Summary

In this lab, students carry out an acid-base reaction to calculate the heat of neutralization based on experimental data. This lab will reinforce the concepts of exothermic and endothermic processes, system and surroundings, and heat of reaction (specifically, neutralization).

Grade Level

High school

NGSS Alignment

This lab will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
    

AP Chemistry Curriculum Framework

This lab supports the following units, topics, and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.2: Net Ionic Equations
    • 4.2.A: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
  • Topic 4.7: Types of Chemical Reactions
    • 4.7.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
• Unit 6: Thermochemistry
  • Topic 6.4: Heat Capacity and Calorimetry
    • 6.4.A: 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 6.6: Introduction to Enthalpy of Reaction
    • 6.6.A: Calculate the heat q absorbed or released by a system undergoing a chemical reaction in relationship to the amount of the reacting substance in moles and the molar enthalpy of reaction.

Objectives

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

• Explain the difference between exothermic and endothermic processes.
• Experimentally determine the heat of neutralization of an acid-base reaction.

Chemistry Topics

This lab supports students’ understanding of:

• Heat of neutralization
• Acids and bases

Time

Teacher Preparation: 20-30 minutes (maybe a little longer, if you need to prepare the solutions)

Lesson: 45-60 minutes

Materials

For each group:

• Foam cup
• Foam cup with hole (for thermometer)
• 2 clean, dry 50-mL graduated cylinders
• Thermometer
• 30 mL 1.0-M HCl
• 30 mL 1.0-M NaOH
• Beaker (optional)

Safety

• Always wear safety goggles when working with chemicals.
• When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• 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

• Prior to completing this lab, students should be familiar with the concepts of writing and balancing chemical equations, acids and bases, neutralization reactions, heat, exothermic and endothermic processes, enthalpy, heat of reaction, system and surroundings, calorimetry, and specific heat capacity. Heat of neutralization is a subcategory of heat of reaction and does not necessarily need to be introduced ahead of time if they are familiar with heat of reaction already.
• Locate/prepare the solutions for this lab at least a day in advance, and store them in the same place so they will be at approximately the same temperature when it is time for students to start measuring temperatures. It is important that they start out at the same temperature (or very close) so the temperature increase can be measured from the same baseline (room temp).
• To increase the level of difficulty of this lab, you could remove the calculations table and the scaffolding it provides so students must figure out on their own how to determine the heat of neutralization from the data they collected. (You could also have them determine what data they need to collect and create their own data table as part of that.)
• To calculate the heat released by the neutralization reaction, students will need to use the equation q = mc∆T. This lab assumes the densities and specific heat capacities of the solutions are the same as those of water (D = 1.0 g/mL and c = 4.184 J/g°C) since the concentration of the acid and base aren’t too high. (The lower the concentration, the higher the percentage of water in the solution and the closer to true these assumptions will be.) The density is needed to convert from volumes to masses of each solution, which are used with the specific heat capacity, c, and temperature change, ∆T, to calculate heat, q. These assumptions are not perfectly accurate, but they will be close.
• If it has not been covered previously, or if students need a review, a conversation about sign conventions (+/- q and ∆H values) will be necessary for students to end up with the correct sign for the exothermic reaction in this lab. First, be sure students understand the difference between system and surroundings. In the case of a chemical reaction, the system is the chemicals that are reacting, and the surrounds are everything else, including air, beakers, thermometers, water molecules that are not part of the reaction (such as if the reaction is in an aqueous solution). To ensure students fully understand this, it may be useful to review the answers to the prelab questions before students actually complete the lab. Then, students will need to understand that positive values for q and ∆H indicate an endothermic reaction/process, and negative values indicate an exothermic process. This is always from the perspective of the system. Since this lab (and many other labs involving energy transfer) measures the increase in temperature of the surroundings, that means heat is leaving the system, and so the q value calculated with the positive ∆T is the q value of the surroundings, which is positive. Therefore, that same amount of heat, q, must have been lost by the system and would be reported with a negative sign.
• As part of their analysis, you could ask students to find the percent error of their experimental value compared to the accepted value of -57.5 kJ/mol.
• One extension activity would be to complete the procedures a second time but with either different volumes of solution or different concentrations of solution (as suggested in one of the Analysis questions), or you could assign different volumes/concentrations to different groups from the outset. Students should see that the temperature changes would be different, but ultimately result in the same heat of neutralization. Since it is Joules per mole of reaction, it is an intrinsic property and doesn’t change with the amount of reactants used. The amount of heat generated would change, but the amount of heat per mole of reactant would be a constant.
• Another extension of this lab would be to try a similar experiment but with heats of solution (instead of neutralization) involving substances that cause energy changes when dissolved in water (such as the examples given in the background information on the student handout). You could also, for either the acid-base reaction or a dissolution, give students a target temperature change to create in a set amount of water. They should select the appropriate acid-base reaction or salt dissolution (based on positive or negative temperature change) and use proportional math to calculate the amount of acid and base or salt needed to create that temperature change.
• Since both solutions are 1.0 M strong acid or base and are used in the same volume, the contents of the cups at the end of the reaction should be neutral salt water and can be dumped down the drain. To encourage good lab safety habits, you could have students add a couple drops of universal indicator or use pH paper to ensure it is neutral before pouring it out.
• This is an activity that I designed with a colleague over the past school year. We have several other target labs that we do through the year. Neither of us has seen a lab like this previously, and together we have nearly 20 years of experience.