Question

Lattice energy of NaCl is $- 186kcal/mol.$If solution enthalpies of $N{a^ + }$ and $C{l^ - }$ are $- 97kcal/mol$ and $- 85kcal/mol$ respectively. The enthalpy NaCl solution in kcal will be:
A) -4
B) 182
C) 368
D) 4

Answer 1

At first we will write about what is given in the question and then we will write the total heat of the solution. We will need only a simple calculation to find the enthalpy of NaCl. Never forget to write the correct unit.

Complete solution:
The lattice energy of Nacl is $- 186kcal/mol.$
The enthalpy of $N{a^ + }$is $- 97kcal/mol$
The enthalpy of $C{l^ - }$ is $- 85kcal/mol$
We have to find out the enthalpy of NaCl.
Step2: The enthalpy change of a solution is the amount of heat that is absorbed or released when the dissolving process happens. It must be at constant pressure. The enthalpy can be either positive or negative. If the enthalpy is negative then the reaction is exothermic means it releases the heat and when the enthalpy is positive it is an endothermic reaction means it will absorb the heat.
Step3. The enthalpy of NaCl will be
The enthalpy of solution will be the addition of the enthalpies for cation and anion subtracted by the lattice energy of NaCl.
$\Delta {H_{solution}} = \Delta {H_{cation}} + \Delta {H_{anion}} - \Delta {H_{LaaticeNaCl}}$
$\Delta {H_{solution}} = ( - 97) + ( - 85) - ( - 186) = 4kcal/mol$
So the enthalpy of NaCl will be $4kcal/mol$.
Hence the correct answer is D.

Note: The enthalpy is the thermodynamic quantity which is equivalent to total heat of the system. It can be explained as the system’s internal energy and the product of its pressure and volume. It is used as measurements in physics, biology and chemistry. The volume and pressure express the work. The enthalpy does not depend on the path. It only depends on the pressure, volume dn internal energy.