# Heat of Neutralization: HCl(aq) + NaOH(aq)

Equal volumes, 50.0 mL, of 3.0 M hydrochloric acid and 3.0 M sodium hydroxide solutions having an initial temperature of 20.0°C react in a calorimeter.  The resultant solution records a temperature of 40.0°C.  The heat gained by the resultant solution can be calculated using

qsolution = m c ∆T    where m is the total mass of the resultant solution and c is the specific heat capacity of the resultant solution

Since the solutions are mostly water, the solutions are assumed to have a density of 1.0 g/mL and a specific heat of 4.18 J/g°C.  The reaction of an aqueous hydrochloric acid solution with an aqueous sodium hydroxide solution is represented by the neutralization chemical equation

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) + heat

Curriculum Notes

HCl(aq) + NaOH(aq) --> NaCl(aq) + H2O(l) + Energy

Thermochemistry  determine the heat exchanged at constant pressure,

q = m c ∆T.

Calculating the limiting reactant, the change in enthalpy of the reaction, ∆Hrxn, can be determined since the reaction was conducted under conditions of constant pressure

∆Hrxn = qrxn / # moles of limiting reactant

This reaction is classified as an exothermic reaction.

This demonstration is usually performed when topics in thermochemistry or thermodynamics are being discussed. The reaction of HCl(aq), a strong acid, with NaOH(aq), a strong base, is an exothermic reaction. The big idea for most calorimetry themed demonstrations is energy is conserved.  Energy cannot be created or destroyed, but it can be exchanged.

qlostqgain = 0   or   qreleased + qgain = 0

This demonstration also illustrates how the formation of water (one of the driving forces) can act to drive a reaction to spontaneity. This is a neutralization reaction with the hydroxide ion acting as the base and the hydronium ion acting as the acid.

Making this demonstration interactive - active learning

The instructor should "frame" the demonstration and guide the discussion.  After students observe the initial conditions of the solutions and observe the results of the demonstration, it is important for the students to be allowed to discuss what gains heat and what loses heat in this chemical process before the instructors tells the students the answers. Students should be asked to identify what gains heat and what looses heat - use a series of Clicker Questions.  Ask "What gains heat?"  "How much energy, as heat, is released or gained by the solution?"  "How much energy, as heat, is released or gained by the reaction?" "What are the primary species present in each solution before the reaction?"  "What are the species present in the solution after the reaction?"  "How is the heat manifested - what are the water molecules doing differently while the reaction occurs?"

Student difficulties with thermochemistry concepts

Students have difficulty distinguishing the terms temperature and heat. Students have difficulty with the idea that the bulk material they can see is NOT the chemical reaction. A chemical reaction has no mass, has no specific heat, and does not change temperature.  A chemical reaction consists of bonds breaking and bonds forming and this is a form of potential energy.  In this demonstration, the chemical reaction releases heat to the immediate the surroundings.  The water and dissolved chemicals gain heat - heat is transferred into the solution, which is mostly water. gained heat.  When heat is transferred into the surroundings, the solution, from the chemical reaction, the solution increases in temperature.  The water molecules being formed by the reaction have higher kinetic energy compare to the original water molecules in the solution. The newly formed water molecules collide with the original water molecules causing some of the original water molecules to move faster, there is a net increase in kinetic energy of the water molecules.

Kinetic energy = (1/2 mv2)average  = (3/2) kT   T is the temperature and k is the Boltzmann constant

Students have a difficult time understanding that through the vibration and movement of atoms and or molecules heat is exchanged and this is a form of kinetic energy.

There is a computer animation available depicting the rapid movement of newly formed water molecules as a result of an acid-base reaction to accompany this demonstration.

There is a calorimetry computer simulation available to accompany this demonstration.

There is an in-class POGIL-like activity to accompany this demonstration.

There are a set of interactive guided-inquiry Power Point slides to accompany this demonstration.

Showing acid-base neutralization calorimetry demonstration, the computer animation at the particle level, and the chemical equations helps students connect the macroscopic, microscopic (particle), and symbolic levels of representation - Alex Johnstone's Triangle - which leads to a more in-depth understanding of the concepts associated with thermochemistry.

Calorimetry Computer Simulation

https://media.pearsoncmg.com/bc/bc_0media_chem/chem_sim/calorimetry/Calor.php

©2016 Greenbowe, Abraham, Gelder  Chemistry Education Instructional Resources.  University of Oregon, Oklahoma State University, University of Oklahoma, Pearson

Learning Objectives

After observing the demonstration and doing the in-class activities, students should be able to

1. Identify the system and the surroundings for a given calorimetry experiment.

2. Identify what is releasing heat and what is gaining heat for a given calorimetry experiment.

3. Calculate the heat gained or released by a solution, qsolution, involved in a given calorimetry experiment: total mass of the solution, specific heat of the solution, change in temperature of the solution: q = m c ∆T

4. Apply the law of conservation of energy to calorimeter experiments,  qreaction qsolution= 0

5. If the calorimetry experiment is carried out under constant pressure conditions, calculate H for the reaction.

6.  Given either the initial and final temperature measurements of a solution or the sign of the ∆Hrxn, identify if a reaction is endothermic or exothermic.

7. Given the change in enthalpy for a reaction, the amounts of reactants, and a balanced chemical equation, calculate the heat exchanged for a reaction.

One day of lead time is required for this project.
Discussion

Students must have experience working with physical processes involving calorimetry prior to learning about chemical reactions involving calorimetry and thermochemistry.  Calorimetry is the process by which the heat exchanged in a chemical or physical process can be determined. The apparatus is the calorimeter. A coffee cup calorimeter made of styrofoam is effective in preventing heat transfer between the system and the environment. Because the solution in the calorimeter (the cup) is open to the atmosphere, as long as the pressure does not change while performing the demonstration, this is constant pressure calorimetry. The heat exchanged by the reaction, qreaction,  can be used to determine the change in enthalpy of the reaction.

The balanced chemical equation representing the neutralization of hydrochloric acid with sodium hydroxide is:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) + heat

Since theses are dilute solutions and are mostly water, assume that the densities of the solutions and the specific heat capacities of the solutions are approximately 1.0 g/ml and 4.18 J/g°C, respectively.  In order for students to grasp the main concepts associated with this demonstration, assume that the calorimeter is very well insulated and that no energy, heat, is lost to the surroundings or walls of the container

qsolution = m c ∆T    where m is the total mass of the resultant solution, c is the specific heat capacity of the solution,

and ∆T = Tf -Ti

qsolution = (50. g HCl + 50. g NaOH)(4.18 J/g °C)(40.0°C - 20.0 °C) = +8,360 J

The energy released by the reaction is qreaction. By the law of conservation of energy:

qreaction + qsolution = 0     qreaction = -qsolution = -8,360 J

The limiting reactant is either the HCl or the NaOH since there are equimolar amounts present

0.050 L HCl x 3.00 mole HCl/L HCl   =  0.150 mole HCl

At constant pressure, the enthalpy change for the reaction for the amounts of acid and base that react are

∆H rxnqreaction / # moles of limiting reactant = -8,360 J / 0.150 mole HCl  = - 55,730 J/mole HCl or -55.7 kJ/mole HCl

Materials
• Two Styrofoam coffee cups – nested
• Lid
• Digital Thermometer or a Vernier Temperature Probe or Thermocouple with interface  to computer* Logger Pro or Logger Lite software
• PC or Mac lap-top with appropriate software for displaying the temperature
• 50 mL 3.0 M HCl
• 50 mL 3.0 M NaOH
• 2 100 mL graduated cylinders
• ring stand and 2 clamps  (see diagram and digital image of the set-up)
Safety Precautions

The 3.0 M HCl solution is corrosive. The 3.0 M NaOH solution is caustic. Both the acid and base solutions can cause burns to exposed skin and damage to eyes. Use gloves and eye protection while preparing and performing the experiments.

Footnotes

References

1. Greenbowe, T.J. and Meltzer, D.E. (2003). “Student Learning of Thermochemical Concepts in the Context of Solution Calorimetry.” International Journal of Science Education, 25(7), 779-800.

2. J. Kotz, P. Treichel, J. Townsend ( 2009) Chemistry & Chemical Reactivity 7th ed. Instructors Edition; Brooks/Cole.

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© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group