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Discoveirng Hess's Law

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Cody Keller Period 2 Examining Hess's Law Introduction Hess's law is named after Germain Hess, a Swiss born Russian Scientist. Hess's law is a law of physical chemistry used in predicting the enthalpy change and conservation of energy of chemical reactions. In particular, this law state that the total energy change for a chemical reaction is not dependant on the route the reaction takes place, assuming the initial and final conditions are the same. Research Question How do we find the enthalpy of the reactions NaOH + HCL � H2O + NaCl, KOH + HCL � H2O + KCl, and NH4Cl + Ba(OH)2 � NH3 + BaCl2? Hypothesis Part 1 If we have 51.1 grams of NaOH mixed with 48.9 grams of HCl, then their reaction should produce -55.7 KJ/mol in an exothermic reaction, according to our theoretical math. Part 2 If we have 7.1 grams of diluted HCl and 11.7 grams of diluted KOH, then the reaction created should produce -56.7 KJ/mol in an exothermic reaction, according to our theoretical math. Part 3 If we have 1.5 grams of both NH4Cl and Ba(OH)2 , then the occurring reaction should produce 91.32 KJ/mol in an endothermic reaction, according to our theoretical math. Variables Part 1 Independent o The independent variables were the amounts of reactant used (51.1 grams of NaOH and 48.9 grams of HCl), and their molarity, because although they were predetermined, they were determined by us, not given to us. ...read more.


First we got all the necessary equipment and tools to perform the lab. 2. We then took the masses of the 50mL beakers. 3. Then, after solving for the amounts needed of the reactants, we acquired the amount of reactants needed. 4. We then placed each reactant (HCl and KOH) into the separate 50mL beakers and their masses were then subtracted by the original mass of the 50mL beakers to find the masses of the reactants. 5. Both were diluted with 5mL of water while being stirred with a stirring rod. 6. We then placed both of the solutions into a single, 100mL beaker and we recorded the changes in the temperature. 7. After all this had transpired, we recorded our data and solved for ?H. Part 3 1. First we got all the necessary equipment and tools to perform the lab. 2. We then made sure that we had our safety goggles, gloves, and a fume hood to let out the ammonium gas, due to the harmfulness associated with these chemicals. 3. Before any lab reactions were done, we solved for the correct amount of reactants (1.5 grams of both NH4Cl and Ba(OH)2) and placed them in separate 50mL beakers. 4. We then took their masses and subtracted the original 50mL beaker weight to find the masses of the reactants. 5. Then, we put both of the reactants in the mortar and we began to mash the substance with the pestle while recording the temperature and observations. ...read more.


While none of our actual ?H values matched up exactly with the theoretical ?H values, all of the values successfully and accurately portrayed whether the reactions were endothermic or exothermic, which is principally what the lab is about, and none of our actual ?H values were astronomically off, or at least not off enough to make a significant difference. Furthermore, we were able to calculate the ?H's with a moderate amount of accuracy. This lack of exact accuracy indicates several errors in this lab. Firstly, human error is a gigantic factor. With so many calculations, sign changes, eyeball measuring, and other human inaccuracies undetected and therefore unaccounted for indefinitely contribute to the lack of accuracy. Also, inaccurate measuring tools and devices may have played a role. Finally, a lack of understanding of what was actually transpiring would have been tremendously helpful. When you don't fully understand the concepts being employed, you are going completely on "empty" formulas without an understanding of what you are actually doing, making it virtually impossible to detect error. If this lab were to be conducted again, there would be several different things I would do different. I would alter my methods of conducting this lab to minimize human error. Also, I would use more accurate measuring tools and devices. Finally, I would make my best effort to come into the lab with as much prior knowledge about the subject as possible, and also bring to the table a superb grasp of what was actually going on. ...read more.

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