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Application of Hess(TM)s law to Determine the Enthalpy Change of Hydration of Magnesium Sulphate(VI)

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Experiment 6 Date: 18-10-2005 Application of Hess's law to Determine the Enthalpy Change of Hydration of Magnesium Sulphate(VI) Objective To determine enthalpy change of hydration of magnesium sulphate (VI) . Introduction The enthalpy of a substance refers to its total energy content. During reactions, the enthalpy of the substances involved changes. This is known as the enthalpy change of a substance. Enthalpy change of substances from their initial states to their final states is independent of the pathways. This means that no matter how many reactions the reactants undergo to form the same products, the change in enthalpy is always the same. This principle governing the change of enthalpy of substances is known as the Hess's Law. For example, a certain amount of magnesium sulphate (VI) powder is dissolved in distilled water, and the enthalpy change, H1 is recorded. Magnesium sulphate (VI)-7-water is also dissolved in water and its enthalpy change, H2 is also recorded. By Hess's Law, the enthalpy change for magnesium sulphate to turn to magnesium sulphate -7-water is equal to H1 - H2, as shown in the above born-haber cycle diagram. The enthalpy change for the crystallization of magnesium sulphate is not measured directly because the reaction does not favour the direct measurement of the enthalpy change. ...read more.


Discussion 1. When the temperature of the solution has a different temperature than its surroundings, energy will be transferred from the higher area to the lower area. So, the temperature of the solution, after the reaction, will eventually restore the temperature of its surroundings. The longer the time used in the reaction, the more the heat lost to the surroundings. Therefore, the longer the time used in the reaction, the more inaccurate the results would be. For example, it took a long time for the anhydrous magnesium sulphate to dissolve fully in water. So, a large amount of heat is lost to the surroundings. Although only the greatest temperature change is needed, the temperature loss still has a bad effect on the accuracy of the experiment. In order to reduce the errors in this experiment, we need to shorten the time. How can we shorten the time? We can speed up the dissolution process of the anhydrous magnesium sulphate by adding a larger volume of water. With a larger volume of solute, the magnesium sulphate crystals will be more easily dissolved. Also, we can speed up the reaction by using heated distilled water. ...read more.


This is because while I was doing my experiment, the sun was rising and the temperature was rising higher and higher. Second, I assumed no heat loss to the surroundings. If there were heat loss, which was actually the case, I would have to consider the heat loss to the surroundings. There is a method to take into account the heat loss to the surroundings, but it is not a good method when the reaction takes a long time to complete. Also, as the final temperature does not have great difference from the surroundings, the heat loss can be neglected. 6. Why cannot the molar enthalpy change of hydration of magnesium sulphate (VI) be measured directly in the laboratory? The reason why the molar enthalpy change of hydration is not measured directly can be traced back to how crystallization is carried out. The saturated solution of magnesium sulphate is to be left in a certain place for a long time until the water evaporates and crystals are formed. 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 constant temperature throughout the whole experiment. So, the enthalpy change cannot be measured directly. ...read more.

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