cold pack lab - 2010 ColdPackDesign Lab...

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Cold Pack Design  Lab Effect of Barium Hydroxide to a solution  of Ammonium Chloride and water Comparing the endothermic reaction of ammonium chloride and water with  the addition of barium hydroxide  2010 Alice Wang IB Sl Year 2 2/21/2010
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PURPOSE  To test the reactivity of an endothermic reaction with divergent sample sizes of ammonium chloride to water and determine what variables affect the effectiveness of a cold pack To establish if different quantities of barium hydroxide will affect the reactivity the endothermic reaction of barium hydroxide and water BACKGROUND Cold packs depend on spontaneous reactions, reactions that begin as soon as the reactants are placed together and continue on their own. There are two driving forces for all chemical reactions are enthalpy and entropy. Chemical changes tend to go from orderly arrangements of molecules and ions to disorderly arrangements. Nature tends to increase the amount of messiness, or disorder, or entropy. The natural tendency to increase entropy sometimes opposes the tendency to release heat. When the increase in entropy is great enough, it can drive the heat flow “backward.” The drive for high entropy overpowers the drive to release heat. Endothermic reactions happen spontaneously only when the reactions permits a large increase in entropy. In other words, for a process to take place spontaneously at constant temperature and pressure, the change in free energy must be negative. An endothermic reaction may thus be spontaneous at constant pressure if the positive value of the heat absorbed is offset by a sufficient increase in entropy (randomness). To fully understand how a hot or cold pack works, we must examine how the particles interact when a salt dissolves in water. The salt crystal is made of positive and negative ions that hold the it together by electrostatic attractions, the attraction of opposite charges. When a salt dissolves in water, the electrostatic attractions between the ions are broken and each ion forms new electrostatic interactions with the water molecules. In the case of the instant cold pack, the starting material was highly ordered: The water was pure in its own beaker, and the ammonium ions and chloride ions were arranged in an orderly pattern in solid crystals. The substances were sorted and organized—everything in its place. When the water was dissolved, the orderly arrangement of the ions was disrupted. The ions were dispersed randomly throughout the water, and the once-pure water became disordered. The system went from very ordered to very disordered, and the reaction was partly driven by this increase in entropy. Ammonium chloride is classified as a salt. Chemically speaking, there are thousands of salts in addition to sodium chloride, common table salt. Salts contain ions, particles with electrical
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charges. Because the ions with positive charge are strongly attracted to those with negative charge, they form a sold crystal. In the first step, the solid crystal separates into ions. Breaking
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This note was uploaded on 04/19/2010 for the course CHEM 1 taught by Professor Chemistry during the Fall '10 term at Academy of Design Tampa.

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cold pack lab - 2010 ColdPackDesign Lab...

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