E5 Partial Molar Enthalpy
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Salvador, Sharlene Kim L. San Diego, Josheff Sanders
EXPERIMENT NO. 5: THERMODYNAMICS OF MIXTURES DETERMINATION OF PARTIAL MOLAR ENTHALPY Abstract Partial Molar Enthalpy is an extensive property that measures the heat of enthalpy per mole of a component substance of a certain mixture. The thermodynamic property of enthalpy in a mixture depends on the molar amount and properties of each individualcomponent ay constant temperature and pressure. In this experiment, the partial molar enthalpy of the mixture of Ammonium Chloride-H2O system is to be obtained at varying concentrations (0.5 g increments). The prepared ammonium chloride is then placed in Teflondish. It was dissolvedin a 100 g of water placed in a Dewar flask inside the set-up Par Solution Calorimeter after pushing the rod at first beep. The second beep reveals its enthalpy of solution and the temperature rise. Then we plot the molar enthalpy vs. mole fraction and we get the equation of the trend line. Differentiating the equation will result to partial molar enthalpy with respect to the mole fraction of water. The result showed that the partial molar enthalpy directly varies with its mole fraction of the component. On the other hand, the molar enthalpy of the solution approaches to -1200 cal/g and the values are fluctuating dependent on the molar component in mixture. The graphical analysis shows that the maximum point of the curve has the least mole fraction of water.
Keywords: Partial molar enthalpy, density, derivative, extensive property, graphical analysis Introduction This experiment intended to determine the effect of compositions of the different components of the mixture on the molar enthalpy of the solution and to evaluate the partial molar enthalpy of the different components of the mixture. In this case, thermodynamic property of ammonium chloride-water binary system which is its molar enthalpy of the solution relates to its partial molar enthalpy of ammonium chloride. The partial molar enthalpy is a thermodynamic quantity which indicates how an extensive property like U,H,S of mixture varies with change in the molar composition of the mixture at constant temperature, pressure and other thermodynamic parameters considered. Thus essentially, it is a partial derivative with respect to the number of moles of the component under study. Consider a homogeneous fluid solution comprised of any number of chemical species. For such a PVT system let the symbol M (internal energy, enthalpy, entropy) represent the molar (unit mass) value of any extensive thermodynamic property of the solution. A total system property is thennM, where n= ∑iniandi is the index identifying the chemical species. The solution property M is not solely related to the properties Miof the pure chemical species which comprise the solution. The relationship must be established experimentally for every specific system. Although the chemical species which make up a solution do not in fact have separate properties of their own, a solution property may be arbitrarily apportioned among the individual species.
Methodology The apparatus needed in this experiment were PARR 1455 Solution Calorimeter, 100 ml graduated cylinder, wash bottle, analytical balance and thermometer. For the reagents, only ammonium chloride and water were used. Initially, we weighed 0.05 g of ammonium chloride directly in a Teflon dish and we filled 100 g of distilled water in a Dewar flask. Then we set-up the PARR solution calorimeter with Teflon dish and Dewar flask mounted. And then press start and note the sample ID and press ENTER. We input the exact weight of the sample and press ENTER. After the first beep, we pushed the rod and then we press ENTER. Finally, the second beep shows the enthalpy of the solution and the temperature rise. We repeat the procedure for the next samples with 0.05 g increment up to 0.5 g of ammonium chloride. Set-up of apparatus: PARR 1455 Solution Calorimeter
The obtained results will be used to evaluate the relationship of molar enthalpy of the solution and the mole fraction of water by plotting the data. We evaluate the partial molar enthalpy of the components of the ammonium chloride-water system mixture using partial differential equation. Finally, we plot the partial molar enthalpy of each component vs. mole fraction of ammonium chloride.
Results and Discussion In this experiment, we determined the effect of composition of each component of ammonium chloride –water mixture on the molar enthalpy of the solution and its respective partial molar enthalpy. The mass of the ammonium chloride varies with 0.05 g of increment up to 0.5 g. while the water has constant mass of 100 g. The experiment is taken at constant room temperature at 30°C and also pressure. The small massof the ammonium chloride has little effect for internal variable like partial molar volume but in this experiment the solution of ammonium chloride and water exhibits an exothermic reaction andit turns out that the molar enthalpy of the solution changes significantly. On the other hand, the molecular weight of the mixture approaches to the molar mass of water and also increases as seen in table 5-1 since water has greater molar composition than ammonium chloride in the mixture. But the experiment exhibits the molar enthalpy of the solution which ammonium chloride contributes greatly on it. As expected, the mole fraction of water decreases as the mole fraction of ammonium chloride increases because water has constant mass throughout the experiment. It also serves as the solvent for the ammonium chloride as solute. As observed in table 5-1, the molar enthalpy of solution is greater at the lowest molar fraction of ammonium chloride and has smaller molar enthalpy at lowest enthalpy of solution which also belongs at the lowest molar fraction of ammonium chloride. This means that the molar enthalpy of the mixture at varying concentration of solute fluctuates. The trend of the curve mole fraction of ammonium chloride vs. molar enthalpy of solution fits a polynomial with a degree of four. As observed in graph 5-1 , there is a huge drop of molar enthalpy of the solution from 0.0005 mole fraction of ammonium chloride up to 0.001 mole fraction of ammonium chloride . It slopes right upward and oscillates like of a polynomial function.
References 1. Atkins, P., De Paula, J., (2006) Atkin's Physical Chemistry 8th edition, Oxford University Press ISBN 978-0-19-870072-2 2. Perry, Robert H. , Green D.W., (2001) Perry’s Chemical Engineers Handbook 7th edition, McGraw-Hill Co. ISBN 0-07-049841-5 3. F. Ralloa, F. Rodantea and P. Silvestronia .Calorimetric determination of partial molar enthalpies of solution of water and dimethylsulfoxide in their mixtures Retrieved from http://www.sciencedirect.com/science?_ob=Ar ticleURL&_udi=B6THV-44FMT4V4. Determination of Partial Molar Enthalpies. Retrieved from http://docs.google.com/viewer?a=v&q=cache: pDBnjDgxrpkJ:www.chem.boun.edu.tr/webpag es/courses/chem356/EXP5procedure.pdf+deter mination+of+partial+molar+enthalpy+experim ent
Appendix B. Graphs, Tables and Figures Graph 5-1
Appendix A. Sample Computation -Consider Run 1
( ) ( ) ( )
( ) ( )
( ) Graph5-2
Mole Average Specific Fraction MW enthalpy of (b) of solution (cal/g) 0.99983 18.0059 -67.2234 0.99966 18.0120 -63.6964 0.99949 18.0179 -61.6798 0.99932
Table 5-1 Ammonium Chloride (a) and Water (b) Solution
Mass of (a) (g)
Mass of (b) (g)
Mole of (a)
Mole of (b)
Mole Fraction of (a)
Molar enthalpy of Solution () -1210.42 -1147.30 1111.340 1157.087 1170.823 -1208.67 1176.190 1198.212 1174.811 -1190.65
Partial Molar enthalpy (a)
Partial molar enthalpy (b)
1168635.277 731346.025 755869.344
67.0579 209.392 347.104