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# Enthalpy Change of the Hydration of Magnesium Sulphate

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Introduction

Enthalpy Change of the Hydration of Magnesium Sulphate Objective To determine the enthalpy change of the hydration of magnesium sulphate (MgSO4) using Hess's law. Procedures 1. The mass of a clean and dry polystyrene foam cup was weighed and recorded. 2. 50.0 cm3 of de-ionized water was measured by using a measuring cylinder 3. The 50.0 cm3 of water measured was poured it into the polystyrene foam cup. 4. The foam cup was then fitted in a 250 cm3 beaker. 5. A thermometer was put through the hole in the lid. 6. The temperature of the de-ionized water was measured by using the thermometer. 7. The result was recorded in the table provided. 8. The mass of 0.025 mole of anhydrous magnesium sulphate was calculated. 9. The mass of a weighing bottle was weighed. 10. According to the mass calculated in step 8, anhydrous magnesium sulphate was added to the weighing bottle carefully with a spatula. 11. The mass of the weighing bottle with anhydrous magnesium sulphate was weighed. 12. The masses weighed were recorded. 13. The weighed anhydrous magnesium sulphate was added to the water in the foam cup. 14. The mixture with the thermometer was stirred to dissolve the solid as quickly as possible. 15. The highest temperature of the solution was recorded The apparatus used with water was washed and dried. ...read more.

Middle

7H2O(s)]) =0.425902 /0.023178532 = 18.3748479 ~ +18.3 kJmol-1 4. Using the enthalpy changes of the reactions calculated above, construct an energy cycle and determine the enthalpy change of the hydration of magnesium sulphate. MgSO4(s) + 7H2O(l) MgSO4? 7H2O(s) ?H?soln [MgSO4(s)] + nH2O + nH2O ?H?soln [MgSO4? 7H2O(s)] Mg2+ (aq)+ SO42-(aq) The enthalpy change of the hydration of magnesium sulphate(?H?r ) = ?H?soln [MgSO4(s)]- ?H?soln [MgSO4? 7H2O(s)] =-36.73623506-18.3748479 =-55.11108296 ~-55.1kJmol-1 5. The true standard enthalpy change of the hydration of magnesium sulphate is -104.0 kJ mol-1. Point out some factors leading to any difference between the experimental value and the true value. There are several sources of error which causethe difference between the experimental value and the true value: 1. Heat loss to the surroundings It was assumed that the temperature of the surroundings remain constant throughout the whole experiment.However, there might be actually a change of conditions(e.g. temperature). For instance ,if the temperature is changed during the experiment, thre will be heat transfer from the higher temperature region to the lower one.Moreover, Hess's Law only holds when the final and initial reaction conditions are the same. Thus,it led to a very significant difference between the experimental value and the true value Moreover,the heat absorbed by other material used in the experiment(e.g. ...read more.

Conclusion

This method takes a long time. As mentioned above, the longer the time of reaction, the larger the heat loss. So, in this method of preparation involved a large amount of heat loss and the results would be extremely unreliable; the solution would maintain a more or less the same temperature throughout the whole experiment as the heat tranfer in a very long time.At the same time, the condition of experiment is also difficult to control.Eventually, it will be extremely difficult ,if not impossible,to measure the exact temperature change in the reaction. Furthermore,the nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level can also make the temperature change measured in the process inaccurate. Moreover,crystallization usually occurs at a lower temperatures. This can only mean that a crystal is more easily destroyed than it is formed. Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. Therefore,normally, the enthalpy change of the hydration of magnesium sulphate will not be measured directly. * Conclusion The enthalpy change of the hydration of magnesium sulphate (MgSO4) founded in the experiment is -55.1kJmol-1. * Reference http://www.wonderhowto.com/how-to-determine-enthalpy-hydration-mgso4-284169/ http://wiki.answers.com/Q/Why_the_enthalpy_change_of_hydration_of_anhydrous_magnesium_sulphate_cannot_be_measured_directly http://www.studyzones.com/questionzone/answer/49437x1149/Enthalpy-change-of-MgSO4 http://www.righthealth.com/topic/Magnesium_Sulphate?p=l&as=goog&ac=404 http://www.instructables.com/id/Determine-the-Enthalpy-change-in-hydration-of-Anhy/ http://www.chemicalforums.com/index.php?topic=5730.0 F.6 Chemistry Notes Section III by Ms Sin W L THE END ?? ?? ?? ?? F.6 Chemistry/TAS 5/P.1(6) 1 ...read more.

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