Heat of Neutralization Reaction II: HCl(aq) + NaOH(aq) Amounts of Reactants
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Compare the temperature change when 50 mL of 1.0 M of HCl and 50 mL of 1.0 M NaOH are mixed in a calorimeter to the the temperature change when 50 mL of 3.0 M of HCl and 50 mL of 3.0 M NaOH are mixed in a calorimeter. The amounts of reactants that react influences q, the heat exchanged during an acid-base neutralization reaction.
A way to organize the data and the calculated values of ∆T, qsoluntion, qreaction and ∆Hrxn are given in the table below.
| 50 mL + 50 mL | ∆T solution | q solution | ∆Hrxn (kJ/mol) |
1 M HCl + 1 M NaOH | |||
3 M HCl + 3 M NaOH |
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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 demonstration illustrates how the amount of reactants reacting influences the heat exchanged, q, but does not influence ∆H. This demonstration is usually performed when topics in thermochemistry 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 demonstrations is energy is conserved. Energy cannot be created or destroyed, but it can be exchanged.
qlost+ qgain = 0 or qreleased + qgain = 0
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.
Student have difficulties with thermochemistry concepts: amounts of reactants
Students do not have difficulty with the idea that the amount of reactants influences the heat exchanged, but they do have difficulty apply this concept.
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.
There are clicker questions that can accompany this demonstration.
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. 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
2. Apply the law of conservation of energy to calorimeter experiments, qreaction + qsolution= 0
3. If the calorimetry experiment is carried out under constant pressure conditions, calculate ∆H for the reaction.
4. Given the change in enthalpy for a reaction, the amounts of reactants, and a balanced chemical equation, calculate the heat exchanged for a reaction.
5. Show that the amounts of reactants that react influences q, the heat exchanged during an acid-base neutralization reaction.
A way to organize the data and the calculated values of ∆T, q, and ∆Hrxn are given in the table below.
| 50 mL + 50 mL | ∆T solution | q solution | ∆Hrxn (kJ/mol) |
1 M HCl + 1 M NaOH | +6.7°C | +2,800 J | -56 kJ/mol |
3 M HCl + 3 M NaOH | +20°C | +8,400 J | -56 kJ/mol |
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The heat exchanged is proportional to the amount of moles of HCl and NaOH that react. ∆H is reported as a per mole value. The value of ∆H is not influenced by the amount of reactants.
- Four Styrofoam coffee cups – two nested and two nester to
- 2 Lids
- 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; 50.0 mL of 1.0 M HCl
- 50 mL 3.0 M NaOH; 50.0 mL of 1.0 M NaOH
- 4 100 mL graduated cylinders
- 2 ring stands and 4 clamps (see diagram and digital image of the set-up)
