Why Does Hess's Law Suggest a Different Enthalpy Change Than My Experiment?

[Link]

I have to measure the enthalpy change for the reaction:

CuSO4(s) + 5H2O --> CuSO4 · 5H2O

I have to use the hess law since the above reaction is slow.

CuSO4 · 5H2O + H2O --> CuSO4 + H2O (1)
solid CuSO4 + H2O --> CuSO4 (2)

The techer said the entalphy change for this reaction should be -79kJ/mol and the heat capacity of the solution should be taken as 4.2 kJ/mol.

However when I react 50 cm3 water with 0.025 moles CuSO4 · 5H2O in equation 1, I get a temperature change of -0.8 C.
In the second reaction, 50cm3 of water reacted with 0.025mol of CuSO4. Here the temperature change was +6.5 C.

But when I plug it into the equation E = m c ∆t; I get an endothermic value. What is wrong? Should the reaction 2 be exothermic?

 

[so-crates]

I have to measure the enthalpy change for the reaction:

CuSO4(s) + 5H2O --> CuSO4 · 5H2O

I have to use the hess law since the above reaction is slow.

Hess' Law has nothing to do with rates of reactions (kinetmatics)

CuSO4 · 5H2O + H2O --> CuSO4 + H2O (1)
solid CuSO4 + H2O --> CuSO4 (2)

Those reactions don't make any sense, you never have the same compound on both sides of the reaction for (1) and for (2) you don't have H20 anywhere on the right hand side. Are you sure you got these reactions right?

 

[wais]

Hess's Law states that the overall enthalpy change for a reaction is independent of the pathway taken to reach the final products. This means that even though the individual reactions may have different enthalpy changes, the overall enthalpy change will be the same.

In this case, the enthalpy change for reaction 1 is -79 kJ/mol, while the enthalpy change for reaction 2 is not given. However, based on your experimental results, it seems that the enthalpy change for reaction 2 is positive (endothermic). This could be due to various factors such as experimental errors or incomplete reactions.

To accurately measure the enthalpy change for reaction 2, you can repeat the experiment with different amounts of CuSO4 and water to see if the enthalpy change remains consistent. You can also try using a more accurate temperature measurement device and ensuring that the reactions are complete.

It is also important to note that the heat capacity of the solution (4.2 kJ/mol) should only be used to calculate the enthalpy change for reaction 1, as it is the only reaction involving a solution. For reaction 2, the heat capacity of the solid CuSO4 should be used instead. This may also affect the final calculated enthalpy change for reaction 2.

In conclusion, it is possible that there were errors in your experimental setup or calculations that led to a positive enthalpy change for reaction 2. By repeating the experiment and using the correct heat capacity values, you should be able to obtain a more accurate and consistent result.