Thermodynamics - Thermodynamics-Enthalpy of Reaction and Hesss Law I Purpose The release or absorption of heat energy is a unique value for every

Thermodynamics - Thermodynamics-Enthalpy of Reaction and...

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Thermodynamics-Enthalpy of Reaction and Hess’s Law I. Purpose The release or absorption of heat energy is a unique value for every reaction. Are all these values experimentally determined? No, many are calculated using Hess’s law. This lab demonstrates the principle of Hess’s Law; if several reactions add up to produce an overall reaction, then the heat transfers of the reactions will add up to the value of the heat transfer for the overall reaction. In this experiment, the enthalpy changes for the reaction of ammonia and hydrochloric acid will be determined using Hess’s law. So, the purpose of this experiment was to verify Hess’s Law. Three acid-base reactions, chosen so that the third reaction equals the first reaction minus the second, are measured for temperature change by calorimetry. The values of heat change and enthalpy of reaction are calculated for each reaction. The measured value for the third reaction is then compared to the value calculated by subtracting the enthalpy of reaction for reaction two from the reaction of reaction one. II. Procedures First we have to determine the heat capacity of the calorimeter (part 1). First I set up the calorimeter by using Styrofoam cups with a cover having a hole, to allow for a thermometer. Then I measured 50.0 mL of distilled or deionized water in a 50-mL graduated cylinder and I transferred the water into the calorimeter. Then I placed the calorimeter assembly on a magnetic stirrer, and I also added a magnetic stirring bar, and set the bar spinning slowly. Once I measured the temperature of the water I heated 75 mL of distilled or deionized water to about 70 o C in a 250-mL beaker. Then I measured 50.0 mL of the 70 o C distilled water in a 50-mL graduated cylinder. After I measured the temperature of the hot water, I immediately poured the hot water into the room temperature water in the calorimeter. I, then, covered the calorimeter and inserted the thermometer and also stirred the water. After I recorded the temperature every 20 seconds (for 3 minutes), I emptied the calorimeter and dried the inside of the calorimeter for the second part of the lab. Now there are three reactions in which I have to determine the heats of reaction (part 2). For the first reaction ( HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) ) I did the following steps. First I measured 50.0 mL of a 2.0 M HCl solution in a 50-mL graduated cylinder and transfer to the calorimeter. After I recorded the temperature of the HCl solution I rinsed the 50-mL graduated cylinder with distilled water. Then I measured 50.0 mL of a 2.0 M NaOH solution in a 50-mL graduated cylinder. After I recorded the temperature of the NaOH solution, I put a magnetic stirring bar into the calorimeter and started to slowly spin the HCl solution. I then quickly added the 50.0 mL of 2.0 M NaOH solution to the calorimeter, then I covered it and also inserted the thermometer. I recorded the temperature every 20 seconds for a total of 3 minutes. Now I thoroughly rinsed and dried the calorimeter, thermometer, stirrer bar, and graduated cylinder used. I redid the steps of
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