Heat of Solution
Solubility Method

PURPOSE:

The purpose of this experiment is to determine the heat of solution of an organic acid by measuring its solubility at three temperatures and applying the Van't Hoff equation to the data obtained.

DISCUSSION:

Since the solubility of a substance is a special case of the equilibrium constant, the Van't Hoff equation is applicable. This equation, when applied to solubility, may be expressed in the following two forms:
lns= H/RT+I
in which s is the solubility at certain temperatures, H is the average heat of solution over the temperature range used, and R is the gas constant expressed in joules per mole per degree. The solubility is usually expressed in grams of solute per 100 g of solvent.
The heat of solution so calculated is approximately the average heat of solution over the temperature range studied, and it is the heat of solution at the saturation concentration. Moreover, it is the integral heat of solution as discussed in elsewhere. The value obtained may vary slightly from the value of a more dilute solution.
In this experiment the student will measure the heat of solution of oxalic acid in water at 35, 30, and 25.

APPARATUS AND CHEMICALS:

Constant-temperature baths capable of regulation to 0.2 ; 50 thermometers calibrated in 0.10 intervals; magnetic stirrer and stirring bar; two weighing bottles; three 5-ml pipettes; two 255-ml Erlenmeyer flasks, buret, two 400-ml beakers, stand and buret clamp; oxalic, succinic or tartaric acid; 0.25 N sodium hydroxide; phenolphthalein indicator.

EXPERIMENTAL PROCEDURE:

A saturated solution of oxalic acid is prepared at a temperature of about 40. Pour 150 ml of this solution into a 250 ml flask and place it in a constant-temperature bath at 35. The flask should previously have been heated to the bath temperature. As the solution cools, crystallization takes place. Shake frequently and at the end of 10 minutes withdraw a sample of approximately 5 ml. A pipet assembly as shown in Figure 16-1 (only tip of pipet and the filter are shown but a safety pipet filler should be used) is used to withdraw the sample. The pipet and filter should be at a temperature above the bath temperature so that no crystallization occurs between the glass wool plug and the tip of the pipet or in the pipet. Discharge the sample into a weighed weighing bottle and weigh. Wash into a 400-ml beaker, add about 100 ml of water, and titrate with standard sodium hydroxide solution using phenolphthalein indicator. Carry out the determination in duplicate. If you are in doubt whether equilibrium has been established in the solution, a second pair of samples can be withdrawn and titrated after an additional 5 minutes. If the grams of oxalic acid associated with a given number of grams of water has not changed, equilibrium may be assumed.
The bottle or flask with the oxalic acid solution is now placed in another constant-temperature bath at 30, or the bath previously used can be reset to operate at this new temperature. When thermal equilibrium is attained, samples are withdrawn, weighed, and titrated. Repeat the procedure at 25.

UTILIZATION OF DATA:

From the volumes of sodium hydroxide used and its normality, the weight of the sample, and the known equivalent weight of oxalic acid, the grams of oxalic acid per 100 g of water is calculated for each temperature. Average the determinations at the same temperature.
Plot log s versus l/T on regular coordinate paper, or use semi-logarithmic paper and plot the solubility (on the log axis) as a function of l/T. Draw the best straight line through the points. Select two pairs of S - T data from the curve an~ substitute them in equation (2). Calculate the value ofH.

COMMENTS TO THE INSTRUCTOR:

If succinic acid is used, it is better to take l0-ml samples and to determine the solubilities at 45,35, and 25.
It is most convenient to pull a number of filter tubes and use a different one for each sample. All filter tubes are cleaned at the end of the experiment and left to dry for the next time of use.
The 0.250 N sodium hydroxide is readily diluted from commercial 5 N sodium hydroxide solution.