Question

During the formation of \[{\text{NaCl}}\], which of the enthalpies are negative?
A. Lattice enthalpy
B. Electron affinity
C. Both A and B
D. None of these

Answer 1

Hint: The enthalpy will be negative where the energy is released. The more strong and stable the lattice formed more negative will be the lattice energy. Electron affinity is the electron taking tendency of the element.

Complete step by step answer:
Ionic solids are generally very strong solids and they have strong intermolecular forces of attraction. Hence are always found in solid state. Lattice enthalpy is the amount of energy that is required to separate one mole of ionic compounds in solid state into its constituent gaseous ions. In this case the lattice energy will always be positive.
It can also be defined as the amount of energy released when 1 mole of solid ionic lattice forms from its constituent gaseous ion. In this definition the lattice energy will always be negative.
\[{\text{NaCl}}\] Crystal lattice is a very strong lattice and hence when it forms energy is always released and the lattice enthalpy will always be negative.
Electron affinity is the amount of energy released when an electron is added to the neutral atom.
Chlorine has very high electron affinity that is - 349 kilojoule per mole and sodium has a positive electron gain enthalpy of 52 kilojoule per mole. The total electron affinity in the formation of NaCl will still be negative.

So, the correct option is both ‘a’ and ‘b’ that is the option C.

Note:
\[{\text{NaCl}}\] forms a face – centered cubic lattice, in which chlorine occupies the corners and face center and the sodium ion occupies the octahedral voids. The number of atoms that are present in one unit cell of \[{\text{NaCl}}\] is 4.