# AACT Member-Only Content

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

# Heat of Neutralization Mark as Favorite (4 Favorites)

LAB in Acid Base Reactions, Molarity, Net Ionic Equation, Mole Concept, Dimensional Analysis, Measurements, Heat of Neutralization, Calorimetry, Specific Heat, Temperature, Exothermic & Endothermic, Bond Energy. Last updated March 02, 2022.

### 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).

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
• TRA-1.B: 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
• TRA-2.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
• 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 6.6: Introduction to Enthalpy of Reaction
• ENE-2.F: 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.

### Background

A chemical reaction involves breaking bonds and forming new bonds. This change is always accompanied by an exchange in energy. If energy is released from the system (the reaction) to the surroundings, then the reaction is exothermic, and the products have less energy than the reactants. If energy is absorbed from the surroundings, then the system (the reaction) is endothermic, and the products have more energy than the reactants.

Because energy cannot be created or destroyed, only transferred, the quantity of heat, q, released or absorbed by the system is equal to the amount of heat that is absorbed or released by the surroundings. Thus, the quantity of energy will be the same but the sign (+ or -) will be reversed depending on the frame of reference (system or surroundings). Mathematically, this can be written as qsystem = -qsurroundings. By definition, exothermic reactions use a negative sign (system releases energy) and endothermic reactions use a positive sign (system absorbs energy). The amount of heat released or absorbed per mole of reaction is called the heat of reaction, ∆Hrxn.

Physical changes can also release or absorb energy. For example, energy is exchanged when a substance dissociates and dissolves in water. When sodium hydroxide pellets are added to water, the temperature of the water can become so hot that the solution steams. Hot packs and cold packs used by athletes contain compounds that release or absorb heat when dissolved in water. The heat associated with a process like this is called the heat of solution, ∆Hsoln.

Hot Pack: CaCl2 + H2O → Ca2+ + 2Cl- + H2O + Heat

Cold Pack:   NH4NO3 + H2O + Heat → NH4+ + NO3- + H2O

In this lab, you will look at a chemical reaction that causes a change in temperature. You will combine aqueous hydrochloric acid and aqueous sodium hydroxide, and as the neutralization reaction between this acid and base occurs, you will see a change in temperature. You will use the data you collect to determine the heat of neutralization, ∆Hneut, a subcategory of heat of reaction specific to acid-base reactions.

### Prelab Questions

1. Define neutralization reaction.
2. Write the balanced molecular equation, complete ionic equation, and net ionic equation of the reaction between sodium hydroxide solution and hydrochloric acid solution. (You do not need to include heat in the equation.)
Molecular Equation:
Complete Ionic Equation:
Net Ionic Equation:
3. Identify what substances/materials/equipment etc. make up the system and the surroundings for the neutralization reaction that will be carried out in this lab.

In the questions below, circle the word/phrase in bold that best completes each sentence.

1. An exothermic process occurs when energy is (absorbed from/released to) the surroundings, while an endothermic process occurs when energy is (absorbed from/ released to) the surroundings.
2. If the system loses energy, the quantity of energy gained by the surroundings is (greater than/less than/equal to) the amount of energy lost by the system.
3. The energy released when solid sodium hydroxide is dissolved in water would best be described as the heat of (reaction/neutralization/solution).
4. The dissolution of CaCl2 (absorbs heat from/releases heat to) the surroundings while the dissolution of NH4NO3 (absorbs heat from/releases heat to) the surroundings.
5. Hot packs use (endothermic/exothermic) processes while cold packs use (endothermic/exothermic) processes.
6. The thermite reaction occurs between iron oxide and aluminum and produces molten iron, aluminum oxide, and a flame so hot that it will even burn underwater. This evidence suggests that the thermite reaction highly (endothermic/exothermic).

### Objective

Experimentally determine the heat of neutralization of the reaction between hydrochloric acid and aqueous sodium hydroxide.

### Materials

• Foam cup
• Foam cup with small 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 handling chemicals in the lab.
• When working with acids and bases, if any solution gets on your skin, immediately alert your instructor and thoroughly flush the area with water.
• Wash your hands thoroughly before leaving the lab.
• Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

### Procedure

1. Obtain two foam cups, one with a hole and one without a hole.
2. Obtain a clean, dry 50-mL graduated cylinder. Dispense exactly 30.0 mL of the 1.0-M NaOH solution in the graduated cylinder. Insert a thermometer into the solution and record the temperature to the nearest tenth degree.
3. Obtain a second clean, dry 50-mL graduated cylinder. Dispense exactly 30.0 mL of the 1.0-M HCl solution. Rinse and dry the thermometer used earlier, then take the temperature of HCl to the nearest tenth degree.
4. If both solutions are the same temperature, then proceed. If the temperatures do not agree, wait 5 minutes and retake temperatures. Repeat until the solutions are within 0.1°C of one another, then proceed to step 5.
5. Transfer the NaOH into the foam cup (without a hole). Then pour the HCl into the same foam cup. Quickly, place the cup with the hole in it upside down on top of the cup containing the solutions as a sort of “lid” to prevent heat loss. Insert the thermometer through the hole and gently stir. Stabilize the cup by having one person stir while another holds the cup, or by placing the cup in a beaker.
6. Record the peak temperature reached by the resulting solution.

### Data

 Temperature of NaOH solution °C Temperature of HCl solution °C Peak temperature of solution formed by mixing NaOH and HCl °C Molarity of HCl solution Molar Volume of HCl solution mL

### Calculations

Show your work for each calculation in the table below. Assume that both the density and the specific heat values for the solutions are the same as water: 1.0 g/mL and 4.184 J/g°C, respectively.

 Temperature change of solution mixture (surroundings) °C Total mass of solutions grams Heat absorbed by solution mixture (qsurroundings) Joules Heat released by neutralization reaction (qsystem) Joules Moles of HCl used moles Heat of neutralization in Joules per mole of HCl J/mol Heat of neutralization in kilojoules per mole of HCl kJ/mol

### Analysis

1. Two assumptions were made in the calculations section. What are they, and why are these assumptions reasonable?
2. Based on the temperature change you saw in this reaction, is the neutralization reaction of HCl and NaOH exothermic or endothermic? Explain.
3. Based on your categorization of the reaction as exothermic or endothermic, rewrite the complete balanced equation for the reaction that took place in this lab, including heat on the appropriate side of the equation.
4. How would your temperature data have been affected if you had used twice the volume of each solution? If you had used 0.1-M solutions instead of 1.0-M solutions? Would these changes have affected your heat of neutralization calculations? Why or why not?

### Conclusion

Summarize what you have learned about exothermic and endothermic reactions, temperature change, and heat of neutralization.