To determine enthalpy change of hydration of magnesium sulphate(VI)

After that, by using a suitable energy-cycle diagram, and applying Hess’s law, the heat formation of formation of calcium carbonate can be calculated.

5. Procedure: Please refer to the lab. manual
6. Data:

1st Part:  Reaction of calcium with dil. Hydrochloric acid

Volume of hydrochloric acid used: 100cm3  

2nd Part:  Reaction of calcium carbonate with dil. Hydrochloric acid

Molar mass of calcium carbonate = 100 g/mol

Volume of hydrochloric acid used = 100 cm3

7. Calculation:

Note: No temperature drop can be observed after the max temperature is attained for 4 readings.

Note: No temperature drop can be observed after the max temperature is attained for 4 readings.

Extrapolation procedure cannot be done.

   Therefore, the temperature change of 1st part exp. = 28-25 = +3oC

            the temperature change of 2nd part exp.= 26-25 = +1oC

Heat evolved in 1st part :

=(mc  T) x (no. of mol)-1  =(100 x 4.2 x 3) x (0.53 / 40.1)-1  =-95332 J mol-1

Heat evolved in 2nd part:

=(mc  T) x (no. of mol)-1  =(100 x 4.2 x 1) x (1/100.1)-1  = -42042 J mol-1

Using enthalpy cycle:

Ca(s) + C(graphite) +3/2O2(g)                CaCO3(s)

Ca2+(aq) + H2(g) + C(graphite) + 3/2O2(g)

Ca2+(aq) + H2O + C(graphite) + O2(g)

                               Ca2+(aq) + CO2(g) + H2O(l) 

Note:

   H1 = Enthalpy change in 1st part’s reaction

   H2 = Enthalpy change in 2nd part’s reaction

Hf [H2O(l)] = -242000 J mol-1

Hf [CO2(g)]= -393500 J mol-1          source: physical chemisty I of Fillans

By Hess’s law:

Hf[CaCO3(s)] =  H1  +   Hf [H2O(l)]  +   Hf [CO2(g)]  -   H2

          =  -688.79 kJ mol-1

8. Discussion

1. Why is the exact concentration of hydrochloric acid unimportant?

The exact concentration of the hydrochloric acid was unimportant as it was present in excess, that was, to provide enough H+(aq) to react with calcium. At the same time, the exact concentration of hydrochloric acid would affect the calculation afterwards.

        

2. State the law which you use for calculating the heat of formation of calcium carbonate. On which thermodynamic principle does it depends?

The law used was the Hess’s law.

It’ depends on the standard enthalpy change of a reaction is independent of the route by which the chemical reaction takes place, which only depends on the difference between the standard enthalpies of reactants and products, i.e. conservation of energy.

3. Why is this law useful?

This principle is useful as the standard enthalpy change of some reaction might not be able to find directly, and thus Hess’s law can be used and the standard the enthalpy change of the reaction can be calculated indirectly by this method.

4. Discuss the possible sources of errors. State how to minimize the possible errors.

1. The solid labeled “calcium” is suspected to be calcium oxide, which show white color but not grey shinny color.
2. Heat produced the reaction might heat up the air inside the beaker, which cannot be avoided.
3. Error in reading thermometer
4. Error in weighing