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Thermochemistry And Hess Law Preliminary Lab Assignment 4-5

Unformatted text preview: Bruno 1 Lexi Bruno 10/25/14 AP Chemistry Thermochemistry and Hess's Law Objective: The purpose of this lab is to calculate the ∆H of the reaction of ammonia (NH3) and hydrochloric acid (HCl) to form ammonium chloride (NH4CL) using the principals of Hess’s law applied to calorimetry. Lab Team: Nicole Song Equipment: 2 Molar HCL 2 Molar NaOH 2 Molar NH3 2 Molar NH4CL NaHCO3(s) Baking soda HC2H3O2(aq) Vinegar Styrofoam cups Thermometer (2) 50mL graduated cylinders Distilled water Stirring rod Beaker Ring stand Hot plate Procedure: 1. Find the heat capacity of the calorimeter Take two Styrofoam cups and put them together with a cover on top. Put a hole in the top of the cover so a thermometer can be placed in it. Measure 50.0 mL of distilled water at room temperature into the calorimeter. Stir the liquid. record the temperature. Heat about 75 mL of distilled water to about 70 degrees Celsius. Measure 50.0 mL of the heated water into a different cup and record the temperature. Immediately pour the hot water into the room temperature water, cover it and record the temperature after every 20 seconds for the next three minutes. 2. Find the heat of the reactions Determine the temperature change that occurs when 50.0 mL of 2.0 M HCL reacts with 50.0 mL Bruno 2 of 2.0 M NaOH. Measure the temperature of each solutions(rinsing and drying the thermometer before transferring it to the next solution. Measure out 50.0 mL of 2.0 M HCL and put it in the calorimeter. Stir the solution. Measure out 50.0mL of 2.0 NaOH and add it to the acid. Quickly cover and record the temperature after 20 seconds and every 20 seconds for the next three minutes. Using the same process, combine solutions of 2.0 M NH4CL and 2.0 M NaOH and for the third reaction, combine solutions of 2.0 M NH3 and 2.0 M HCL Observations: 70 (degrees C) water and room temperature water (22.5 degrees C) time(seconds) temperature(Celsius) NH4Cl (22.0 degrees C) and NaOH (22.2 degrees C) time(seconds) Calculations: HCl (22.2 degrees C) and NaOH (23.0 degreestemperature(Celsius) C) time(seconds) NH3 (22.0 degrees C) and HCl (22.0 degrees C) temperature(Celsius) time(seconds) temperature(Celsius) 70 degree Water and room temperature water Bruno 3 temperat ure(Celsius) 50 45 40 35 30 25 20 15 10 5 0 0 20 40 60 80 100 120 140 160 180 200 time(seconds) 40 HCl and NaOH 35 30 25 temperat ure(Celsius) 20 15 10 5 0 0 20 40 60 80 100 120 time(seconds) 140 160 180 200 Bruno 4 NH4Cl and NaOH 12 10 8 temperat ure(Celsius) 6 4 2 0 0 20 40 60 80 100 120 time(seconds) 140 160 180 200 Bruno 5 NH3 and HCl 40 35 30 25 temperat ure(Celsius) 20 15 10 5 0 0 20 40 60 80 100 120 time(seconds) 140 160 180 200 Bruno 6 Discussion Questions 1. Calorimetry is the science of measuring the heat of chemical reactions or physical changes 2. The graphical temperature analysis' capability to graph linear regression allows the data to be more precise. The line of best fit synthesized by the program shows the most accurate average of the measurements listed on the graph. Bruno 7 3. The negative sign in front of the bracket means that the reaction is exothermic. This shows that water is the surrounding environment for the reaction. The water/ surrounding receives heat because the system around it loses heat. 4. The values I have found do in fact support Hess's law more or less. They were not 100% accurate being that would be nearly impossible without the correct technology. The percent error calculated was 8.25% meaning the values found are close to precisely supporting Hess's law. 5. To achieve greater accuracy, a better calorimeter could be used rather than one constructed out of Styrofoam cups. The Styrofoam calorimeter was not perfectly closed. The hole where the thermometer was placed did not completely seal in the heat. 6. Pre-Lab Questions 1. Delta H signifies the difference in enthalpy. Enthalpy refers to the sum of the internal energy of a system plus the product of the system's pressure and volume 2. Specific heat is the heat required to raise the temperature of the unit mass of a given substance by a given amount 3. Bruno 8 4. 5. Hess's Law states that if two chemical equations can be algebraically combined to give a third equation, the values of H for the two equations can be combined in the same manner to give …H for a third equation. If a reaction can be carried out in a series of steps, the sum of the enthalpies for each step should equal the enthalpy change for the total reaction. Conclusion Overall the outcome of this lab was rather successful. The ∆Hrxn of ammonia (NH3) and hydrochloric acid (HCl) to form ammonium chloride (NH4CL) was -47.1 kJ/mol. The ∆H of the first reaction between NaOH and HCL was -45.6kJ/mol. The second reaction between NH4Cl and NaOH was -2.1kJ/mol. Since the reaction of NH3and HCl to form NH4Cl was carried out in a chain of steps and no external work was done, the principals of Hess’s Law, which states that ∆H or the heat energy concerned with a chemical reaction will be equal to the sum of the enthalpy changes for the individual steps and is the same whether the reaction occurs in one or several steps, can be employed to find the enthalpy change. Some errors that could have been probable in this lab would include incomplete mixing. If the solutions were not mixed completely, the temperature would fluctuate. Also, if the thermometer was not completely at Bruno 9 equilibrium, the temperature we read could possibly be off by a mere tenth of a point. If a temperature probe interfaced with a computer were to be used in this experiment, more data points would be able to be collected in a shorter period of time. Another spot for error was in the calorimeter itself, the top of it was not sealed perfectly to prevent the heat from escaping. The Styrofoam cups were also probably not best to use for a calorimeter. All in all this experiment was pretty much a success regardless of some small room for error ...
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Unformatted text preview: Conclusion: The purpose of this experiment was met because the group was able to verify Hesss Law. From the calculations, H 3 was found to be -26 kJ/mol, and when Hesss Law was used, H 3 was found to be -18.6 kJ/mol. The percent error calculated from these values ended up being -39.78%. Since the calculated answers were close, the purpose was met. Questions 1. The direct method worked better to find H 3 than the indirect method because in order to use Hesss Law, the data had to be nearly error free. If the group does have an error in the calculations, than the Hesss Law answer will not match the answer the group calculated. The Hesss Law answer had a -61% error compared to the calculated answer. Therefore, the direct method worked better than the indirect method. 2. Hesss Law states that if a reaction is carried out in a number of steps, H for the overall reaction is equal to the sum of the Hs from each individual step. 3. H means the heat or enthalpy change for a chemical reaction. This energy change is equal to the amount of heat transferred, at constant pressure, in the reaction. This change represents the difference in enthalpy of the products and the reactants and is independent of the steps in going from reactants to products. 4. The true initial temperature is found by using a modified linear fit because when excluding any points that are skewed, it is giving the results a more accurate reading. The first couple points that were skewed are due to incomplete mixing and lack of equilibrium with the thermometer. 5. All of the solutions did not have the same initial temperature because they were not contained in the same area of the room. These areas could have a slightly different temperature causing the different initial temperatures. Also, transfer of heat from holding the graduated cylinder could have increased the initial temperature, and depending on how long the graduated cylinder was held, could have determined how high the initial temperature rose....
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