switching the sign of enthalpy of rxn

[Rosalindbungs]

this is in examkracker 1001 gen chem chapter of solutions

what's given is:
heat of hydration of nacl is -783 kj/mol
change in enthalpy for forming nacl from its ions is -786 kj/mol

question: what is the heat of solution when nacl is dissolved in water?
the answer is +3 kj/mol.

why do you reverse -786 to make it positive?

i thought this:
na + cl = nacl
nacl + h2o= na + cl + h2o

the nacl's and na's and cl's cancel; therefore, the answer is -1569 kj/mol.

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[normtheniner]

this is in examkracker 1001 gen chem chapter of solutions

what's given is:
heat of hydration of nacl is -783 kj/mol
change in enthalpy for forming nacl from its ions is -786 kj/mol

question: what is the heat of solution when nacl is dissolved in water?
the answer is +3 kj/mol.

why do you reverse -786 to make it positive?

i thought this:
na + cl = nacl
nacl + h2o= na + cl + h2o

the nacl's and na's and cl's cancel; therefore, the answer is -1569 kj/mol.

I'm not super sure about this, but the question is asking the heat of the solution when NaCl is dissolves... as in the ionic bond breaks forming the seperate ions Na and Cl. The formation of NaCl from the ions is an exothermic reaction -786 kj/mol, therefore the reverse reaction (the formation of the ions) would be an endothermic reaction +786 kj/mol.

So that would give you a net of +3 kj/mol making the reaction endothermic (as far as I can tell).

 

[Rabolisk]

If you had canceled like you did, you would have gotten
H2O -> H2O with an enthalpy of -1569 kJ/mol.. clearly not right.
NaCl (s) -> Na+(g) + Cl-(g)
Na+(g) + Cl-(g) + H2O(l) -> Na+(aq) + Cl-(aq)
Now the overall reaction is
NaCl(s) + H2O(l) -> Na+(aq) + Cl-(aq).

 

[FrkyBgStok]

yeah these are pretty good answers above but confusing to me. simply put, the change in enthalpy for FORMING NaCl from its ion is -786 kj/mol (given) and the question is asking about forming the IONS from NaCl so it must be reversed.

Na+ + Cl- --> NaCl = -786
NaCl --> Na+ + Cl- = +786