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To determine the enthalpy of hydration of magnesium sulphate.

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Introduction

Experiment 22 Date: 6th May 1999 Object: To determine the enthalpy of hydration of magnesium sulphate. Apparatus: Polystyrene foam cup, thermometer, cardboard, electronic balance. Materials: Both anhydrous and hydrated magnesium sulphate. Procedures: I. Enthalpy change of solution of magnesium sulphate 1. An empty polystyrene cup was weighted. 2. 50mL of distilled water was poured from a measuring cylinder into the polystyrene foam cup and the temperature was measured. 3. 0.025 mole of anhydrous magnesium sulphate was measured accurately and was added into the water in the polystyrene foam cup. The solution was stirred to dissolve the solute as quickly as possible. The highest temperature of the solution was recorded. 4. ...read more.

Middle

c. Hydration of anhydrous magnesium sulphate to form magnesium sulphate-7-water: MgSO4(s) + 7H2O(l) --> MgSO4.7H2O(s) The energy cycle below can be used to link the corresponding enthalpy ?H MgSO4(s) + 7H2O(l) --> MgSO4.7H2O(s) ?H1 -?H2 Mg2+(aq) + SO42-(aq) Where ?H = enthalpy change of hydration of magnesium sulphate, ?H1 = enthalpy change of solution of anhydrous magnesium sulphate, ?H2 = enthalpy change of solution of magnesium sulphate-7-water. By Hess Law, the molar enthalpy change of hydration of anhydrous magnesium sulphate to form magnesium sulphate-7-water is calculated as shown below: ?n?H = m.s.??? ?0.025?H1 = energy change of solution in cup + energy change of cup = 0.05 x 4.2 x (-1) ...read more.

Conclusion

Heat loss was assumed to be equal to heat gained. The main source of error of the experiment is the heat loss to the surroundings was severe. Though a cardboard was used to cover up the setup, there is heat loss through conduction to our body as the cup was held throughout the experiment. Also there was heat lost through radiation to the surroundings. As the solution was stirred with the thermometer in the initial stage of the experiment, extra amount of energy might be supplied to the system. All these factors may affect the results of the experiment. The molar enthalpy change of hydration of magnesium sulphate CANNOT be measured directly in the laboratory because the reaction involved is very difficult to proceed. The method used in this experiment can be used as an alternative for the measurement of the enthalpy change. ...read more.

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