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Question

Calculate the hydration enthalpy of ions for NaCl(s) from the following data.
ΔlatticeH=+788 kJmol1
ΔsolH=+4 kJmol1

A
784 kJmol1
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B
1084 kJmol1
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C
+784 kJmol1
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D
+1084 kJmol1
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Solution

The correct option is A 784 kJmol1
We know that,
ΔsolnH=ΔhydH+ΔlatticeH
but given that,
ΔlatticeH=+788 kJmol1
ΔsolnH=+4 kJmol1

putting the above values in the formula we get,
+4 kJmol1=ΔhydH+788 kJmol1
ΔhydH=784 kJmol1


Theory:

Lattice enthalpy ΔLatticeH:
Energy required to completely separate one mole of a solid ionic compound into its gaseous constituent ions.

Standard lattice enthalpy ΔLatticeHo:
Energy required to completely separate one mole of a solid ionic compound into its gaseous constituent ions at unit pressure (1 bar) and temperature of 298 K.

Standard enthalpy of hydration ΔhydHoConditions specified = 1 bar and 298 K
Enthalpy change when one mole of anhydrous or partially hydrated salt combines with the requisite amount of water to form a new hydrated stable salt at unit pressure (1 bar) and 298 K.

Integral enthalpy of solution/enthalpy of dissolution:
Enthalpy change when one mole of the solute is dissolved in a definite amount of the solvent to make a solution.
ΔsolnH=ΔhydH+ΔlatticeH


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