Two titrations are conducted by the incremental addition of acid and base. When a base is titrated with an acid, the acid is added in several portions. the temperature is recorded for each increment. A plot is made of temperature rise from the beginning of the titration to that point versus the volume added. The titration is then carried out with the other order of addition. The intersection of the two temperature/ volume curves is related to the combining stoichiometry of the acid and base.
Acids and bases react to produce water and liberate heat. The amount of heat is related to the amount of acid and base reacted.
In an earlier experiment, the amount of heat released when equimolar amounts of acid and base were mixed all at once was measured. In this experiment, the same kinds of reactions are conducted except that the solutions are mixed in increments, and the temperature changes are measured in increments.
For polyprotic acids (acids with 2, 3, or more titratable protons), the amount of heat released depends upon the ratio of moles reacted.
A scientist named Job (pronounced J-oh-b, rhymes with ear lobe) used a technique for studying quantitative relationships in cases of these reactions. First, prepare mixtures of reacting chemicals in such a way that you keep track of the mole ratios, and measure the property change of interest. For Job, this was usually intensity of color formed. In this experiment it is temperature change. Try to use ratios from zero moles of the first reactant and one of the second up to one of the first reactant and zero of the second. Second, make a graph where the x-axis is the number of moles of each reactant (one goes from 0 to 1, while the other goes from 1 to 0.)
Job noted that these graphs often consisted of two straight lines. These lines usually intersected at the mole ratio corresponding to the ratio of the coefficients of the balanced chemical equation. Because this straightforward graphical analysis preceded the development of computers by several decades, the technique has been used often, and Job's name has been associated with it.
Acids and bases, especially strong bases like sodium hydroxide, are caustic to eyes and skin.
Handle acids and bases cautiously. Wear eye protection.
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Allow sufficient time for the chemicals and apparatus to become equilibrated to room temperature, about 1 hour.
Clean two 50-mL burets. Place them in a two-sided buret clamp on a support stand. Rinse one buret three times with 5-mL portions of 1.0 M sodium hydroxide. Fill the buret, and remove air from the tip by twisting the stopcock very rapidly. Drain until the meniscus is below the 0.00-mL mark. Prepare the second buret in a similar way using 1.0 M acid. (See DCExperiment 102 to see any of these steps. Click here to see the experiment.)
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On a separate sheet of paper, record the exact buret reading.
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Prepare a calorimeter by inserting two expanded styrene cups into one another, and inserting these in a 250-mL beaker for support. Prepare a cardboard calorimeter cover. Insert the thermistor through a hole in this cover.
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++Setting up the Apple II:
Install a pair of thermistors by connecting their 9-pin D-connector to the game port at the rear of the computer.
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Boot the computer with the HEATS OF REACTION software.
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Select option 3, Heats of Reaction. Respond to the instructions on the title screen. (It is necessary to calibrate thermistors with this software. Follow the calibration procedure.) Check to see that calibrated thermistors are installed. Provide the information requested. Select NEGLECT CALORIMETER CORRECTION. Select ACID INTO BASE.
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Withdraw exactly 50.0 mL of base from the buret into the calorimeter. Replace the calorimeter cover. Swirl gently to equilibrate.
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Press the space bar when ready. A temperature reading (Kelvin) appears at the bottom of the screen. When the temperature stabilizes, the Celsius temperature is reported. Press the space bar to continue.
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Press A. Remove the calorimeter cover. Quickly add 5.0 mL of acid. Restore the cover, and stir gently. Intermediate temperatures appear at the bottom of the screen. Continue stirring gently.
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Once the temperature becomes stable, a screen appears indicating that the temperature has been recorded.
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On a separate sheet of paper, record the exact buret reading.
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Press A. Remove the calorimeter cover. Quickly add 5.0 mL more of acid. Replace the cover, and stir gently until the temperature stabilizes. Record the exact buret reading for this increment below the first recorded increment.
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Once the temperature stabilizes, the computer records it.
Continue to add increments and follow screen directions until all 50 mL of acid has been used. Press F after the last addition of acid has been completed. Press space to save the results. If the increments differ greatly from 5 mL, press Q to abort and collect a new data set.
If the data seem satisfactory, enter the exact buret readings as instructed. The buret readings will appear on the screen as they are entered. The computer will expect the same number of buret readings as there were temperatures recorded. There is an opportunity to validate the data entries and correct typographical errors.
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Enter a file name to save the data. Once the file is stored, follow the screen instructions to return to the menu.
Run the experiment in the opposite order, base added to acid. Clean and rinse the calorimenter with distilled water. Withdraw exactly 50.0 mL from the acid buret into the calorimeter. Replace the calorimeter cover. Swirl gently to equilibrate.
Proceed just as before, adding increments, waiting for temperature equilibrium, and recording volumes. Enter the data. When finished, name the file.
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Once both data sets (acid to base and base to acid) have been saved, select "Exit Gracefully."
Select "Option 4 - Job's Plot." Follow the instructions to get to the Job's Plot screen. Select option 1, INPUT DATA FROM FILE. Enter the data in the manner indicated. The computer will indicate if the retrieval was successful.
The data will be summarized. Press return to continue. Three options are provided. (Stray points can be marked for deletion.) These data seem linear to the eye. Select option 3, CONSTRUCT BEST LINE TO FIT DATA.
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Follow the instructions to provide appropriate input for the graphing program. Provide the input point numbers for the acid into base titration. After an appropriate response and a few moments, the result will be displayed. Select option 1 to fit another line. Define the range of points to be graphed for the base into acid titration.
After an appropriate response and a few moments, a Job's plot results. Select option 3, DISPLAY LINE INTERSECTION DATA. Results calculated from a data analysis are displayed. Record this data.
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When you are ready, return to the menu. Select exit.
++Macintosh with ULI:
Start the "Data Logger" program. Load the calibration file for your temperature probe.
Select "Data Rate" from the "Collect" menu. Type 0.02 points per second (50 seconds between readings) to set the data rate slow enough to allow you to read the buret. You may wish to make this number larger if you are waiting for the points to be collected.
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Select "Averaging..." from the "Collect" menu. Pick "None".
Withdraw exactly 50.0 mL of base from the buret into the calorimeter. Replace the calorimeter cover. Swirl gently to equilibrate.
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Click "Start" to begin data collection. Wait for the first data point to be recorded.
Remove the calorimeter cover. Quickly add 5.0 mL of acid. Restore the cover, and stir gently. Continue stirring gently.
The next point will plotted.
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On a separate sheet of paper, record the exact buret reading.
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Remove the calorimeter cover. Quickly add 5.0 mL more of acid. Replace the cover, and stir gently until the temperature stabilizes. The computer will continue recording points every 50 seconds. Record the exact buret reading for this increment below the first recorded increment.
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Continue to add increments until all 50 mL of acid has been used.
Select "Data A Table" from the "Window" menu. Copy the data to the clipboard with command-C or the "Edit" menu. Paste the data into a graphing program. Enter your volume data.
A spreadsheet may be used to convert the volumes to millimoles. Graph volume or millimoles of acid versus the temperature.
Run the experiment in the opposite order, base added to acid. Clean and rinse the calorimeter with distilled water. Withdraw exactly 50.0 mL from the acid buret into the calorimeter. Replace the calorimeter cover. Swirl gently to equilibrate.
Proceed just as before, adding increments, waiting for temperature equilibrium, and recording volumes. Treat data as you did the acid data. Plot acid and base data on the same graph so that two intersecting lines are present. Note the mole ratio at the intersection.
Name ___________________________ Class ________
Teacher__________________________
DoChem 127 Job's Method Thermal Titration of Acids and Bases
Watch the movies and answer the questions.
Closure Questions:
1. The Job's plot for the titration of citric acid with sodium hydroxide shows a maximum near the region of 0.75 mol NaOH per 0.25 mol citric acid. On the basis of this result, predict the number of titratable protons per molecule of citric acid.
2. An unmentioned assumption of this experiment is that the reaction is very rapid, and the heat liberated is produced quickly -- nearly instantaneously after mixing. Predict the effect of using substances that take 2 minutes to reach 90% of complete reaction.
To examine the relationship between the energy changes and stoichiometric relationships in an acid/base reaction.
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(per probe)
1 cardboard square
1 support stand
2 50-mL buret
1 two-sided buret clamp
1 100-mL beaker
2 styrofoam cups, nested
100 mL 1.0 M acid (HCl, HNO3, H2SO4, H3PO4)
100 mL 1.0 M sodium hydroxide
++either
1 suitable computer
1 suitable adapter box
1 suitable SERAPHIM software floppy disk
1 calibrated thermistor probe
++or
Macintosh Computer with ULI or serial box interface from Vernier software
1 "Budget Temperature Probes" (TPB-DIN)
1 "Data Logger" software
Check that the thermistor probes have been calibrated with the computers to which they are attached. The student must have the thermistor calibration file on a floppy disk.
Allow at least one hour for the solutions to reach room temperature.
Data may be collected with the Macintosh ULI interface, but you must paste data into a different graphing program. Use a slow data rate to collect the temperature data.
Teacher preparation: 15 minutes
Class time: 40-50 minutes
The materials used in this experiment may be disposed of safely at the sink.
From the computer analysis, the mole ratio at the intersection is 0.484, and the temperature increase at this point is 6.72 °C.
Closure Questions:
1. The Job's plot for the titration of citric acid with sodium hydroxide shows a maximum near the region of 0.75 mol NaOH per 0.25 mol citric acid. On the basis of this result, predict the number of titratable protons per molecule of citric acid.
2. An unmentioned assumption of this experiment is that the reaction is very rapid, and the heat liberated is produced quickly -- nearly instantaneously after mixing. Predict the effect of using substances that take 2 minutes to reach 90% of complete reaction.
Answers to Closure Questions:
1. Since the ratio is 3 mol base per mol of acid, the predicted number of titratable protons is 3.
2. If the calorimeter does not leak any heat, this will have no effect. If the calorimeter leaks heat, the effect will be that the temperature reached will be lower than it otherwise would have.
The original module was written for Project SERAPHIM by Malcolm Rasmussen.
Project SERAPHIM is supported by the National Science Foundation.
For additional information write:
Project SERAPHIM
Department of Chemistry
University of Wisconsin-Madison
1101 University Avenue Madison, WI 53706
energy change
stoichiometric relationship
acid/base reaction titration
hermistor
polyprotic acids
calorimeter
titration
Job's method