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Entropy And Enthalpy

Juliet Q Dalagan, PhD
Department of Chemistry
Xavier University-Ateneo de Cagayan
Corrales Avenue, Cagayan de Oro, Philippines

Djamal Nour M. Marohombsar
Roxcil S. Malaque
LeanaDanica S. Orcullo

BSFT - 3 Chem 68 AGA

I. Abstract
The experimenters in this experimented using a simple set-up with a testube, heater and a suspended thermometer, monitored the temperature and time during the phase change of a heated sample of naphthalene pellets until melted to freezing point. The time and temperature change of the reaction are graphed to identify the changes in enthalpy and entropy that took place as the melted substance solidifies. Using the data we can then predict the change in free energy, ΔG .

Keywords/s: Entropy, Enthalpy, Free energy, naphthalene
II. Introduction
Enthalpy, H, is a thermodynamic property of a system. It is defined as the sum of internal energy U of a system and the product of the pressure and volume of the system:
H = U+PV
The PV term represents the mechanical work done on or by the system. Since we are usually more interested in changes than in absolute values, we could write
ΔH = ΔU+PΔV
Entropy, S, is an another thermodynamic property., which we can consider as a measure of the disorder or randomness of a system. An ordered system has low entropy. A disordered system has high entropy ΔS equals the heat transferred between the system and its surroundings divided by T:
ΔS = Q/T = ΔH/T
Enthalpy and entropy are different quantities. Entropy has the units of heat, joules. Entropy has the units of heat divided by temperature, joules per kelvin.
In this experiment, the group would like to compare the entropy, ΔS and enthalpy, ΔH by monitoring the temperature & time during phase change of a melted naphthalene to freezing point. And using the data to predict ΔG.
III. Experimental Section

Prepare 250 ml of half full tap water in a beaker to be placed in a ring stand, light the Bunsen burner to begin heating of water. In a large test tube fill half full of naphthalene and insert thermometer into crystals, ensure to to attach the cotton thread at the end of the test tube to let it hang in an iron clamp. Place the test tube into the beaker of hot water .Notice the melting of solid crystals.
Carefully remove the water bath apparatus and burner from beneath the test tube and begin recording the temperature every after 30 sec, until the naphthalene passes the freezing point of molecules.

IV. Results and Discussion

Initial Room Temp: 34 ͦ c

Temperature vs. Time

Based from the results the naphthalene freezes at 840 seconds in 77.8 degrees Celsius. Freezing reaction is an exothermic process; energy is lost from the water and dissipated to the surroundings. Therefore, as the surroundings get hotter, they are gaining more energy and thus the entropy of the surroundings is increasing. During the process of melting naphthalene, the reaction is said to be endothermic because the system absorbs the energy from its surroundings. The relationship between the time and temperature is inversely proportional as the time increase the temperature decreases. The Gibbs Free Energy of reaction (DG) is an indicator of reaction spontaneity. Obeying the Entropy change and enthalpy change together influence the spontaneity of a chemical reaction. A spontaneous process is capable of proceeding in a given direction without needing to be driven by an outside source of energy. In Second law of thermodynamics, the naturally occurring reactions always move toward a state of lower potential energy. Thus, a reaction with a negative delta G, like the one in the graph, is said to be spontaneous because there is no heat applied during the reaction. A reaction that is spontaneous is always accompanied by the net release of free energy (energy available to do useful work). However, some spontaneous reactions require added energy to get started. The energy they finally release includes both this added energy and the calculated free energy of the reaction.
V. Conclusion
The cooling the reaction is said to be spontaneous because there is no heat applied during the process and enthalpy seemed to be the dominating factor in finalresult. The spontaneity of the process appears to be associated with a highly ordered system going to a less-ordered, uniform state. The spontaneity of a process depends not only upon the enthalpy change but also upon how the disorder of the system changes.
VI. Experimental Error
One of the possible caused of error in this experiment is the inconsistent recording of temperature in time.

VII. References
Carmichael, L.N., Haines
D.F., and Smoot, R.C. Laboratory Chemistry. Merrill Publishing Co. 1983.

(2014). What is the difference between entropy and enthalpy?. Retrieved from http://socratic.org/questions/what-is-the-difference-between-entropy-and-enthalpy