Determination of Thermodynamic Variables from Solubility Product, Cheat Sheet of Chemistry

Download Determination of Thermodynamic Variables from Solubility Product and more Cheat Sheet Chemistry in PDF only on Docsity! filtrates & residues edited by JAMES O. SCHRECK University of Northern Colorado Greeley, CO 80639 Solubility and Thermodynamics: An Introductory Experiment Robert G. Silberman S.U.N.Y. at Cortland, Cortland, NY 13045 This experiment is reasonable for an introductory class in chemistry; it allows students to examine an equilibrium reaction, a solubility curve, and the thermodynamics asso- ciated with equilibrium concepts in a simple, quick, safe, and cheap way. There are two simplifying assumptions made in the calculations: (1) the activities of the ions and the ionic strength of the so- lutions are not considered, and (2) the temperature at which the crystals become visible is probably lower then equilibrium temperature. In this experiment several thermodynamic variables (AH, AS and AG) will be calculated for the simple dissolu- tion reaction shown below. KNOs(s) + HzO @ K*(aq) + NO3(aq) The solubility in mole/liter of the compound will be measured for six or seven temperatures over a range of 40-60 °C. These values will then be used to calculate the thermodynamic variables using the method outlined in the calculation section below. Procedure « Caution: Potassium nitrate is a strong oxidizer and skin irritant. Avoid contact with skin, eyes, and mucous mem- branes. Avoid shock, heat, and contact with combustible ma- terials. 1. Weigh 20 g potassium nitrate (KNOs3), and transfer it to a large test tube (25 x 200 mm). Note: Technical grade KNO3 does not seem to work well. 2. Add 15 mL water and heat the test tube in a boiling water bath, with stirring, until all of the KNOg3 has dis- solved. 3. Determine and record the volume of the KNOg solu- tion. (This can be done by filling another large test tube (25 x 200 mm) with water until the volumes in both tubes are the same, then measuring the volume in the test tube filled with water in a graduated cylinder). 4, Remove the test tube with KNOg3 solution from the water bath and allow it to cool while slowly stirring the solution. Insert a thermometer into the solution. 5. Record the temperature when crystals first appear. It is assumed at this temperature the system is at equilib- rium, and it is possible to calculate the equilibrium concen- trations of the ions. 6. Add 5 mL water to the test tube, and warm the mix- ture until the solid has all dissolved. Determine the solu- tion volume as before and record. 7. Cool slowly and record the temperature at which crys- tals first appear. 8. Repeat the cycle (steps 6 and 7) adding 5 mL water until a crystallization point temperature near room tem- perature is reached. (This should take six to seven deter- minations). 426 Journal of Chemical Education 9. When you have obtained all data, record it on the chalkboard for the class to use in their laboratory reports. Calculations Because KNOs is a strong electrolyte, its solution reac- tion will be: KNO3(s) + H20 @ K*(aq) + NO3(aq) The reaction may be considered to be at equilibrium when the solid is in contact with a saturated solution—just the condition we have when crystallization begins. The solu- bility, s, of the salt in moles per liter may be calculated from amount of salt weighed out and the volume of the so- lution. The equilibrium constant K for the reaction will be: K =[ K*I[NO3] = (sXs) = s” (D The equilibrium constant K may be used to calculate AG for the reaction at each temperature using the following relationship AG = -RT In K (2) where T is the Kelvin temperature and FR is the gas con- stant (8.31 J/mole-deg). Using eq 1 calculate K for each temperature, and eq 2 calculate AG for each temperature. The variation of K with temperature is such that if a plot of InK (y axis) versus 1/T is made, the result is a straight line with the slope -AH/R. Construct this graph (see the figure), measure the slope of the line and determine the value of AH for the reaction. At this point you have caleu- lated K and AG for each temperature and a single value for AH. (AH is nearly a constant over a small range of tempera- 4.4 4.24 4.0 4 3.84 3.64 3.4 4 Ink 3.24 3.04 2.84 264 0.0029 © 0.0080 »Ss«i00381~=—s«i032~=Ss—«éi.0083 1/T Agraphical representation of the data shown in the table.