- Enthalpy change of solution of hydrated sulphate
- The experiment in part I was repeated using 0.025 mole of magnesium sulphate-7-water instead of the anhydrous salt.
- The molar enthalpy change of solution of magnesium sulphate-7-water was calculated.
Observation:
Discussion:
The enthalpy of hydration of magnesium sulphate was investigated through the three equations described below:
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Dissolution of anhydrous magnesium sulphate into water:
H2O
MgSO4(s) → Mg2+(aq) + SO42-(aq)
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Dissolution of magnesium sulphate-7-water in water:
H2O
MgSO4．7H2O(s) → Mg2+(aq) + SO42-(aq)
-
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
ΔH
MgSO4(s) + 7H2O(l) → MgSO4．7H2O(s)
ΔH1 -ΔH2
Mg2+(aq) + SO42-(aq)
Where
ΔH = enthalpy change of hydration of magnesium sulphate,
ΔH1 = enthalpy change of solution of anhydrous magnesium sulphate,
ΔH2 = 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:
∵nΔH = m.s. Δθ
∴0.025ΔH1 = energy change of solution in cup + energy change of cup
= 0.05 x 4.2 x (-1) + 0.00172 x 1.3 x (-1)
ΔH1 = -8.49 kJ mol-1
0.025ΔH2 = energy change of solution in cup + energy change of cup
= 0.05 x 4.2 x 2 + 0.00172 x 1.3 x 2
ΔH2 = 16.98 kJ mol-1
∵ΔH = ΔH1 - ΔH2
∴ΔH = -8.49 – 16.98
= -25.47 kJ mol-1
In the calculations, it was assumed that there was no heat loss to the surroundings during the experiment. Also the specific heat capacity of the solution was assumed to be the same as that of water. 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.
