To determine the heat of formation of calcium carbonate

After that, by constructing a suitable enthalpy cycle, i.e.

MgSO4(s) + 7H2O(l) MgSO4 . 7H2O(s)

MgSO4(aq)

and applying Hess's law, the molar enthalpy change of hydration of magnesium sulphate(VI) ( H) can be calculated. ( H = H1 - H2)

5) Data:

Anhydrous MgSO4(s)

MgSO4(s)-7-water

Molar mass / g mol-1

20.4

246.4

Mass of copper(II) sulphate / g

3.01

6.16

Initial temp. of water / oC (T1)

25

24

Highest/Lowest temp. attained / oC (T2)

34

22.5

Change in temperature / oC (T2-T1)

+9

-1.5

Volume of deionized water used: 100 cm3

6) Calculation:

The molar enthalpy change of solution of anhydrous magnesium sulphate(VI)

= mc T x (no. of mol)-1

= (100 x 4.2 + 3.21 x 1.3) x (-9) x (0.025)-1

= -152.70 kJ mol-1

The molar enthalpy change of solution of magnesium sulphate(VI)-7-water

= mc T x (no. of mol)-1

= (100 x 4.2 + 3.21 x 1.3) x (1.5) x (0.025)-1

= 25.45 kJ mol-1

By Hess's law, H = H1 - H2

= -178.15 kJ mol-1

The molar enthalpy change of hydration of magnesium sulphate (VI)

= -178.15 kJ mol-1

Discussion:

(1)~(3) is answered above

4) What assumption have you made in your calculation?

Assumed:

* The thermal capacity of the beaker is negligible.

* The specific heat capacity of the 2 resulting solutions are the same as water, and they also weight 1 g/cm3

* The weight of the solids which dissolved in deionized water does not affect the total mass of the solution.

* The solids are in pure state.

* The solid are dissolved quickly so that the heat losses are negligible.

* The resulting solutions were infinitely diluted, that is further addition of water would not cause any further heat change.

* There was no heat lost to surrounding in 1st part and no heat gained in 2nd part.

5) What are the sources of error in this experiment?

* The specific heats of the solutions cannot be exactly 4.2 J g-1 oC-1 as that of water. The error could be minimized by using electrical compensation method.

* The solid night not be in pure state, that was, the anhydrous magnesium sulphate(VI) may be completely anhydrous, which could be improved by heating the sample for several hours in an oven and then use immediately.

* Some solid might not dissolve quickly enough and thus heat was lost. The improvement was using the reactants in powder form or tiny crystal form abut not bigger one, such that they could be dissolved quickly enough.

* Some heat might e lost to 1st part or gained in 2nd part. The improvement was insulate the beaker or use a vacuum flask calorimeter.

6) Why cannot the molar enthalpy change of hydration of magnesium sulphate(VI)

be measured directly in the laboratory?

Because it was not possible to form magnesium sulphate(VI)-7-water from

anhydrous magnesium sulphate(VI) and water because the anhydrous saly may

dissolved in water instead.

Conclusion:

By using the experimental result, the molar enthalpy change of hydration of magnesium sulphate(VI) is -178.15 kJ mol-1

-End of report-