In this lesson students will determine the ΔHrxn of an unknown reaction using a calorimeter and Hess’s Law.
High School (AP Chemistry)
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.
- 5.6 The student is able to use calculations or estimations to relate energy changes associated with heating/cooling a substance to the heat capacity, relate energy changes associated with a phase transition to the enthalpy of fusion/vaporization, relate energy changes associated with a chemical reaction to the enthalpy of the reaction, and relate energy changes to PΔV work.
- 5.7 The student is able to design and/or interpret the results of an experiment in which calorimetry is used to determine the change in enthalpy of a chemical process (heating/cooling, phase transition, or chemical reaction) at constant pressure.
- 5.8 The student is able to draw qualitative and quantitative connections between the reaction enthalpy and the energies involved in the breaking and formation of chemical bonds.
By the end of this lesson, students should be able to:
- Perform a calorimetry experiment to successfully calculate ΔHrxn.
- Apply Hess’s Law to successfully calculate ΔHrxn of an unknown reaction.
- Successfully perform error analysis.
This lesson supports students’ understanding of:
- Energy & Thermodynamics
- Hess’s Law
- Heat Calculations
- Error Analysis
Teacher Preparation: 30 minutes
Lesson: 100 minutes
Materials (per group)
- 100 mL 1.0 M HCl
- 0.400 g Mg ribbon (Ca is a suitable alternative)
- 0.600 g MgO solid (CaO is a suitable alternative)
- 1 Thermometer
- 2 Styrofoam coffee cups for use as a calorimeter
- 1-50 mL Graduated cylinder
- Stirring rod
- Balance capable of measuring to 0.01 g
- 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.
- When diluting acids, always add acid to water.
- This lab is designed to help address two common issues students have in AP Chemistry; calorimetry experiments and applying Hess’s Law. The students should have mastered constant pressure calorimetry and Hess’s law calculations before performing this lab. Ideally this would occur at the end of a traditional thermochemistry unit.
- Calorimetry: Essential Knowledge 5.B.4: Calorimetry is an experimental technique that is used to determine the heat exchanged/transferred in a chemical system.
- Calorimetry takes advantage of the 1st law of the thermodynamics which states that in an isolated system, energy can be transformed from one form to another but cannot be created or destroyed.
- Since we are studying constant pressure calorimetry, the change in energy will be equal to the heat change.
- The implications of 1 st Law of Thermodynamics when applied to calorimetry is the following equation: q universe = qsystem + q surroundings = 0
- The calorimetry experiment is designed in a way where no heat is exchanged between the system and the surroundings, therefore any calculation of heat will be of the system.
- The system will gain or lose heat based on the heat exchanged by the chemical system being studied and will be calculated using the following equation: q=mcΔT
- The student can then calculate the heat transferred by a chemical system by measuring its effects on a system.
- The sign convention is where most students have difficulty, as they forget to change their signs when converting from the q solution to qreaction. To decrease this error they should always check their sign against their definitions of endothermic and exothermic. If the system increased in temperature, the chemical system should have released heat indicating it should have a negative sign for q.
- Using calorimetry to calculate ΔH rxn in units of kJ/mol is a skill which is frequently assessed on the AP chemistry exam. I recommend working the legacy exam 2002 #5 question as a class to help students understand how to approach the dimensional analysis of this concept.
- The legacy exam 2010 Free-Response Question #2 is a great example of the skills a student should have mastered before being able to complete the student lab activity.
- There is a formative quiz available with this lesson which can be given to assess your students’ readiness for this activity.
- Hess’s Law: Essential Knowledge 5.C.2: The net energy change
during a reaction is the sum of the energy required to break the bonds in the
reactant molecules. The net change in energy may be positive for endothermic
reaction where energy is required, or negative for exothermic reactions where
energy is released.
- Hess’s Law appears only in two different points in the AP chemistry course description, but the concept is directly addressed on the practice exam multiple choice question #38.
- According to the AP chemistry course description, the knowledge of state functions is not required for the course, but the application of Hess’s law is a concept for assessment.
student should understand the following Hess’s Law ideas:
- When a reaction is reversed, the sign of the enthalpy of the reaction is changed.
- When two (or more) reactions are summed to obtain an overall reaction, the enthalpies of reaction are summed to obtain the net enthalpy of reaction.
- There is a formative quiz available with this lesson that addresses basic Hess’s Law skills.
- The legacy exam 2005 Form B Free-Response Question #7 is a great example of how Hess’s Law is applied on the AP exam.
- Lesson Outline:
- Day 1 (50 minutes): During day 1, use the formative quiz to review the concepts in this lab and to assess mastery of the concepts. After the formative quiz, divide the class into lab groups and have the students work through the pre-lab questions. Be sure to leave at least 5 minutes of class to go over the solutions to the pre-lab questions with the class to ensure the concepts in the lab will go smoothly.
- Day 2 (50 Minutes): The lab activity should be completed.
- Any grouping strategy will suffice for this lab but groups of 2-4 will be necessary.
- Prior to the lab, the teacher should have the 1.0 M HCl solution prepared and the Mg and MgO measured.
- Each trial should take approximately 5 minutes to complete including all of the data collection. With the concentration of acid used in this lab, the reaction takes approximately 1-2 minute to reach a maximum temperature.
- Nested coffee cups are used to ensure that less heat is lost during the reaction. A lid can also be used in this experimental setup to ensure less heat is lost, but perfect data is not the absolute goal of the lab.
- If Mg is replaced with Ca, then the mass should be changed to 0.660 grams.
- If MgO is replaced with CaO, then the mass should be changed to 0.835 grams.
- Either reagent will produce similar results with and the products and the waste solution can be disposed of using the same guidelines (Flinn #24b because of excess HCl).
- Example of the lab set-up can be seen in the image.
- Sample lab data and answer key have been provided for teacher reference.