn of an unknown reaction.
  • Successfully perform error analysis.

Chemistry Topics

This lab supports students’ understanding of:

  • Calorimetry
  • Heat Calculations
  • Enthalpy
  • Hess’s Law
  • Error Analysis

Time

Teacher Preparation: 30 minutes
Lesson: 45–60 minutes

Materials (per group)

  • Beaker, 250 mL
  • Electronic Balance (2 decimals)
  • Graduated cylinder, 50 mL
  • Styrofoam cup, 2
  • Thermometer or temperature probe
  • Weighing boats or paper towels, 2
  • Deionized water
  • HCl, 1.00 M, 100 mL
  • Sodium bicarbonate, 2.50 g
  • Sodium carbonate, 2.50 g

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 materials and dispose of waste.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • When diluting acids, always add acid to water.

Teacher Notes

  • This lab can serve as a summative assessment for a unit on thermochemistry and thermodynamics. Students should have a working knowledge of calorimetry calculations and Hess’s Law.
  • Two student versions for this lab have been included, a traditional version, and an inquiry version.
  • There is a PowerPoint presentation that can be used as a prelab lesson. There is a prelab student worksheet to use with the presentation. It includes the following:
    • A lab overview.
    • Assumptions students should make during the lab.
    • A short review of coffee cup calorimetry.
    • Information about data collection and how to use it.
    • Sample data and calculations.
  • The prelab activity should allow students to design a data table for the lab. If you would prefer that a data table is included, download the Traditional Student document.
  • A lab data calculator is included in the sidebar as an Excel spreadsheet. You can use it to input student lab data and check their calculations.
  • Students begin by collecting mass and temperature data for two reactions: sodium bicarbonate and hydrochloric acid and sodium carbonate. They use that data to calculate the heat involved in each reaction using q = mC∆T. They then use the calculated value and the moles of the limiting reactant to determine ∆H for each reaction.
  • After completing the lab, students use their calculations and Hess’s Law to determine ∆H for the decomposition of baking soda.
  • The AACT resource, Hess’s Law, is a good resource to help students practice Hess’s Law calculations.
  • A different version of this lab, called Hess’s Law Application, which includes expanded teacher notes is also available from AACT.
  • Use the Energy Changes in Chemical Reactions simulation help students evaluate the energy changes in endothermic and exothermic reaction.
  • The picture of the coffee cup calorimeter found in the PPT is attributed to Community College Consortium for Bioscience Credentials, and available via Wikimedia Commons.
  • If available, use temperature probes with at least one decimal and electronic balances with two decimals. This lab usually produces good results with percent error less than 5%.
  • The Answer Key provided for the prelab activity can also be used for the lab.