Test Hess' law by converting NaOH to NaCl by using two different routes and measuring the enthalpy change for each one.

2. 1 gram of solid NaOH was added to it. The apparatus was stirred

3. The final temperature was recorded. Since the reaction is slow the final temperature was found by plotting a cooling curve.

4. The heat given out by the reaction was noted using the formula

Heat given out by the reaction= heat absorbed by water

5. The enthalpy change is noted using the formula

H2=heat absorbed by water/ no. of moles of the limiting reactant

 

Reaction 3- NaOH(aq) +HCl(aq)                      NaCl +H2O(l)

1. The 25 cm3 aqueous NaOH is taken in an insulated calorimeter . The initial temperature is noted.
2. Aqueous 25 cm3 HCl in added to it. The apparatus is stirred and the final temperature is noted.
3. The heat evolved ion the experiment in noted by the formula

Heat given out by the reaction=heat absorbed by water

4. The enthalpy change is calculated using the formula

H3=heat absorbed by water/no .of moles of the limiting reactant

After the following enthalpies have been calculated the Hess’ law is proven if

                                      H1       =    H2      +     H3

                                Data collection and data processing

Enthalpy change=(heat absorbed by water)/no.of moles of the limiting reactant

For reaction 1-

Heat evolved by the reaction    =   heat absorbed by water

Heat absorbed by water- mass of water  x    sp. heat capacity of water    x    change in                      temperature    

the maximum temperature reached as seen in the graph= 36oC

therefore

heat absorbed by water=mass      x      sp. heat capacity       x      change in temperature

                                     =  50          x       4.18                         x        (36-28)

                                     =   1672 joules

                                     = 1.672 KJ

no .of moles of NaOH= 0.025

therefore the enthalpy change per mole = 1.672/0.025

                                                               =66.88 KJ/mol

reaction 2-

Heat evolved by the reaction    =   heat absorbed by water

Heat absorbed by water= mass of water  x    sp. heat capacity of water    x    change in                      temperature    

final temperature as is seen from the graph=34oC

heat absorbed by water= 25     x       4.18       x       (34-27.5)

                                    =679.25  Joules

                                    = 0.67925 KiloJoules

enthalpy change=-27.17 KJ/mol

reaction 3-

Final temperature= 35.5oC

The heat absorbed by water     =      25        x       4.18          x        (35.5-30)

                                                =      574.75 joules

                                                =      0.5745 KJ

The enthalpy change              =    - 22.99KJ/mol

H1= -66.88 KJ/mol

H2+H3=-50.16 Kj/mol

Total error possible= 1.56

All the three reactions were exothermic, i.e., heat was given out in all the three reactions therefore the values will be negative.

Conclusion and evaluation- the following factors would have caused a shift from the actual values-

1. All the heat would not be transferred to the water. Some heat would be lost to the surroundings, which would cause a decrease in the value.

2. Also it is assumed in reaction 1 and 3 that HCL(aq)  would have the same sp. Heat capacity as that of water which is not true.

3. The stirring was not adequate.

A cooling curve was plotted when the reaction was slow. This would give us the value of the final temperature which would be obtained had the reaction been fast.

The difference between both the enthalpies is significantly large. This could be due to the above stated reasons.

It was expected that enthalpy value reaction 1 would be equal to that of reaction 2 + reaction3 (since Hess’ law states that the total enthalpy change on converting a given set of reactants to a particular set of products is always constant irrespective of the pathway it takes).