Experiment 8

Thermochemistry - Enthalpy of Neutralization

In this experiment you will mix an acid (hydrochloric, sulfuric or phosphoric acid) and a base (NaOH) and measure the heat of reaction (enthalpy of neutralization; ΔH_rxn) for this titration reaction.  After you determine the amount of heat produced in your experiment, you will calculate the heat produced for each mole of water formed.

The acid solution that you will use for this experiment will be either 2M HCl, 1M H₂SO₄, or 0.667M H₃PO₄.  Do not be concerned that the concentrations of these acids are different, because the amount of water (H₂O) formed in each reaction will be the same, 0.200 moles.  Since the concentration of protons (H⁺ ions) in each acid solution is 2.000 M, equal volumes of these solutions (100 mL) will produce the same amount of water (0.200 moles).  You will be assigned which acid you will use.  There will be an excess of OH^- (hydroxide) ions (>0.200 M) in the NaOH solution.

A description of how to do the experiment is contained in your published protocols.  During the first day of this lab, you will do your calorimetry experiments by hand and calculate the heat of reaction for each of your experiments.  The information included here is supplemental to the published protocol and describes some of the analyses you will perform.  If you were assigned to use sulfuric acid, the following equation shows your reaction, and the theoretical heat of reaction (enthalpy of neutralization; you should calculate the DH_rxn for this reaction to verify the heat of reaction shown).

H2SO4 (aq) + 2 NaOH (aq) → 2 H2O (l) + Na2SO4 (aq)

ΔHrxn = -110.0 kJ

Please note that the heat of reaction  (ΔH_rxn) for this reaction is -110 kJ, and is for the synthesis of two (2) moles of water, not just one.  In fact, when you determine the heat of reaction for the other two acids used in this experiment (phosphoric and hydrochloric acids), you will produce different amounts of water for each reaction.  You should complete the reactions for the other two acids, and show your results in the table below (the balanced equation and heats of reaction are shown for sulfuric acid).  In order to compare the reaction enthalpies for each acid, you need to divide the calculated heat of reaction by the number of moles of water actually produced.  The values you determine are the theoretical heats of reaction for each of the acids used in these experiments, and should be close to each other, although not identical.

Use the values in Table 2 to calculate the heat of reaction for the other two acids. The values you calculater here for the ΔH/mol H₂O will be used for your percent error calculations.

Please remember that for your experiments, you will actually produce the same amount of water (0.200 moles) regardless of which acid is used in your experiment.  It is for this reason that you must adjust the heat of reaction for each of the reactions above, to adjust for the number of moles of water formed.  Since you will produce 0.200 moles of water in each of your experiments, after you determine the amount of heat produced in each of your individual reactions, you must multiply the observed enthalpy of neutralization by 5 (five) to calculate the amount of heat produced for each mole (1.00 moles) of water formed.   To determine the ΔH_rxn (heat of reaction) for each of your individual experiments, use the equations shown in Table 4.  After you calculate your observed values, calculate the ΔH for each mole of water formed (multiply the ΔH_rxn by 5 [or divide by 0.200]).  Calculate the ΔH_rxn for each of your reactions in the same manner.

Using the chart above, enter the value for the experiment results you obtained, but only for the acid that you used using the equation shown. Remember that q_water = - q_reaction

The equations and information you will need are shown below (ΔH_rxn = q_p)

Experimental Procedure

To do your experiment follow the instructions listed below.

• Go to the reagent cart and dispense your acid and base solutions.
  • Pour about 250 mL of your assigned acid into a labelled beaker and take back to your bench.
  • Pour about 250 mL of the NaOH solution into a labelled beaker and take back to your bench.
• Measure 100 mL of your assigned acid into your calorimeter (consisting of two or three styrofoam cups)
• Measure the temperature of the NaOH solution in the beaker using a digital thermometer
• After measuring the temperature of the NaOH solution, wash the thermometer probe with DI water, and dry it thoroughly prior to placing it into the acid solution in the calorimeter.
  • If the temperature of the two solutions are not the same, average the temperatures and use this averaged temperature for T_i (initial temperature).
• Measure 100 mL of your NaOH solution in a graduated cylinder.
• Quickly pour entire NaOH solution (100 mL) into the calorimeter and swirl gently to mix the contents.
  • The thermometer probe should be in the acid solution before the NaOH addition, and kept there during mixing.  When a maximum temperature is observed on the thermometer, record this value as T_f (final temperature) for the experiment.
• Calculate q_p (ΔH_rxn), which is the enthalpy of neutralization, using the equation shown in Table 4 (please note that this q_p that you calculate is for the formation of 0.200 moles water).  
• Calculate the ΔH  per mole water by multiplying the q_p calculated above by 5 (or divide by 0.200).
  • This value is the experimental heat of reaction for the synthesis of one mole of water.
• Repeat the titration experiment another time (a total of two experiments will be performed) and calculate the ΔH_rxn for each experiment.
• Calculate the ΔH for the synthesis of one mole of water, as described above, for each experiment.

After you have finished each of your two titration experiments, calculate the heat of reaction for each.  After you have determined the enthalpies of reaction for these experiments and have calculated the ΔH for the synthesis of one mole of water, enter these experimental values into the spreadsheet.  Be sure to enter your experimental values in the correct section of the spreadsheet, and enter the values for synthesis of one mole of water.

Calculation of experimental error.

In order to calculate your experimental error for each of your reactions, use the equation below.  The theoretical value is the heat of reaction, for your acid, per mole of water formed, shown in Table 1.  The experimental value is the actual heat of reaction you determined in each of your experiments (per mole of water formed).

To obtain %Error for a measurement, you need to know the Theoretical and Experimental values for your measurement, or set of calculations.  Use this formula to obtain the %Error, which is always a positve (absolute) value.

Computer-assisted calorimetry

The collection of the calorimetric data in this part of the experiment (second day of lab) will be performed using LabWorks.  An explanation of this procedure will be described in lab.  The analysis of your data will be performed during the next lab period.

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