Experiment 8 Lab Report PDF

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Professor: April French
Experiment 8 Lab Report...

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Jessica Flesch CHE 111 SEC 014 Isoiza Ojo Emily Hamilton 11/26/2016 Lab 8: Determining Enthalpy of Calorimetry Reactions Introduction: The purpose of this lab is to determine the enthalpy of a chemical reaction three different ways for a company. The heat energy change of a system cannot always be measured directly, but the heat energy change of the surroundings can be found by monitoring temperature changes. The heat change that occurs when two or more substances are combined can be measured by a device called a calorimeter. The insulation of the device allows it to lose the least amount of heat to the surroundings as possible. The calorimeter does absorb a small amount of heat, so it must be calibrated in order to determine how much heat is absorbed in order to take that number into account when doing calculations of heat change in a reaction. The first step in this experiment is to find the calorimeter's heat capacity by mixing samples of hot and cold water together. This will reveal how much heat is lost to the calorimeter, which can be used to determine heat loss during the acid base reactions. When it is assumed that no heat is lost to the surroundings, the equation below is true: qcold water + qhot water + qcal = 0 The equation can also be rearranged, as shown below, to determine the heat of the calorimeter (French, et al. 104). qcal = -qhw -qcw Two formulas will be used during this experiment to calculate the individual terms of the equation. The first formula is q=msΔT This formula is used when the specific heat of the substance is know, but the masses vary. The second formula, shown below, is used for an object with a constant mass and an unchanging composition (French, et al. 104). C is heat capacity that is unique to each particular substance. q=cΔT When the above formulas are substituted into the third equation, the following equation can be produced:

CcalΔTcal= -(mcw)(swater)(ΔTcw)-(mhw)(swater)(ΔThw) To expand this equation, ΔTcal can be replaced by (Tf-Ti,cw), ΔTcw can be replaced by (Tf-Ti,cw), and ΔThw can be replaced by (Tf-Ti,hw) (French, et al, 104). Tf values will all be equal because the cold water, hot water, and calorimeter all eventually become the same temperature. All of the values in the final equation other than Ccal can be found from experimental data or reference values, so the result will be Ccal. This value will be used to determine the heat of reaction for the three acid base reactions involving the calorimeter. In an aqueous solution reaction, any heat gained or lost by the reaction will be absorbed by the calorimeter or the solution. Measuring the temperature change allows for the determination of how much heat is absorbed individually by the solution and the calorimeter. The sum is the heat change from the chemical reaction (French, et al. 104). The following equation can be used to find the heat of reaction: qrxn=-qsoln-qcal This can be expanded into the following form: Qrxn=-msolnsΔT-CcalΔT Above, msoln is the total mass of the combined solution, s is specific heat of water, and ΔT is the temperature change of the solution and calorimeter (French, et al. 105). The initial temperatures of the solutions and the calorimeter are equal. The maximum temperature of the solution is the final temperature, which will also be the calorimeter's final temperature. The heat of reaction will be found using the heat capacity determined in the calibration process. Heat capacity is only relevant when using identical amounts of reactants in the same calorimeter, so the more universal value of enthalpy is considered. It is measured in kJ/moles of the limiting reagent and can be determined by the following equation: ΔH=q/n q is the heat change and n is the number of moles of limiting reagent. Enthalpy can be compared to the same reaction with different amounts of a substance or to proven enthalpy values (French, et al. 105). For each of the three reactions observed in this experiment, the heat of the reaction will be found by using the change in temperature of the solution, as well as the heat capacity. Hess's law will be used to determine two different enthalpy values of the same reaction, which will be compared to the accepted value.

Methods:

No changes to the published procedure. The procedure was found in CHE 111 General Chemistry Lab Manual written by University of Kentucky Department of Chemistry on pages 103-108.

Discussion: The purpose of this lab is to determine the enthalpy of a chemical reaction three different ways for a company. A calorimeter will be used to measure the heat change, and this data can be used to calculate the enthalpy. The enthalpy of the first reaction is –69.6 kJ/mole. The enthalpy of the second reaction is –8.94 kJ/mole. The enthalpy of the third reaction is –71.39 kJ/mole. Each of these values was calculated using the value of the respective qsol and qcal. The enthalpy change for reaction 3 calculated using Hess's Law is –60.6 kJ/mole. The percent error in reaction 3's enthalpy (calculated using q=msΔT) is 34.67%. The percent error in reaction 3's enthalpy (calculated from Hess's Law) is 14.31%. These results support the original purpose of the experiment. This experiment was intended to determine the enthalpy of a reaction three different ways for a company, and this was accomplished. There were some sources of error present in this experiment. One source of error occurred when the calorimeter lost heat to the surroundings when it was not supposed to. This could have happened when the cardboard the lifted off the top of the cup. This could have affected the results by lowering the initial temperature during the three different chemical reactions. This error could have been avoided by performing steps 15-18 quicker in order to minimize the amount of heat lost to the surroundings. A second source of error may have occurred when the cup in the calorimeter was swirled. Some of the solution in the cup may have been lost during this process. This would have resulted in a lower volume of whichever substance was being measured at the time. This error could have been avoided by being more careful when swirling the Styrofoam cup. A third error could have occurred when rinsing the Styrofoam cup between trials. The same cup was used for each reaction, so there may have been water left over in the cup from when it was rinsed. This could have affected the results of the experiment by making the initial temperature lower if cold water was used to rinse the cup. This error could have been avoided by drying the cup more thoroughly in between trials.

Conclusion:

From this experiment, I learned how to determine the enthalpy of a chemical reaction using a calorimeter. I also practiced the skills of writing a net ionic equation, using Hess's Law to determine enthalpy. This experiment taught me that a calorimeter is a device used to measure the heat changes when two or more substances are combined. The skill of calorimetry has real life applications, such as use in chemical plants and laboratories to make sure heating and cooling systems can withstand heat changes. A bomb calorimeter can be used to calculate the amount of calories in the food we consume. This device can also measure calorific content for coal, fuel oil, combustible waste, explosives, gasoline, nutritional supplements, and more.

Works Cited: French, April N, Allison Soult, Stephen Testa, Pauline Stratman, Meral Savas, Francois Botha, Carolyn Brock, Charles Griffith, Daria Hood, Robert Kiser, Penny O’Conner, William Plucknett, Donald Sands, Diane Vance, and William Wagnor “Experiment 8: Enthalpy of Reaction” General Chemistry Laboratory Manual. Plymouth Hayden-McNeil Publishing, 2015. 103-108. Web. 23 November 2016...