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Enthalpy Change of a Displacement Reaction

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Introduction

Chemistry Investigation--Enthalpy Change of a Displacement Reaction Data Collection 1. Table(1):Uncertainties Thermometer �0.5? Cylinder �1mL 2. Table(2)Quantitative observations of cold water and warm water Volume Temperature Cold water 50mL�1mL 20.2?�0.5? Warm water 50mL�1mL 61.8?�0.5? Table(3)Temperature changes when warm water was added into cold water Time intervals Temperature 30s 30.0?�0.5? 30s 38.2?�0.5? 30s 38.0?�0.5? 30s 37.8?�0.5? 30s 37.5?�0.5? 30s 37.2?�0.5? 30s 37.0?�0.5? Table(4)Temperature changes when Zn(5g�0.1g) was added into 100mL�1mL CuSO4(0.500M) Initial temperature 19.8?�0.5? 8min 35.0?�0.5? 0.5min 25.0?�0.5? 8.5min 35.0?�0.5? 1min 25.8?�0.5? 9min 35.2?�0.5? 1.5min 26.8?�0.5? 9.5min 35.2?�0.5? 2min 27.2?�0.5? 10min 35.2?�0.5? 2.5min 27.2?�0.5? 10.5min 36.0?�0.5? 3min 27.8?�0.5? 11min 35.8?�0.5? 3.5min 28.1?�0.5? 11.5min 35.8?�0.5? 4min 27.8?�0.5? 12min 35.8?�0.5? 4.5min 28.2?�0.5? 12.5min 35.8?�0.5? 5min 32.0?�0.5? 13min 35.8?�0.5? 5.5min 35.0?�0.5? 13.5min 35.6?�0.5? 6min 35.0?�0.5? 14min 35.2?�0.5? 6.5min 35.0?�0.5? 14.5min 35.2?�0.5? 7min 35.2?�0.5? 15min 35.2?�0.5? 7.5min 34.8?�0.5? 3. Table(5)Qualitative Observations CuSO4 solution Blue aqueous Zn Black powder Phenomena 1. After the lid of the calorimeter was covered and the mixture in it was stirred, the outer wall became warm. 2. When the reaction in the calorimeter was complete, the mixture in it was colorless and there was excessive zinc powder left. Data processing and presentation This experiment includes two parts, the determination of a calorimeter constant and then the enthalpy change of the reaction between Zn and CuSO4. Part A 1. The heat absorbed by the cold water. ...read more.

Middle

The total heat released by the reaction In this reaction, only using the data obtained in this experiment to calculate the heat absorbed by the solution is not accurate. Because the calorimeter was not completely insulated, it absorbed some heat from the reaction. Therefore the total heat released by the reaction should also include the heat absorbed by the calorimeter. The total heat energy=Qwater+Qcalorimeter =7.81kJ +1.1kJ =8.9kJ Absolute uncertainty=�0.5kJ + �0.71kJ=�1.2kJ % uncertainty=1.2/8.9 � 100%=13% 4. The enthalpy change of the reaction The heat will vary with the amount of the limiting reagent, so by convention it is given for molar amounts in the equation as it is written. Therefore, ?H=the heat absorbed or released/the number of moles=Q/n In the reaction between CuSO4 and Zn, the limiting reagent needs to be determined. a) The limiting reagent The chemical formula of the reaction is: Zn(s) + CuSO4(aq)==> Cu(s) + ZnSO4(aq) The amount of CuSO4=0.500M�0.100L=0.0500mol The amount of Zn=m/M=5.0g/65.39gmol-1=0.076mol According to the balanced equation, the molar ratio of Zn and CuSO4 should be 1 : 1, but here the amount of Zn is more than that of CuSO4, thus Zn was excessive in the reaction and CuSO4 is the limiting reagent. b) The amount of the limiting reagent Since the limiting reagent is CuSO4, the number of moles of it needs to be taken. ...read more.

Conclusion

This would accumulate the errors since I have calculated ms?T twice. For this reason, the error I got is bigger. 3. Improvements a) During the reaction, there was still a lot of heat lost to the surroundings. To prevent this, next time, I can use plastic or glass stick as the stirring rod since they can conduct less heat than the copper wire. Moreover, next time, cotton needs to be used to fill the openings on the calorimeter. Towel is also necessary because it is needed to wrap the calorimeter to prevent the heat lost. b) The time required for the reaction needs to be extended. By convention, the temperature would gradually fall as there is more heat lost to the surroundings. But according to the graph, the line is still very smooth, this means that 17 minutes is not enough. If the time is extended, the graph can be plotted more accurately and the extrapolating line can point to a more accurate temperature value. c) Next time, I should use more combined formulae. Since the more steps you use, the bigger the errors you have. The number of the steps should be reduced. d) Using a more accurate thermometer since the accuracy of the thermometer is the main reason that causes the % uncertainty to be so high. ...read more.

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