Determination of Heat of Solution Essay

ABSTRACT

Thermodynamic values can be determined using the Vant Hoff isochore method. This method entails the use of equilibrium systems to determine the change in enthalpy of the solution, which can b related to the change in internal energy of the solution. The van’t Hoff isochore relates the equilibrium constant of a chemical reaction at one temperature to the equilibrium constant of the same reaction at a different temperature, allowing it to be worked out for all temperatures if it is known for one.

The experiment used the solution of toluene and naphthalene to determine the change in enthalpy. The mole fractions and the tempterature of recrylstallizations were graphed and the slope was determined. From the slope the change in enthalpy was determined to be 3.45J. This implies that energy is absorbed by the system.

INTRODUCTION

Solutions are very common in nature and in the chemistry lab. They provide the environment in which many chemical reactions occur. Thus, in the chemistry classroom and lab, we are immensely interested in solutions, especially liquid solutions.

Solutions are defined as homogeneous mixtures of pure substances in which no precipitation or settling occurs. We often think of solutions as liquids, but we can have solutions of solids (alloys), gases (air is a solution of nitrogen, oxygen, carbon dioxide, and a number of other gases), and a combination of states such as liquid and solid metals (amalgams) and liquids and gases (nitrogen in the blood, carbonated beverages). The ease of dissolution is dependent on two factors: (1) the change in disorder or randomness (entropy) of the system and (2) the change in the energy of the process (heat of solution). The process is generally favored when the degree of randomness increases and the energy of the system decreases (exothermic). When dissolution occur the entropy of the system increases. For example, the ions in crystals are highly ordered. Once dissolved, these same ions in solution are relatively disordered.

When a polymer is dissolved in a solvent, the heat measured is a sum of a polymer-solvent interaction term and a term related to the structure that existed in the solid polymer relative to its amorphous liquid state. This latter contribution, termed the “residual” heat, can have an endothermic contribution due to the fusion of crystalline regions and an exothermic contribution due to the disruption of structure in noncrystalline amorphous regions. Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. Chemically it is a mono-substituted benzene derivative, i.e. one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3. It is an aromatic hydrocarbon that is widely used as an industrial feedstock and as a solvent. Like other solvents, toluene is sometimes also used as an inhalant drug for its intoxicating properties; however, this can potentially cause severe neurological harm.

Figure 1: Structure of Toluene

Naphthalene, also known as naphthalin, bicyclo[4.4.0]deca-1,3,5,7,9-pentene or antimite is a crystalline, aromatic, white, solid hydrocarbon with formula C10H8 and the structure of two fused benzene rings. It is best known as the traditional, primary ingredient of mothballs. It is volatile, forming a flammable vapor, and readily sublimes at room temperature, producing a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass.

Figure 2: Structure of Naphthalene

EXPERIMENTAL
A. Compounds tested
Naphthalene, Toluene

B. Procedures

An accurate quantity of 15 g of naphthalene was weighed and placed into a test tube. An accurately measured volume of 5mL of toluene was also added. The stopper, thermometer, and stirrer were fitted in the set-up. The test tube was warmed in a water bath until all the solute was dissolved. The solution was allowed to cool in air, and was stirred continuously until an appearance of a solid was observed. The temperature at which the solid was observed was also recorded. Warming the mixture until the entire solid was re-dissolved and allowing the solution to be cooled in air also did a second determination. Another 1mL of toluene was added and steps 3 to 5 were repeated. Another four more 1mL portions of toluene were also added. The mole fractions were determined and graphed with the temperature of recrystallization. From the graph, the slope was determined and from the value of the slope, the enthalpy change of the solution.

RESULTS AND DISCUSSIONS

The mole fraction of the naphthalene to the toluene was determined and the temperature at which is recrystallized is tabulated below:

Table 1: Mole Fraction and temperature of recrystallization

Mole fraction| Temp.|
0.45| 67|
0.41| 60|
0.37| 59|
0.34| 56|
0.31| 56|
0.29| 54|

Besides calorimetric method of analysis, using the equilibrium system may also be utilized to determine thermodynamic values. This is dependent to the enthalpy change. The mole fraction also interferes with the equilibrium system. Changes in the mole fraction will cause the equilibrium system to shift from one form to another. The process is in isochore, denoting no change in volume. The volume of the naphthalene is not diminished in the process of the experiment. The change in enthalpy is determined by determining the slope of the graph between the inverse of the emperature and the mole fraction.

Figure3: Graph of Mole fraction versus the inverse of the recrystallization temperature

From the grpah above, there is an inverse relationship between the recrystallization temperature and the mole fraction of naphthalene. As the mole fraction of the naphthalene is decreased, the inverse of the temperature of crystallization is increased. From the slope of the graph, we are able to determine the enthalpy change of the solution. Using the formula: Figure 4: Equation for enthalpy change

From the equation above the determined enthalpy change is 3.45 J. Since the enthalpy change is a positive value the transfer of energy is towards the system.

REFERENCES
[1] Rossotti and H. Rossotti, The Determination of Stability Constants, McGraw-Hill, 1961

[2] Atkins, Peter; De Paula, Julio (2006-03-10). Physical Chemistry (8th ed.). W.H. Freeman and Company. p. 212.

[3] NonLinear vant hoff solubility. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T7W-479CM6F-3M&_user=10&_coverDate=01%2F31%2F1984&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1474224563&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=bc9cac9220a53d018a21b381170d2732&searchtype=a. taken September 28, 2010

[4] Vant Hoff Isochore. http://www.scenta.co.uk/tcaep/nonxml/science/equations/details/van’t%20Hoff%20isochore.htm taken September 28, 2010.