[mrxtothaz]

For this question, I'm pretty much expected to use Hess's Law to algebraically add two equations, and then to compare their sum with a third reaction.  The problem is that while the equations seem to add up (reactions 1+2= reaction 3), the heats of reaction don't.  This could be experimental error (the lab results are provided with the question), but the enthalpy is off by quite a bit.  If anyone could point out any error on my part, or affirm my results, I would very much appreciate it.

I have been provided with the results of a lab:

Reaction 1:
NaOH(s) --> Na⁺(aq) + OH^-(aq)
Reaction 2:
Na⁺(aq) + OH^-(aq) + H⁺(aq) + Cl^-(aq) --> H2O(l) + Na⁺(aq) + Cl^-(aq)
OH^-(aq) + H⁺(aq) --> H2O(l)
Reaction 3:
NaOH(s) + H⁺(aq) + Cl^-(aq) --> H2O(l) + Na⁺(aq) + Cl^-(aq)
NaOH(s) + H⁺(aq) --> H2O(l) + Na⁺(aq)

The enthalpies for each reaction:
∆H°r: -5.80 kJ (Reaction 1)
∆H°r: -5.06 kJ (Reaction 2)
∆H°r: -16.7 kJ (Reaction 3)

I have been asked to add the equations of 1 and 2 (Hess's Law), and to compare the sum with equation 3:

NaOH(s) --> Na⁺(aq) + OH^-(aq) ∆H°r: -5.80 kJ (Reaction 1)
OH^-(aq) + H⁺(aq) --> H2O(l) ∆H°r: -5.06 kJ (Reaction 2)

NaOH(s) + H⁺(aq) + OH^-(aq) --> Na⁺(aq) + H2O(l) + OH^-(aq) ∆H°r: -10.9 kJ (Reaction 1+2 intermediate)

NaOH(s) + H⁺(aq) --> Na⁺(aq) + H2O(l) ∆H°r: -10.9 kJ (Reaction 1+2)

NaOH(s) + H⁺(aq) --> Na⁺(aq) + H2O(l) ∆H°r: -16.7 kJ (Reaction 3)

So, as can be seen, the equations for reactions 1&2 are added and are identical to that of reaction 3.  However, the enthalpies of reaction are way different.  I do not know how I can account for this steep a difference, as all the lab results were provided with the question.  The only difference is that, in the addition of reactions 1&2, there was a hydroxide ion on both sides of the equation that was cancelled out (since I was asked for the net ionic equations for each reaction).  Could this account for the difference, or does it have no role in the enthalpy of reaction, given that it is a spectator ion?

If I did nothing wrong, I suspect I am supposed to identify this discrepancy between reaction 1+2 with reaction 3. The only thing I can think of is that, in calculating the enthalpy of reaction, the specific heat capacity of each solution (density, as well) as assumed to be the same as that of water.

[Zeppos10]

the literature says that the enthalpy for reaction 1/2/3 are 44/56/100 kJ/mol.
errors in the lab involve quantities measured and heats measured.
What is the actual question here ?