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Application of Hess's Law to determine the enthalpy change of hydration of Magnesium sulphate

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Introduction

? Name: Penguin Chow Cheuk Yan ? ? Class: 6S Class Number: 14 ? ? Date: 04 - 11 - 2002 ? Chemistry Full Report Experiment 5 Title: Application of Hess's Law to determine the enthalpy change of hydration of Magnesium sulphate (VI) Aim: Using Hess's Law to find the enthalpy change of hydration of magnesium sulphate (VI) Theory: Hess's Law can be defined as the heat given off or absorbed by a reaction is independent of the route taken. In this experiment, the enthalpy change of hydration of Magnesium sulphate (VI) cannot be directly measured by calorimetry in the laboratory as hydration is a very slow process. ?H1 = enthalpy change of hydration of MgSO4(S) ?H2 = molar enthalpy change of solution of hydrous MgSO4(S) ?H3 = molar enthalpy change of solution of MgSO4� 7H2O(S) According to Hess's law, the enthalpy of the overall reaction should be equal to the sum of the enthalpies of the two sub-reactions. ...read more.

Middle

Using a thermometer, the temperature of the water in the cup was measured 3. 0.025 mole of MgSO4� 7H2O(S) was weighed accurately by the balance instead of MgSO4(S)and was added into the foam cup 4. The solute was stirred to make sure all anhydrous magnesium sulphate (VI) were completely dissolved into the water as quickly as possible 5. The highest temperature of the solution was taken down 6. The molar enthalpy change of solution of MgSO4(S) was then calculated ( Assume: The specific heat capacity of the solution in the foam cup = 4.2kJkg-1K-1 The specific heat capacity of the foam cup = 1.3kJkg-1K-1 ) Result: Weight of the polystyrene cup = _________ g (A) Anhydrous (B) Hydrated Weight of magnesium sulphate added /g Initial temperature of water / C Highest temperature of the solution / C Lowest temperature of the solution / C Change in temperature / C Calculation: 1. ...read more.

Conclusion

/ ( 0.025 mol ) = kJ/mol Molar enthalpy change of solution of MgSO4� 7H2O(S) = - kJ/mol Using Hess's Law , ?H1 = ?H2 - ?H3 = [ ( ) - ( ) ] kJ/mol = kJ/mol The true enthalpy change of the hydration of magnesium sulphate = -104.0 kJ Thus , the percentage error = [ ( ) - ( ) ] / ( ) = % Discussion: 3. Point out some factors leading to the difference between the experimental value and the true value ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ 4. Explain why the enthalpy change of the hydration of magnesium sulphate cannot be measured directly in the laboratory The reason is that this reaction, hydration, is a very slow process which is impossible to obtain directly in the laboratory ____________________________________________________ 5. Explain why it is not necessary to plot a temperature-time graph to determine the ?T ____________________________________________________ ____________________________________________________ ____________________________________________________ 6. Draw an enthalpy level diagram for the reactions involved in the enthalpy cycle used ...read more.

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