HEAT OF SOLUTION - I

(Last Revision: Sept. 24, 2003)
  • ABSTRACT:


A modification of the text procedure is used to determine the heat of solution of KNO3 in water. A computer is used both to control and record the data from the heat of solution calorimeter.

  • TEXT REFERENCE:

    • "Experiments in Physical Chemistry", Nibler, Garland, and Shoemaker, Seventh Ed., McGraw-Hill, 2003, pp.167-171.


OTHER REFERENCES:

  • GENERAL DESCRIPTION AND THEORY:


The heat of solution of a salt in water may be determined by dissolving a known amount of the salt in a known amount of water in a calorimeter. If the heat capacity of the calorimeter and contents are known the heat of solution can be determined from the temperature change. A similar experiment is described in the lab text and the differences in procedure will be described in these notes.

  • EQUIPMENT:


Dewar flask calorimeter with heater, thermistor, stirrer and connector block (Figure 1). Powerpak  with circuit block containing circuits for the thermistor bridge and the heater. (Figures 2 - 4) Computer connector block (Figure 5) and computer running the LabView program with the vi program, heatsol.vi . 31/2 Floppy disk, stop watch or timer.

  • PROGRAMS:


Lab View program running heatsol.vi.

  • CHEMICALS:


KNO3

  • DIAGRAMS:


Figure 1.

Figure 2.

Figure 3 

Figure 4.

Figure 5.

  • LABORATORY PROCEDURE:


The procedure is similar to that described in the text with some exceptions. The calorimeter should contain 180 ml of distilled water ( the water should be brought to 25°C in a thermostat before use) and samples of 0.5 to 0.8 g of KNO3 should be weighed into the sample holder, which is a plastic syringe with a neoprene plug in the end. First make sure the circuits are connected correctly according to the diagrams. The numbers in the circuit diagram refer to the numbers on the external wiring block to the computer. The power for the heater is obtained from a power pack which is connected to the wiring board. The power for the thermistor circuit is obtained from a battery.

Assemble the apparatus and turn on the stirrer. Allow the stirring to continue for several minutes. Then connect the battery and with the voltmeter connected to the leads going to 5 and 6 on the computer wiring block, adjust the potentiometer on the circuit board so that zero volts are being produced by the bridge circuit. Then start the program, heatsol.vi.  Each run consists of the following steps: When you run the program you must indicate a file name for the data from that run at the end of the run. For each run, after an initial curve is obtained for about 5 minutes, turn on the heater and stop watch and heat for five minutes. The heating time should be measured exactly with the timer. Allow an additional five minutes or more to obtain a linear curve and then introduce the sample into the calorimeter. When the curve has reached a steady state, stop the program by clicking the switch up. Repeat this procedure for each of four samples, which should be added to the same solution.
 

  • CALCULATIONS:


The temperature changes for the heating and sample reaction are obtained from the data recorded by the program. Import the data into Lotus 123, Quattro Pro or Excel and do a linear regression on the before and after temperature- time curves. (The temperature will be listed in mv ) The heat capacity of the calorimeter can be determined by dividing the energy introduced by the temperature change introduced in mv, (J/mv). The heat of solution for the sample is then found by multiplying the heat capacity by the temperature change caused by the sample dissolving in the water. The temperature change may be found from the regression equations evaluated at the mid-point of the heating or reaction. The energy is equal to the average voltage across the heater squared divided by the resistance of the heater (5.47 ohms) times the time.

The heat of solution per mole is then found by dividing the total heat energy produced or absorbed (by all of the samples that are in the solution) by the number of moles of sample (all that are in the solution.) The heat of solution should then be plotted versus the square root of the molal concentration to find the heat of solution at infinite dilution. The differential heat of solution may be found by using :

DHdiff = DHint + (m1/2/2)(dDHint/dm1/2)

  • LITERATURE VALUES: DH at inf. dilution for KNO3 is 8549 cal/mol.